Occasionally a sap-sucking insect known as Brown or Black Olive Scale will be seen on olive trees. It is rarely a problem if the trees are in good health. We usually only spray our mature trees for scale every two to three years and only then if they need it. However, certain areas of Australia are more prone to the scale.

If your olive tree has black spots on branches or an infestation of black scale, it's crucial to act quickly. Scale on olive trees, including black olive scale, appears as dark bumps that weaken growth. For black scale treatment, use a proven treatment, introduce beneficial insects, and prune for better air circulation. If you're wondering how to get rid of black scale on an olive tree, early detection and prompt action are key to protecting your grove.

About

The adult females are very easy to recognise on the olive tree stems. They are dome shaped, dark brown to black in colour, and about the size of a match head.

The tiny eggs laid under the female, look like piles of very fine sand. Mainly during the summer, these eggs hatch into tiny, six-legged, cream coloured ‘crawlers’. The crawlers move up the stems and usually settle along the veins of young leaves. At this stage they don’t have the impervious shell of the adult and can usually be killed with one or two applications of white oil about two weeks apart. White oil should be used only as directed on the label by the manufacturers (and by your agricultural department) and never during the hot part of the day. It puts an oil film over the young ‘crawler’ and suffocates it. If applied in the hot part of the day it also stops the leaves from breathing properly and can be detrimental to the tree. The White oil application will also tend to rid the tree of ‘sooty mould’ as discussed soon.

If the crawlers are allowed to live, they will moult after about one month and then migrate to the young stems and twigs of the tree. Here they will mature and lay more eggs and their protective brown shells will be impervious to white oil. Squash the scale between your fingers to see if it is alive. If it is alive, then your fingers will be wet from the juices squeezed out. If it is dead then your fingers will be dry and dusty.

Bad infestations of live mature scale may need spraying with an insecticide such as Supracide. (Important: See note regarding “Treatment”) In Greece, Supracide is the main spray used for most olive problems. Once again, check with your local agricultural chemical supplier and the product label, for directions.

Probably the damage done by the scale itself to the tough olive tree is negligible compared with what happens next.

As the scale feeds, the ‘manure’ they excrete is a sweet, sticky, ‘honeydew’. This excreted sticky liquid can finally cover the leaves of the entire tree. A fungus known as sooty mould feeds on this food and multiplies until the entire tree may be covered with the black sooty mould. This is where the real problem lies.

The leaves are coated with the black deposit, so the sun’s light can’t penetrate the leaves properly. Therefore photosynthesis can’t take place efficiently. Therefore, ‘root producing’ food is not manufactured in the leaf. Therefore roots don’t develop properly. Therefore the poor root system can’t collect enough food and water from the soil to send up to produce more leaves, which in turn will produce more root. Once the vicious cycle begins, a stunted and unhealthy tree with poor crops is the result.

To make the problem worse, sweet ‘honeydew’ on the leaves also attracts large numbers of ants. It appears that as the ants constantly move over the scale, they frighten away the small wasp parasites which in normal cases would keep the scale under control.

Black Olive Scale Gallery

The good news is that healthy olive trees don’t get the scale, sooty mould, and ant infestation to any great extent. More good news is that heavily infested trees are easily fixed.

Normally, one thorough spraying of the entire tree and soil below with a systemic insecticide will be adequate. Nevertheless, to be sure, a second spray about two weeks later may be worthwhile.

Now, if there is no more live scale, there is no more eating, therefore no more ‘honeydew’ excreta, therefore no more sooty mould and ants. Over a period of time the dead sooty mould deposit will peel off the leaves from exposure to the rain, wind and sun. The green leaf surface will be exposed and growth will continue as normal. Treat the tree to an occasional feeding of Seagold fertilizer/mulch and foliar application and some water and watch its health come back.

Admiral Advance by Sumitomo for control of Black Olive Scale
** DELIVERY INCLUDED in Australia
EAdmiral
$315.00(ex tax)

Pybo - Natural Pyrethrum Contact Insecticide 24 hour withholding period!
Solvent-free botanical insecticide for fast-acting, broad-spectrum pest control.
EP0
$246.90(ex tax)

Pyganic Insecticide - Organic Pest Control
Certified Organic Botanical Crop Protection
EA.PyGa
$149.25(ex tax)

Seaweed Extract Composted Seaweed Fertiliser - SEAGOLD
100% natural powdered seaweed extract
EA09
$75.00(ex tax)

Sumi-Alpha Flex 20L
Dual-use insecticide for controlling insects and mites
EA.ALPHA
$480.00(ex tax)

Scientific Name:  Saissetia oleae

DESCRIPTION OF THE PEST

Black scale adult females are about 0.20 inch (about the size of a match head) in diameter. They are dark brown or black with a prominent H-shaped ridge on the back. Young scales are yellow to orange crawlers and are found on leaves and twigs of the tree. Often, a hand lens is needed to detect the crawlers. Black scale usually has one generation per year in interior valley olive growing districts. In cooler, coastal regions multiple generations occur. Black scale prefers dense unpruned portions of trees. Open, airy trees rarely support populations of black scale.

DAMAGE

Young black scale excretes a sticky, shiny honeydew on leaves of infested trees. At first, affected trees and leaves glisten and then become sooty and black in appearance as sooty mould fungus grows on the honeydew. Infestations reduce vigour and productivity of the tree. Continued feeding causes defoliation that reduces the bloom in the following year. Olive pickers are reluctant to pick olive fruits covered with honeydew and sooty mould.

CULTURAL CONTROL

Pruning to provide open, airy trees discourages black scale infestation and is preferred to chemical treatment.

BIOLOGICAL CONTROL

A number of parasites attack the black scale, the most common are Metaphycus helvolus, Metaphycus bartletti, and Scutellista cyanea. These parasites, combined with proper pruning, provide sufficient control in northern and coastal orchards. In other regions, biological control is often ineffective because the black scale’s development pattern hampers parasite establishment.

ORGANICALLY ACCEPTABLE METHODS

Cultural and biological control and oil sprays. Organic pyrethrum sprays like Pyganic ( Pybo is no longer organically certified).

WHEN TO TREAT

If infestations are resulting in honeydew, treat the crawlers. In interior valleys, delay treatment until hatching is complete and crawlers have left protection of the old female body. Once crawlers have completely emerged, a treatment can effectively be made in summer, fall or winter provided the scales have not developed into the rubber stage (later second instar, which are dark, mottled grey, and leathery, with a clear H-shaped ridge on the back).

TREATMENT

Due to the chemical nature of the treatments, Please check with your agricultural chemical supplier as to the suitability, application and safety precautions of your chosen scale treatment for olives. Some growers have used Summer or Petroleum Oil and Supracide.  Californian olive growers use Oil Emulsions, Diazinon 50WP, Methidathion and Carbaryl. The use of chemicals reduces the microbial population in your soil and can inhibit the uptake of certain nutrients to your trees.  Harmful residues of chemicals can also build up in your soil structure.

A new product Admiral has become available which acts as an insect growth regulator rather than a kill-on-contact pesticide, it has been quite effective and like any treatment of scale; timing is essential.  Ants can be controlled with an Ant Bait suitable for Horticultural use.  We suggest Distance Plus Ant Bait.

• Admiral Advance
• Pybo
• Pyganic
• Distance Plus Ant
  

References

“Olives – Pest Management Guidelines” (UCPMG Publication 8, 1994). These guidelines cover the major olive problems found in Australia and California and are available for free from their website http://www.ipm.ucdavis.edu/PMG/selectnewpest.olives.html . (The information comes from California so all references to places, seasons, months and treatments are Californian). If you have any questions, please contact The Olive Centre, PH: 07 4696 9845, Email: sales@theolivecentre.com.au

Admiral Advance by Sumitomo for control of Black Olive Scale
** DELIVERY INCLUDED in Australia
EAdmiral
$315.00(ex tax)

Pybo - Natural Pyrethrum Contact Insecticide 24 hour withholding period!
Solvent-free botanical insecticide for fast-acting, broad-spectrum pest control.
EP0
$246.90(ex tax)

Pyganic Insecticide - Organic Pest Control
Certified Organic Botanical Crop Protection
EA.PyGa
$149.25(ex tax)

Seaweed Extract Composted Seaweed Fertiliser - SEAGOLD
100% natural powdered seaweed extract
EA09
$75.00(ex tax)

Sumi-Alpha Flex 20L
Dual-use insecticide for controlling insects and mites
EA.ALPHA
$480.00(ex tax)

The Olive Lace Bug (Froggattia olivinia) has become an increasingly significant concern for olive producers. These sap-seeking insects primarily feed on the undersides of olive leaves, causing distinct yellow mottling on the leaf surface. If left unmanaged, affected leaves typically turn brown, leading to premature leaf drop. Severe infestations can result in substantial loss of tree vitality, defoliation, and notably reduced fruit yields. 

Native to New South Wales and Southern Queensland, the Olive Lace Bug has been recorded across other Australian states as well. New infestations can occur frequently throughout the growing season, with the pest capable of producing three to four generations annually. 

Proactively identifying and managing these pests is crucial to safeguarding your groves and maintaining consistent productivity. Our detailed article provides valuable insights and practical strategies for effectively controlling and preventing Olive Lace Bug infestations.

#OliveLaceBug #OliveGroves #PestManagement #OliveIndustry #Agribusiness #OliveProduction #CropProtection #IntegratedPestManagement #OliveGrowers #SustainableFarming #AustralianOlives #AgricultureNews #OliveFarming #Horticulture #FarmManagement

The Olive Lace Bug (Froggattia olivina) is an Australian native sap-sucking insect posing significant threats to olive groves. It specifically targets olive trees (Olea europaea), potentially reducing yields and causing tree death if left unmanaged. Olive lace bug infestation is considered a serious threat to the olive industry in Queensland, New South Wales, Victoria and across Australia.

Olive Lace Bug (Froggattia olivina) infestation on the underside of an olive leaf, showing multiple life stages 

nymphs, adults, and characteristic black excrement spots.

STAGES OF OLIVE LACE BUG 

Lifecycle

Female Olive Lace Bugs insert eggs into the tissue on the undersides of leaves, usually along the midribs. Eggs hatch into nymphs, which pass through five moults before reaching adulthood. Olive Lace Bug overwinters as eggs, with hatching typically occurring in early spring (September to October). Adults may also overwinter in protected locations on trees. Depending on climate conditions, there may be one to four generations per year, with a lifecycle ranging from 12-23 days in warm weather to up to 7 weeks in cooler conditions.

Distribution and Spread

Originally native to New South Wales and southern Queensland, olive lace bugs have spread throughout Australia, excluding the Northern Territory. The movement of olive plants and industry activities have facilitated this spread. Juvenile bugs, relatively immobile, cluster on leaf undersides and are easily spread through planting materials, workers, and tools. Adults disperse via short flights or wind

Identification and Monitoring

• Regularly inspect leaf undersides from early spring. 
• Early infestations appear as rusty-yellow spots about half the size of a pin-head on the upper surface of leaves, contrasting clearly with the dark green leaf surface. 
• Severe infestations result in leaf browning, premature drop, and twig dieback.

Damage and Symptoms

Heavy infestations significantly affect tree vigor, delaying flowering and fruiting, reducing yields for up to two seasons, and potentially causing young tree death. Mature trees can also be severely affected, with death observed in extreme cases.

Host Plants

Known hosts include native mock olive (Notelaea longifolia) and cultivated olives (Olea europaea).

Integrated Pest Management (IPM) Strategies

• Regular Monitoring: Check frequently to detect early infestations. 
• Cultural Practices: Keep trees healthy through adequate fertilisation, irrigation, systematic pruning, and canopy management. Avoid stress caused by poor soil preparation, proximity to large eucalypts, or nutrient deficiencies.
• Biological Control: Support beneficial predators such as lacewing larvae, ladybird beetles, and predatory mites. Note: Biological controls require a continual supply of the pest to be effective, which can be difficult to achieve in the long term.
• Spray Controls: Apply proven products known to be effective in the control of OLB.  See more:  Olive Lace Bug Products

Effective Spray Regime

• Spray soon after initial detection. Severe infestations may require a second treatment 10-14 days later. 
• Apply thorough coverage on leaf undersides. 
• Prune regularly to open the canopy, improving spray effectiveness and reducing pest habitat.

Long-term Sustainability

• Maintain optimal tree health with regular nutrient checks and soil testing. 
• Remove dead or unwanted branches. 
• Educate staff on proper pest identification and management techniques.

By proactively managing olive lace bug, you safeguard the health and productivity of your olive groves, ensuring sustained profitability.

Olive trees are well adapted to hot and dry Mediterranean climates, but even they can suffer from sunburn (also called sunscald) when exposed to intense sunlight, heat stress, or when bark that was previously shaded becomes suddenly exposed. In mature or neglected/abandoned groves - especially those with heavy pruning or thin canopies - the risk can increase significantly. 

Sunburn damage weakens trees, opens the way for pests and disease, slows growth, and in severe cases may lead to branch dieback or decline. It’s worth recognising early and managing before the damage becomes irreversible.

Sunburn Damage to Olive Tree Trunk -The image above shows classic symptoms: cracked, peeling bark and exposed wood.

Recognising Sunburn Damage in Olives

Here are key symptoms to watch for:

• Bark becomes discolored (straw, pale, or bleached areas) compared to healthy bark.
• Cracking, peeling or flaking bark, sometimes exposing pale wood beneath.
  
• Sunken or collapsed bark patches where the surface is depressed.
  
• In severe cases, sections of bark slough off entirely and expose dead wood.
  
• Cankers or lesions forming in the affected areas.
  
• Reduced leaf vigor, scorching or browning of leaves, especially near the canopy edges.
  
• Fruit drop or shriveling if the tree is already bearing. High heat stress may trigger olive abscission.
  
• Over time, branch dieback or trunk weakness in the sunburned section may appear.
  

One important effect is that sunburned bark is more vulnerable to pest and fungal invasion, such as wood-boring insects or opportunistic pathogens that exploit the compromised protective barrier. 

Because olive trees often live many decades, even older trees can sustain recovery—provided the damage is not too extensive and you intervene early.

Why Sunburn in Your Olive Grove Is a Concern

• Reduced growth and productivity: Damaged bark and cambium reduce the tree’s ability to translocate water and nutrients. The tree may divert energy to healing instead of growth or fruiting.
• Structural weakness: Sunken or damaged trunk areas may become weak points, prone to breakage or cracks later, especially under wind stress. 
  
• Higher susceptibility to pests and pathogens: Exposed or cracked bark invites insects (borers) or fungal pathogens to colonize. 
  
• Delayed recovery: If large patches are affected, the tree may require a long time to compartmentalize the damage, and growth may be permanently affected in that area.
  

Preventative Measures & Remedies

Here’s a set of strategies you can apply now or over seasons to protect your olive trees and help heal existing damage.

1. Maintain or restore shade to the trunk

• When pruning, retain lower branches or scaffold limbs that offer partial shading to the trunk. Don’t prune so aggressively that bark is suddenly exposed. 
• Use ground covers, mulch, or low shrubs around the dripline (but not touching the trunk) so that radiant heat from bare soil is reduced.
• If possible, plant shade species (small trees or shrubs) in-line rows or adjacent to blocks to break sunlight incidence midday or afternoon.

• Whitewashing or painting the trunk with a light, water-based paint (often diluted limewash or similar) helps reflect sunlight and reduce temperature extremes. Many growers use this method on sensitive or newly exposed trunks. 
• Use tree wrap or reflective sleeves on trunks, ideally on the side facing intense sun (often western or northwestern exposure in Australian climates). Wrapping material should allow air movement—avoid tight plastic wraps that trap moisture. 
  
• In olive orchards, kaolin clay sprays (e.g. “Surround” brand or similar) are sometimes used on foliage and trunk to reduce radiant heat absorption and protect against sun and heat stress. Some trials report yield improvements by reducing fruit burn and drop under high-heat conditions.
  

• Ensure the tree is not already under water stress. Provide adequate soil moisture during hot seasons (without overwatering).
• Use mulches (organic materials like prunings, compost, bark chips) to help moderate soil temperature and reduce evaporation, which helps maintain a stable microclimate for roots.
• Avoid practices that leave the soil bare and hot—bare clay can retain and radiate heat back onto trunks.

• Remove loose or dead bark carefully, but don’t over-prune or cut live tissue aggressively. Let the tree compartmentalize the damage naturally.
• For deeper or cankered sections, consult a tree health specialist to assess whether you need to trim back to sound wood or apply wound dressings.
• Monitor the area over seasons; the tree may form callus growth around the margin of the injury and seal it internally if conditions are favorable.
• Avoid additional stress (drought, nutrient deficiency, pests) in damaged trees so energy is available for healing.

• As you rehabilitate your grove, assess tree spacing, row orientation, and tree height to reduce reflective heat loading.
• Avoid creating large expanses of bare, reflective ground under rows. Maintain a cover crop, grass alley, or soil cover to diffuse heat.
• Track which trees show signs of sunburn after pruning or canopy changes. Use careful pruning patterns that don’t suddenly expose shaded bark.

Signs of sunburn in olive trees

Sunburn appears as pale, bleached bark patches on exposed trunk surfaces, cracks or peeling bark, and sometimes sunken or depressed bark areas. In advanced damage, bark may fall off, leaving wood exposed. Leaves near the margins of canopy may show browning or scorching, and fruit may drop prematurely under heat stress.

If you can, have a sample branch punched from just beyond the edge of the sunburn area so an arborist or consultant can evaluate whether live cambial tissue remains. Also, map out which exposures (north, west, etc.) in your grove tend to show sunburn more often—this helps plan protective shading or wrap strategies.

As you re-establish your grove’s health in other areas (soil fertility, drainage, pest and disease management, good pruning), protecting against sunburn becomes part of the maintenance process rather than a standalone issue.

References

• UC IPM (University of California): guidance on whitewashing trunks to prevent sunburn/sunscald on trees. 
• The Olive Centre: overview of sunburn damage in olive trees, risk factors (water deficit, heat), and vulnerability to borers.
• Tessenderlo Kerley technical note: kaolin (Surround) particle film reduces heat load/sunburn and can improve olive yield/quality under high-radiation conditions.
• Peer-reviewed study (Horticulturae/MDPI): mineral clay particles (incl. kaolin) evaluated on olives for effects on yield and oil quality.
• Research summary (IPB/Portugal): field experiments with kaolin 5% on ‘Cobrançosa’ olives under rainfed and deficit irrigation; particle film proposed to reflect heat/irradiance. 
• Australis Plants (AU olive resource): practical tips—white water-based paint (50:50) or trunk wraps on young/renovated olives; risk after hard summer pruning.
• NSW DPI (citrus reworking guide—general orchard practice): after heavy cutting, paint exposed trunks/limbs with diluted white water-based paint to prevent sunburn (principle applicable to olives after renovation pruning). 
• Agriculture Victoria (orchard recovery): recommends whitewash/diluted white paint on trunks/large limbs to minimise sunburn following canopy loss—relevant where olive canopies are thinned or defoliated. 
• Australian Olives (Olives Australia) tutorial: Peacock spot factsheet (context—sunburned tissue predisposes to disease; useful companion reference for disease pressure in humid regions).

What is Phytophthora Root Rot?

Phytophthora root rot is a destructive soil-borne disease of olive trees caused by Phytophthora species (water-mould pathogens). At least seven Phytophthora species have been identified attacking olives in Australia . These pathogens infect roots and can extend into the lower trunk, causing root decay and crown cankers that girdle the tree. If left untreated, Phytophthora root rot can kill olive trees, either through a rapid collapse or a slow decline over several seasons . The disease has been observed in many olive-growing regions worldwide, often linked to periods of excessive soil moisture. 

Symptoms: Infected olive trees typically show a loss of vigor and drought-like symptoms even when soil moisture is adequate. Foliage becomes sparse as leaves wilt, turn yellow, and drop prematurely . Shoot dieback starts at the tips of branches and progresses downward. In advanced cases, entire limbs or the whole canopy may wilt suddenly, especially under stress conditions like hot weather, flowering or heavy fruit load . Root and trunk symptoms include soft brown rot of feeder roots and lesion-like cankers at the crown or lower trunk; peeling back bark at the base often reveals reddish-brown discoloration of the wood. Affected trees may respond by shooting new suckers from the lower trunk or roots as the upper canopy dies back . Over time, the trunk can exhibit cracks or distortions due to the underlying canker damage . In some cases, trees can decline gradually over years, whereas in other cases they collapse quickly when the compromised root system can no longer support the canopy (for example, during a heatwave or late summer) . 

Contributing Factors and Disease Spread

Waterlogging and Poor Drainage: Excess soil moisture is the single biggest contributing factor to Phytophthora root rot in olives. Phytophthora thrives in saturated, oxygen-deprived soils. Australian surveys have consistently found Phytophthora outbreaks correlated with waterlogged conditions, claypan soil layers, or generally poor drainage in groves. Even a short period of waterlogging (as little as 24 hours) in warm temperatures can kill fine olive roots and predispose trees to infection. Low-lying orchard areas, heavy clay soils that drain slowly, or sites with a high water table create ideal conditions for the pathogen. It’s important to note that while waterlogging is a common trigger, Phytophthora can sometimes cause problems even in well-drained soils if the pathogen is present and environmental conditions (temperature, soil moisture) become favorable. In high-rainfall climates or during unusually wet seasons, otherwise well-drained olive blocks may still experience Phytophthora issues if drainage cannot keep up with prolonged rainfall. 

Susceptible Rootstocks: Most olive trees in Australia are grown on their own roots (i.e., not grafted), but in cases where different rootstocks or wild olive (Olea europaea subsp. africana) seedlings are used, susceptibility can vary. Caution is advised when using feral/wild olive trees as rootstocks or nursery stock. These plants often originate from areas where Phytophthora is present in the soil and may introduce the pathogen or be less tolerant to it. There is currently no widely available Phytophthora-resistant olive rootstock, so all varieties should be assumed susceptible. Research by Spooner-Hart et al. noted that the emergence of Phytophthora problems in Australian olives has coincided with the expansion of plantings into non-traditional (non-Mediterranean) climates and heavier soils. This underscores the role of environment and rootzone conditions in disease incidence.

Warm, High-Rainfall Climates: Olives are traditionally adapted to Mediterranean climates (winter rain, dry summers). In parts of Australia with warm temperatures and summer-dominant rainfall (e.g., coastal Queensland and northern New South Wales), the risk of Phytophthora root rot is higher. The pathogen is widespread in soils and waterways in these regions and can easily infect olive roots when wet, warm conditions persist. Growers in such climates must be especially proactive with prevention measures. High humidity and frequent rain not only favor the pathogen but can also mask early drought-stress symptoms – an infected tree might not show obvious distress until a dry period or heat event reveals the extent of root loss.

Disease Spread: Phytophthora produces motile spores (zoospores) that swim in free water, so the pathogen spreads with water movement through soil and runoff. It can be introduced or spread in a grove via infected nursery stock, contaminated soil on equipment, flood irrigation water, or even the boots of workers moving from an infested wet area to a clean area. Once in the soil, Phytophthora can persist for years in root debris or as resilient spores. Thus, any practice that moves soil or water (e.g., tractor implements, drainage flows) from an infected zone to an uninfected zone can facilitate the dissemination of the disease. Growers should avoid transferring mud from known infested blocks and ensure any new trees planted are from disease-free sources (pathogen-free certified nurseries). 

Best Practices for Managing Phytophthora in Olives 

Successful management of Phytophthora root rot in olives relies on an integrated strategy. This includes preventative chemical treatments, supportive nutritional therapies, and cultural practices to improve soil conditions and reduce pathogen spread. The goal is to protect healthy roots from infection, eradicate or suppress the pathogen in soil where possible, and help affected trees recover. Below are the current best practices recommended for Australian growers:

Preventative Use of Phosphorous Acid (Phosphonate) Fungicides

Phosphorous acid (also known as phosphonate or phosphite) is a key fungicide for managing Phytophthora in many tree crops and is a cornerstone of preventative treatment in olives. Phosphonate does not act like a typical fungicide that directly kills the pathogen on contact; instead, it works by inhibiting Phytophthora growth and stimulating the tree’s own defense mechanisms. This dual mode of action makes it most effective as a preventative treatment, applied before or at the very early stages of infection, to help the plant resist invasion. Phosphorous acid is available under various trade names (e.g., Fosject 400, Agri-Fos 600, Foli-R-Fos 200, Yates Anti-Rot) with different concentrations of active ingredient. Always confirm that the product is permitted for use on olives and follow the label or permit directions. 

Application timing and rates: On woody perennial crops like olives, foliar sprays of phosphonate are typically applied approximately every 6 weeks during the growing season for ongoing protection. This ensures a consistent level of the fungicide within the plant, as it is systemic and will move into the roots. Label rates depend on product concentration; for example, products with 600 g/L a.i. are used around 2.5 mL/L, 400 g/L formulations at 5 mL/L, and 200 g/L formulations at 10 mL/L (when applied with an air-blast sprayer to fully cover the foliage). For young or small olive trees, high-volume spraying to runoff ensures good coverage. Crucial timing is just before periods of high risk – e.g., before winter rains or summer wet spells – so that the roots are protected in advance.

In situations where an olive tree has very little foliage left (severe defoliation from root rot), phosphonate can be applied as a bark spray or trunk injection. Spraying a ~10% phosphorous acid solution directly on the trunk or injecting the solution into the lower trunk can deliver the fungicide to the vascular system when leaves are insufficient. Trunk application is usually done in autumn or spring when the tree is actively translocating, to maximize uptake. Always exercise caution with concentrated trunk sprays to avoid phytotoxicity and adhere to recommended concentrations.

Mode of action and benefits: Once absorbed, phosphonate is translocated downward with the sap flow, reaching the roots and inhibiting Phytophthora in infected tissues. It also primes the tree’s immune response. Treated trees often show not only disease suppression but also improved new root development in some cases. Phosphonate is valued for being relatively inexpensive and having low toxicity to humans and non-target organisms, making it a practical choice for routine preventative use. In warm, high-rainfall regions of Australia where Phytophthora is endemic, applying phosphonate prophylactically to young olive trees can protect them until their root systems establish. Many agronomists recommend an initial phosphonate spray or injection soon after planting in such regions, followed by periodic treatments during the wet season.

It’s important to remember that phosphonate is a suppressive, not an eradicant, treatment. It significantly reduces Phytophthora levels and activity in the tree but does not eliminate the pathogen from the soil. Therefore, continuous or at least annual reapplications are needed to maintain protection. If treatments are stopped, Phytophthora can rebound if conducive conditions return. Also, phosphonate works best on preventing new infections and halting early disease – severely diseased trees (with the majority of roots already rotted) may not recover with fungicide alone. In those cases, phosphonate can only prevent further spread while other measures support the tree’s regrowth.

Other fungicides: Another chemical option is metalaxyl-M (e.g., Ridomil Gold), a systemic fungicide specifically targeting oomycete pathogens like Phytophthora. Ridomil can be applied as a soil drench or via injection to kill Phytophthora in the root zone. It has shown effectiveness in olives, but similar to phosphonate, it does not sterilize the soil and must be reapplied periodically to keep the pathogen in check. Phosphonate is often preferred for long-term management due to lower cost and resistance risk, but Ridomil drenches can be useful as a curative kick-start in heavily infested soils or to protect newly planted high-value trees. Always rotate or mix chemical modes of action as allowed, to prevent the development of fungicide resistance in the Phytophthora population. 

Foliar Calcium and Boron to Aid Recovery 

In addition to fungicides, nutritional support plays a critical role in managing Phytophthora root rot – especially for helping infected trees recover. Two nutrients in particular, calcium (Ca) and boron (B), have been observed to assist olive trees suffering from root rot. Calcium and boron are closely associated with the growth of new shoots and root tips; they are essential for cell wall strength (Ca) and cell division/floral development (B). Some olive varieties have relatively high requirements for Ca and B compared to other fruit trees, and deficiencies of these nutrients often manifest as dieback of shoot tips (boron deficiency can cause tip death and poor new leaf growth, while calcium deficiency leads to weak stems and twig dieback).

When roots are compromised by Phytophthora, the tree’s ability to uptake nutrients from the soil is severely impaired. Ailing roots mean even if fertilizers are in the soil, the tree may still suffer from nutrient deficiencies. Foliar feeding can bypass the damaged root system and deliver nutrients directly to the leaves and young shoots. Foliar sprays of calcium and boron have shown positive results in reducing twig dieback and stimulating new growth on moderately affected olive trees. The recommended practice (from field experience in Australia) is to apply calcium and boron together on a regular schedule during the active growing season:

• Calcium nitrate at 10 g/L plus Solubor (boron) at 1.5 g/L, mixed in water, applied as a fine foliar spray every 6–8 weeks. Calcium nitrate provides a readily absorbed form of calcium (along with some nitrogen to spur growth), and Solubor is a common soluble borate fertilizer that corrects boron deficiency. These can be tank-mixed and sprayed to cover the foliage; ideally, apply in the cooler part of the day (morning or late afternoon) to reduce the risk of leaf burn. 

It’s worth noting that while calcium and boron are the focus for tip dieback, other nutrients should not be neglected. Trees battling root rot might also benefit from magnesium (for chlorophyll), zinc (for hormone production), and other micronutrients if deficient. However, over-applying any one element can cause imbalances or toxicity (boron, for instance, can be toxic above recommended rates). Stick to label rates and recommended concentrations for all foliar feeds, and monitor leaf nutrient levels if possible. The Ca+B foliar program should be seen as one component of a broader nutritional management plan for stressed trees. 

Complete Foliar Nutrient Programs for Impaired Roots

Beyond calcium and boron, a complete foliar nutrient program is advised for olive trees with significantly impaired root systems. Because root rot limits uptake of both macro- and micro-nutrients, foliar applications of a balanced fertilizer can supply the tree with essential nutrients until roots recover. Many agricultural suppliers offer soluble foliar fertilizer blends (NPK plus trace elements) that can be sprayed on the canopy. These blends often contain nitrogen, phosphorus, and potassium, as well as micronutrients like zinc, manganese, iron, copper, molybdenum, etc., in plant-available forms. Applying such a foliar feed can green up a chlorotic, declining tree and promote new leaf and root development while bypassing the diseased root system.

A suggested regimen is to spray a complete foliar fertilizer (for example, an NPK 20-20-20 with trace elements, or a product formulated for orchard foliar feeding) on a monthly or bi-monthly schedule during the growing season. This can often be done in conjunction with the calcium nitrate and boron sprays – either by alternating them or, if compatibility is confirmed, combining them in one tank mix. Be cautious when mixing fertilizers with fungicides: phosphonate is generally compatible with many fertilizers, but always jar-test combinations or consult product labels.

Foliar nutrient programs should be tailored to the grove’s specific deficiencies. If leaf analysis or visual symptoms indicate particular nutrient shortages (e.g., yellowing between veins might indicate magnesium or iron deficiency, small, distorted new leaves could indicate zinc deficiency), include or emphasize those nutrients in the foliar mix. Maintaining good overall nutrition will improve the tree’s resilience. Stronger, well-nourished olive trees have a better chance to compartmentalize Phytophthora infections and resume normal growth once conditions improve. Remember that these sprays supplement but do not replace soil fertilization; once roots recover function, reinstating a normal soil fertilizer program (adjusted for any residual soil fertility and the tree’s regained capacity) is important for long-term production.

Improving Soil Drainage and Grove Management 

Cultural controls that improve the soil environment are fundamental to managing Phytophthora – no chemical or nutrient can fully substitute for a well-drained root zone. Growers should evaluate their grove for any conditions that contribute to waterlogging or poor root health and take corrective action:

• Improve drainage: Ensure that water is not pooling around olive roots for extended periods. For new plantings, select well-drained sites or use raised beds/mounded rows in heavier soils. Building the planting rows as mounds (for instance, 30–40 cm above the aisle) allows water to drain away from root zones more quickly. In existing groves, consider installing drainage solutions such as surface drains, French drains, or deep ripping between rows to break up hardpans. If a hard clay subsoil (clay-pan) is identified, deep rip or auger planting holes through it and backfill with a more friable soil mix before planting, to prevent perched water tables. Also, maintain grassed inter-rows or gentle slopes to channel excess rainwater off the orchard rather than letting it stagnate. After heavy rain, inspect the orchard to identify any spots where water stands and address those with drains or by regrading the soil. 
• Optimize irrigation: Over-irrigation can be just as harmful as poor natural drainage. Adjust your irrigation scheduling and method to prevent waterlogging. Use soil moisture sensors if possible to guide irrigation, and err on the side of “drier” rather than “wetter” when Phytophthora risk is high. For example, instead of one long irrigation set, you might split it into shorter, more frequent sets that allow more oxygen into the root zone between waterings. Microsprinklers or drip emitters should be placed such that they wet the root zone adequately but do not create continuously soggy conditions. Make sure emitters are functioning correctly and not leaking excessively in one spot. Avoid irrigating just before evenings or periods of cool, humid weather – it can extend soil wetness duration. Proper irrigation management is part of integrated Phytophthora control, as noted by Queensland’s Department of Agriculture: avoid both over- and under-watering, since stress from drought can also predispose trees to infection or make symptoms worse.
  
• Soil amendments: Increasing soil organic matter can improve structure and drainage in the long term. Using mulch or cover crops in the inter-row can enhance soil porosity and microbial activity (which can sometimes suppress pathogens). Apply organic mulches under the dripline of olive trees to help soil structure, but keep mulch a few inches away from the trunk to avoid creating a perpetually moist collar around the base. In clay soils, the addition of gypsum can help flocculate clay particles and improve permeability. Gypsum (calcium sulfate) applied under the canopy can also provide calcium to the soil profile, which some studies suggest may reduce Phytophthora spore formation or activity (noting that very high soil pH can actually favor the disease, so use gypsum (pH-neutral) rather than lime unless you need to correct acidity). Always test soil pH before adding lime.
  
• Grove hygiene and design: Treat Phytophthora-affected sections of the grove almost as a biohazard area to prevent spread. Do not move soil from infected areas to clean areas – for example, if you dig out a dead tree, dispose of that soil away from the orchard or sterilize it. Clean farm machinery, tools, and footwear after working in a muddy, suspect area. Restrict access to the orchard when the soil is wet (to avoid picking up mud on tires). If using surface water (from dams or creeks) for irrigation, be aware that it could harbor Phytophthora spores from upstream sources – consider water treatment or use of drip irrigation that limits soil splash. In windbreaks or nearby vegetation, note that some ornamental or wild plants can be hosts for Phytophthora; controlling weeds and alternative host plants may reduce inoculum reservoirs. When replanting where an olive tree died of root rot, it’s wise to improve the site drainage and possibly leave the hole fallow or treat the soil (some growers solarise the soil or apply fungicides like metalaxyl pre-plant) before putting a new olive in the same spot
  
• Adjusting grove practices: Other cultural adjustments can reduce stress on at-risk trees. For instance, avoid heavy pruning of diseased trees (they need as much healthy leaf area as possible to regenerate roots) – only remove dead wood and lightly shape to balance the canopy. Do not remove those water shoots or suckers that often appear on the lower trunk of sick trees; as recommended by Australis Plants, allow these shoots to grow (pruning them back only moderately so they don’t become dominant branches) because they help the tree regain foliage and vigor. They can always be pruned off later once the tree fully recovers. Likewise, be cautious with fertilizing a tree with a severely compromised root system – small, frequent doses or foliar feeds are safer than a heavy soil fertilizer application, which the damaged roots cannot absorb (and which could burn them or leach away). Finally, monitor Phytophthora-affected trees closely. If a tree is not responding to treatments (fungicide + nutrients) and continues to decline, it may be better to remove it and focus efforts on protecting surrounding trees. A rotting stump or roots can continue to harbor the pathogen, so in some cases, stump removal or fumigation might be warranted in patch areas of severe infection.

Phosphorous Acid vs. Calcium–Boron Treatments: Efficacy and Limitations

Both phosphonate fungicides and calcium-boron foliar feeds are important tools in managing Phytophthora root rot, but they serve different purposes and have distinct advantages and limitations. It’s not an either/or choice – in fact, they are complementary in a comprehensive management program. Below is a comparison to clarify their roles for growers:

• Phosphorous Acid (Phosphonate) Fungicide: This is a direct disease-control agent. Its primary benefit is its proven efficacy in suppressing Phytophthora within the tree. Phosphonate is currently the most effective chemical for slowing root rot in olives; it can arrest the progression of the pathogen and protect new growth when applied properly. Advantages of phosphorous acid include its systemic action (it reaches roots from foliar or trunk application), relatively low cost, and safety profile (no significant residue issues in fruit, and safe to handlers when used as directed). It also has some plant health benefits, like promoting new root initiation. However, phosphonate has limitations: it is preventative and works best if in the plant before heavy infection occurs. It will not revive roots that have already been killed, nor eliminate the pathogen from the soil. Continuous use is needed to maintain protection, and over-reliance on any single fungicide mode of action can risk the pathogen developing reduced sensitivity (though Phytophthora resistance to phosphonate has been reported only in a few cases, it’s still a consideration). Also, for certified organic olive production, synthetic phosphonate use is usually not allowed – organic growers have extremely limited options beyond cultural controls and perhaps some biofungicides (which have lower efficacy). So, phosphonate is a powerful tool, but it addresses the cause of the disease (the pathogen) rather than the tree’s weakened condition.
• Calcium-Boron Foliar Nutrition: This is a supportive treatment aimed at the tree’s health, not at killing the pathogen. The calcium nitrate + boron sprays help the olive tree by supplying critical nutrients to emerging shoots when roots cannot do so. The key advantage of this approach is that it tackles the symptoms (tip dieback, stunted new growth) and helps the tree to produce new foliage and roots despite the disease. By strengthening cell walls (Ca) and improving meristem growth (B), the foliar nutrients can reduce twig dieback and fruit drop, thus maintaining yield potential better than if the tree were left to decline. Calcium and boron applications are relatively inexpensive and can be easily combined with other foliar feeds. Crucially, they can improve a tree’s vigor, which indirectly makes it more resilient and better able to recover once the pathogen is suppressed. The limitation, of course, is that calcium and boron do not target Phytophthora at all. If used alone, they would not stop the root rot from spreading; a tree might look momentarily better as new leaves flush, but the disease could still be advancing in the roots unabated. Therefore, relying solely on nutritional sprays would be insufficient in a moderate to severe Phytophthora outbreak. Another limitation is that foliar uptake of nutrients can be affected by weather (rain can wash sprays off, very hot days can cause foliar burn or poor absorption), so timing and repetition are important. Finally, one must ensure that other nutrient needs are met – Ca and B address a specific issue, but a tree might also need nitrogen or potassium, etc., which is why a complete foliar nutrient program is recommended alongside Ca+B. 
  

It’s also worth comparing phosphonate with the other fungicide option, metalaxyl (Ridomil). Phosphonate and Ridomil both suppress Phytophthora, but in different ways. Ridomil is more of a curative, directly toxic to the pathogen, whereas phosphonate has those immune-boosting properties. Ridomil can knock back an active infection faster, but it has a higher cost and a risk of resistance development in the pathogen population with overuse. In practice, phosphonate is often used for regular protection, and Ridomil (if used at all) might be reserved for spot-treating severe cases or as a pre-plant soil drench in known infested sites. Both chemicals require reapplication; neither provides permanent protection. Always follow an Integrated Disease Management philosophy when using these tools – they are most effective when combined with the cultural and nutritional strategies described above.

Integrated Disease Management (IDM) in Australian Olive Groves

Managing Phytophthora root rot requires an Integrated Disease Management approach, especially in Australia’s warm, high-rainfall olive-growing regions. No single intervention is a silver bullet; instead, growers should implement a suite of preventive and remedial measures that together minimize disease impact. Below is a summary of IDM practices for Phytophthora root rot in olives: 

• Start with healthy, disease-free planting material: Only source olive trees from reputable, Phytophthora-free nurseries. Inspect the root systems of new trees (if possible) – healthy roots should be white and fibrous, not brown or foul-smelling. Avoid planting olives that show any signs of root rot or cankers. This prevents introducing the pathogen to your grove.
• Select and prepare sites wisely: Prioritize well-drained sites for new olive blocks. If you must plant in a heavier soil, invest time in soil preparation (deep ripping, adding gypsum/organic matter) to improve drainage. Form planting mounds or raised beds to keep root zones high and dry. Identify any low spots in the field and address them (through drainage tiling or by simply not planting olives in the very wettest spots). Good site selection and preparation are the most cost-effective long-term defense.
  
• Optimize water management: Design irrigation systems and schedules to meet olive water needs without creating waterlogged conditions. Use drip or micro-sprinklers to localize water and avoid overspray. Regularly check that irrigation is not contributing to puddling. During rainy periods, turn off irrigation entirely. Remember that olives are drought-tolerant compared to many fruit trees; slight under-watering is safer than over-watering in Phytophthora-prone areas. Also, avoid planting cover crops or pasture in the orchard that require frequent irrigation – keep the inter-row groundcover something that can survive on minimal water.
  
• Monitor and act early: Train yourself and staff to recognize early symptoms of Phytophthora (e.g., leaf yellowing, tip dieback, unusual leaf drop or wilting that isn’t explained by heat alone). Mark suspects trees and considers taking soil or root samples for lab testing to confirm the Phytophthora species. Early detection allows for prompt phosphonate treatment and targeted drainage fixes before the problem spreads or the tree is too far gone. If one tree in an area shows symptoms, proactively treat neighboring trees – they may be infected but not yet showing severe symptoms. 
  
• Apply chemical controls as part of a program: Use systemic fungicides like phosphorous acid as preventative sprays during high-risk periods (e.g., before and during the wet season). Follow up with repeat applications as per the label to maintain protection. If a tree is identified with active root rot, consider a curative treatment (such as a high-rate phosphonate injection or a metalaxyl drench around the root zone) to immediately reduce pathogen load, then continue with routine phosphonate. Always check the APVMA permits and registrations to ensure the product and method you choose are allowed in olives, and observe any withholding periods if the grove is in production. Rotate chemical modes of action if possible to prevent resistance – although options are limited (essentially phosphonates and phenylamides like metalaxyl), do not rely on just one product year after year without guidance. 
  
• Nutritional and soil health management: Maintain adequate nutrition in the grove to avoid stressing trees. Ensure soil pH and fertility are in the optimal range for olives (pH ~6.5–8, adequate but not excessive nitrogen, and sufficient phosphorus and potassium based on soil tests). Stressed or malnourished trees are more susceptible to infection and less likely to recover. After flooding or waterlogging events, consider applying a broad-spectrum foliar fertilizer to give trees a boost, as waterlogging can leach nutrients and damage roots. Incorporate organic matter through mulching or cover cropping (with species that do not harbor Phytophthora) to improve soil structure and microbial diversity, which can create a more hostile environment for the pathogen. Some growers also introduce biological controls like Trichoderma or mycorrhizal fungi into the soil, aiming to outcompete or antagonize Phytophthora – while scientific results on these are mixed, a healthy soil biota generally benefits root health.
  
• Hygiene and quarantine practices: Treat Phytophthora like you would a contagious disease. Clean pruning tools between trees (a bleach or alcohol dip can kill Phytophthora on tools). After removing dead trees or doing any excavation in an infected area, disinfect equipment and even shoes – soil clinging to a shovel or tractor tire can carry zoospores across the orchard. Avoid moving water from a known infested block to other blocks (for example, don’t pump runoff water from a sick block into your irrigation dam). If possible, keep a footbath or a brush station at the entry to a sensitive grove so that visitors don’t inadvertently bring in mud. Avoid sharing equipment with other farms known to have Phytophthora issues, or insist on thorough cleaning. If you yourself have multiple orchards, visit your Phytophthora-free orchard before visiting the infested one on the same day (not after), to reduce the chance of carrying soil back. These biosecurity measures may sound tedious, but they can save you from turning a localized problem into a farm-wide one. 
  
• Resistant varieties and rootstocks: As of now, there are no olive cultivars immune to Phytophthora, but research is ongoing into relative tolerance. Some anecdotal reports suggest that certain olive varieties handle wet feet slightly better than others – for instance, hardy traditional cultivars vs. some high-oil, fast-growing cultivars – but all will succumb if conditions are bad enough. If establishing a new grove in a high-risk site, consult local olive experts or nursery suppliers about any available rootstock or clone bred for Phytophthora resistance. The olive industry internationally is exploring grafting onto rootstocks of closely related species (like wild olive) for disease resistance, but these are not yet common practice. In the future, planting resistant rootstocks could become part of IDM (as it is in the avocado industry), but for now, Australian growers must focus on the other measures. 
  

Conclusion

Managing Phytophthora root rot in olives is challenging, but with vigilant management, it is possible to minimize losses and even restore affected groves to health. The keys are prevention (through site selection, drainage, and preventative fungicides) and support (through nutrition and careful cultural care for stressed trees). Australian olive growers should view Phytophthora management as an ongoing part of grove management, much like pruning or pest control, especially in regions prone to heavy rainfall. By implementing the integrated strategies outlined above, growers can significantly reduce the impact of Phytophthora root rot, protecting their trees and investment. Remember that every grove is different – monitor your olive trees closely and adapt these recommendations to local conditions, and always reference current guidelines from olive industry research and local agricultural authorities. With a proactive, informed approach, even the threat of “root rot” can be managed, and olive trees can continue to thrive and produce in the Australian landscape.

Sources: 

• Spooner-Hart, R. et al. (2005). Sustainable Pest and Disease Management in Australian Olive Production. RIRDC Publication No. 05/080. 
• Spooner-Hart, R., Tesoriero, L., & Hall, B. (2007). Field Guide to Olive Pests, Diseases and Disorders in Australia. RIRDC (eds.).
  
• Australis Plants Nursery. (2007). Phytophthora Root Rot in Olive Trees – Practical guidelinesPhytophthora Root Rot in Olive Trees
  
• Fruit Tree Lane (Australis Plants). (2023). Managing Phytophthora Root Rot in Olive Trees.
• Bailey, A., Hall, B., & Tesoriero, L. (2017). Symptoms and management of Olive diseases and disorders. The Olive Centre Blog.  
  
• Business Queensland, Dept of Agriculture. (2022). Phytophthora Root Rot – Integrated Management.