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.
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 ascospore. 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 15°C and 25°C, the optimum being 25°C. 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:
- Flowers/Blossom: causing blight
- Leaves leaf spots or peacock spot.
- Fruit before harvest: spotting and rotting
- Post harvest fruit infection
What can be done to help prevent Anthracnose?
In previous years these are some of the products that have been used for olives which include:- copper oxychloride, cupric hydroxide, cuprous oxide, dichloflunid, dimethomorph, mancozeb, metiram, pholoraz, propineb, thiram and zineb. Through the APVMA permits are available generally for the use of copper based products for various olive fungal leaf spots and fruit rots including peacock spot. In relation to products to help prevent Anthracnose, please check out the APVMA Website for the PERMITS: latest approvals OTHER APPROVALS: check pubcris database
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.
*** Please note: it is a legal requirement that label instructions are followed, if olives are not listed on the label of the pesticide, it is either not registered for use on olives or an off label permit is required.