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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
I have a couple of olive trees with a bark problem. This particular tree is a bit stressed from lace bug I believe, I'm not sure they've had their full care for the last couple of years due to the transition. It needs a bit of a prune too which I will take care of before I spray it. But will wait for your comments first. See image below.
Sunburn can occur in olive trees as damage to bark, foliage, fruit which is caused by excessive solar radiation exposure and seriously affect growth.
Sunburn can cause the tree the olive tree to be more susceptible to borers. Olive trees affected by sunburn are typically poor in health and if severe enough can result in premature death.
Sunburn in olive trees is usually associated with warm weather coupled with water deficit.
Although olive trees are well adapted to hot and dry conditions, too much heat can result in lower yields, leaf wilt, and reduced photosynthesis as the olive tree shuts down critical functions to respond to heat stress.
Older trees can be damaged when the bark is newly exposed to the sun because of pruning or premature leaf drop. Heavy pruning of olive trees can lead to increase reflected light or radiate heat around established trees can also lead to sunburn.
Sunburn to trunks can leave the tissue dried, cracked and sunken and the bark may peel away leaving the wood susceptible for borer attack or fungal infections to enter the bark for further damage to the exposed hardwood underneath.
Foliage may be brown and necrosis begins at leaf tips, margins and between veins.
Heat stress usually results in fruit drop if an olive crop is present.
Encourage good soil health and moisture-holding capacity. Encourage branch structure with proper pruning and plant training. Retain branches that will help to shade the trunk and be beneficial for cropping.
Give trees adequate irrigation to reduce stress and do not overwater trees.
For olive trees and fruit to reduce the risk of sunburn use Kaolin clay applied as a foliar film to help offer protection in reducing heat stress and intense solar radiation.
Whitewashing trunks may help prevent sunburn which is usually seen as an application of rubberised tree sealant. Sometimes water-based paints are used.
If leaves have not already been killed, sunburn injury to foliage can often be remedied by adequate irrigation, adding shade or shelter, and improving soil conditions.
Bark has been affected by intense sun radiation exposure and also water deficit.