At the 5th International Conference on Sustainable Agriculture and Biosystems, researchers in Iran presented findings on why fruit and flower drop occurs in fruit trees and what growers can do to reduce losses. Drawing on their work, we explore the phenomenon of fruit and flower drop, the underlying causes, and practical remedies for orchard managers.    

The Role of Fruit Trees in Agriculture

Fruit trees contribute significantly to agricultural economies across the world. Their production supports farm income, provides employment for skilled horticulturists, and underpins industries ranging from fresh produce to processing and food manufacturing. Cultivation involves a complex mix of practices: careful irrigation management, balanced fertilisation, pruning, pest control, and the application of modern technologies. Beyond economics, fruit trees are valued for their nutritional contributions, supplying sugars, oils, proteins, vitamins, and essential minerals through both fresh consumption and processed goods.

The Phenomenon of Fruit and Flower Drop

Fruit and flower drop is a natural occurrence in many tree species. Its extent varies according to cultivar, climate, soil type, and orchard practices. In some cases, drop is beneficial, helping the tree regulate excessive fruit load through “natural thinning.” But when drop is excessive or occurs at the wrong stage, it becomes detrimental, reducing yields and profitability.

Researchers typically divide drop into three categories:

1. Natural Drop: This type results from competition between a very high number of flowers. Up to 90% of flowers and fruitlets may be shed in some species, but the remaining fruit usually develop adequately. Growers sometimes assist by thinning to ensure the fruit that remain receive sufficient nutrition and reach high quality.
2. Abnormal Drop: This type is more damaging because it affects flowers and fruits at various stages of development, including larger fruit, often reducing yields significantly.
   
3. Extreme Temperature Drop: This occurs during periods of hot, dry weather (often around June in many regions). While widespread, it usually has limited impact on the final crop load.

Causes of Abnormal Drop

Environmental Factors

• Extreme cold or heat events can damage blossoms or young fruit.
• Strong winds, particularly dust-laden storms, may strip flowers and fruit from trees.
  
• Heavy rainfall or hail can injure delicate blossoms.
  
• Excessive direct sunlight can cause sunburn, leading to premature fruit drop.
  

Mechanical Factors

• High-pressure pesticide or nutrient spraying can dislodge flowers.
• Over-application or incorrect doses of sprays may also contribute to flower drop.
  

Physiological Factors

• Poor pollination and failed fertilisation result in flowers dropping.
• Over-competition for nutrients between flowers and developing fruit increases drop.
  
• Nutrient deficiencies, especially nitrogen and zinc, play a role.
  
• Irregular irrigation - including overwatering that suffocates roots - can trigger drop.
  
• Shallow soils limit root systems, reducing nutrient uptake and increasing drop.
  
• Weak or diseased leaves caused by pests, fungal infections, or poor tree health reduce a tree’s ability to support fruit.
  
• Misuse of plant growth regulators or tank mix errors in spraying can disrupt flowering.
  
• Infections of flowers or fruit by fungi or insects often lead to drop.
  
• Natural ripening and senescence also account for some degree of fruit shedding.
  

The Science Behind Fruit and Flower Drop

The underlying mechanism of drop is closely linked to plant hormones. As fruits grow, the concentration of auxins (growth-promoting hormones) declines, while ethylene levels rise. This shift lowers the fruit detachment force (FDF), weakening the connection between fruit and tree. The abscission zone (the separation layer) becomes increasingly sensitive to ethylene, causing fruit drop. Environmental conditions such as temperature and humidity interact with these hormonal signals to intensify drop.

Remedies to Reduce Fruit and Flower Drop

Growers can apply several strategies to reduce drop and improve fruit set:

• Fertilisation: Apply fertilisers well before flowering to build soil fertility. Nitrogen should be supplied before bloom, not after, to avoid negative effects.
• Micronutrient Sprays: Foliar applications of calcium, zinc, and boron—adjusted for tree age and timing - can enhance pollination and fruit retention.
  
• Pruning: Moderate winter pruning balances vegetative growth with fruit production.
  
• Girdling: In some fruit tree industries (e.g., apples and pears), carefully removing a ring of bark from branches at flowering can improve fruit set.
  
• Irrigation Management: Avoid excessive watering during bloom and fruit set to prevent root suffocation.
  
• Growth Regulators: The use of auxin-based products, such as seaweed extracts (rich in auxins, cytokinins, and gibberellins), can delay fruit drop and extend the flowering period.
  
• Pest and Disease Management: Prompt control of pests and fungal infections prevents cascading effects on fruit drop.
  
• Pollination Support: For self-incompatible varieties, ensure compatible cultivars or introduce pollinators like bees and butterflies.
  
• Windbreaks: Plant hedges or wind barriers to reduce wind damage and limit flower and fruit loss.

Practical Advice

While some degree of fruit and flower drop is unavoidable, excessive losses can usually be mitigated through careful orchard management. Attention to fertilisation, irrigation, pest control, and pollination provides the best defence against unnecessary drop.

Further Reading and References

• Concentrated Seagold Seaweed Extract
• Nitrogen Fertilisers
  
• Boron Application
  
• Calcium Fertilisers
  
• Zinc
  
• Soil Test & Leaf Analysis
  
• Pruning and Grafting tools
  
• Pruning machines 
  
• Gardening tools
  

Scientific Reference

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) . 

       

 An olive tree showing branch dieback and defoliation due to Phytophthora root rot. Infected trees often wilt, develop yellow leaves that drop, and can either die suddenly or decline over several years. These symptoms frequently manifest when the tree is under stress (e.g., during flowering, fruit development, or hot weather) and correspond to extensive root damage and crown cankers in the lower trunk.    

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. 

Growers have observed that olive trees showing moderate dieback will flush new healthy shoots after a couple of rounds of Ca+B foliar nutrition, as opposed to continuing to deteriorate. By maintaining an every-6-8-weeks program through spring and summer, the recovering tree has a better chance to rebuild its canopy and even some root mass (because improving the canopy’s health allows the plant to allocate energy to root regrowth). This approach is a supportive therapy – it does not attack the pathogen, but rather helps the tree tolerate the infection and outgrow the damage. Calcium also contributes to disease resistance by strengthening cell walls, making it a bit harder for Phytophthora to advance through tissues, while boron is crucial for the healing of damaged tissues and the growth of new meristems.

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. 
  

In summary, phosphorous acid vs. Ca-B foliar feeds are not competing remedies but complementary ones. Phosphonate fungicide is your frontline defense to reduce the pathogen load and protect the roots. Calcium and boron sprays (as part of a broader foliar nutrition plan) are a means to nurse the tree back to health by encouraging new growth and mitigating dieback symptoms. Phosphonate keeps the disease in check, giving the tree a chance to regenerate; the CaB and other nutrients give the tree the resources it needs to actually do that regeneration. Growers have found that using both in tandem yields far better outcomes than either approach alone – phosphonate without nutritional support may stabilize the tree but leave it languishing, whereas nutrition without phosphonate lets the disease continue to destroy roots. Thus, an integrated approach is essential.

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. 
  

By combining these strategies, growers create multiple layers of defense against Phytophthora. Integrated disease management means you are never relying on just one method. For example, good drainage and careful irrigation make the soil less conducive to the pathogen; phosphonate treatments reduce the pathogen’s ability to infect; nutritional sprays help the tree recover faster; and hygiene stops the spread to new areas. Each component reinforces the others. This holistic approach is particularly essential in Australia’s summer-rainfall regions, where Phytophthora pressure can be high –  growers in these areas have learned that only vigilant, year-round management will keep Phytophthora root rot at bay and their olive trees productive.

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.