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During the oil-processing season, Anthracnose can be a problem for some growers in certain areas. It is urgent that this issue is addressed otherwise oil quality may be drastically reduced.
Description
The fungus that causes this disease is called Gloeosporium olivarum. Anthracnose is the general term given to diseases that result in roundish black spots on fruit, leaves and stems of various plant species.
What are the symptoms?
Initially, single roundish ocre-coloured spots appear on the olives. Later these spots grow and may join up. Normally attacks start at the tip of the fruit where rainwater accumulates. As the disease progresses, all or part of the infected fruit starts to rot, dries up, shrivels and becomes mummified.
The oil obtained from olives harvested with anthracnose is poorer in quality because alterations occur in their colour, acidity and organoleptic characteristics.
How the fruit is infected?
There are two main sources of spores produced by a species of Collectotrichum, a type called ‘conidio’ and others called ‘ascospores’. The conidia remain active inside the mummified fruit for a year, which may be the primary inoculum source.
Collectotrichum is a very successful colonizer of plant material. It grows rapidly and can cause large amounts of tissue destruction. Collectotrichum exhibits an alternative growth pattern where they have a two-phase infection process (or secondly infection) in which the initially appear symptomless but the fruit has been infected. After this period of non-destructive growth or if high moisture levels are maintained, the fungus when switches to its normal phase where tissue breakdown occurs.
When rainfall occurs, conidia separate from their fruiting body and then quickly enter the fruit through the epidermis. The conidia always need moisture, whether it is rain, dew or high humidity, to germinate and infect.
Normally infection takes place at between 15oC and 25oC, the optimum being 25oC. At this temperature, the typical symptoms of the disease appear within 2-3 days and the ascospores appear about 3 days later.
Anthracnose can affect various parts of the plant:
What can be done to help prevent Anthracnose?
There are no approved chemicals for olives but there are chemicals registered for controlling Anthracnose which include:- champ dry prill, copper oxychloride (organic growers use this quite successfully), cupric hydroxide, cuprous oxide, dichloflunid, dimethomorph, mancozeb, metiram, pholoraz, propineb, thiram and zineb. Permits are available for the use of cupric hydroxide and copper oxychloride for various olive fungal leaf spots and fruit rots including peacock spot.
Sprays should be applied now and should double as a preventative for anthracnose and peacock spot. It is too late to try to fix the problem after it happens. Prevention is important. Olive oil from anthracnose-infected fruit will be poor in quality and often only suitable for burning in oil lamps.
INFORMATION SHEET - PEST & DISEASES
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.
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.
Adult scale on the underside of olive leaves
Overturned scale with orange crawlers showing.
An olive branch covered in sooty mould.
Closeup of sooty mould on olive leaf.
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.
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.
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.
Pruning to provide open, airy trees discourages black scale infestation and is preferred to chemical treatment.
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.
Cultural and biological control and oil sprays. Organic pyrethrum sprays like Pyganic ( Pybo is no longer organically certified).
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).
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.
“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
Due to Tropical Cyclone Alfred, our operations may be affected over the coming days.
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