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Anthracnose in Olives

Anthracnose

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:

  • Flowers – 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?

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.