Managing Bitter Rot in Apples

Bitter rot is a serious disease of apple fruit caused by the fungal pathogen Colletotrichum fioriniae (formerly C. acutatum). This disease has caused significant losses in apple growing regions that experience very warm and humid conditions such as in some Southern US states and Central and South America. This disease has not been a major issue for Ontario apple growers in the past. However, something has changed recently and more bitter rot has shown up in some Ontario apple orchards over the past couple of years.

Epidemics of this disease occur when warm to hot weather occurs early in the growing season together with rainy periods that extend into the summer. Recognizing the disease is an important first step towards management. The pathogen can infect fruit at any time during the growing season from as early as bloom right up until harvest. However, symptoms often show up mid-late season after a period of warm to hot weather accompanied by rain or a thunderstorm. The initial infection appears as small grey or brown spots that quickly enlarge into circular sunken light to dark brown rots on infected fruit (Figure 1). Warm temperatures hasten the rotting process. Under humid conditions, cream to salmon-coloured masses of spores are produced on the surface of the rotting fruit which is very diagnostic. These spores can then be rain splashed to other fruit resulting in further infections and more rotten apples. If the spores land on fruit just before or during harvest, infection can occur and small bitter rot lesions will develop slowly while in cold storage. These small spots will begin to enlarge within a few days as the fruit warms up. As the fruit lesions enlarge, a diagnostic V-shaped rot may progress towards the core (Figure 2), but this does not always occur and may not be the most reliable symptom for disease diagnosis.

Figure 1. Bitter rot appears as a circular sunken rot with cream to salmon-coloured spore masses produced near the center of the lesion.

Figure 1. Bitter rot appears as a circular sunken rot with cream to salmon-coloured spore masses produced near the center of the lesion.

Figure 2. Bitter rot lesions develop a diagnostic V-shaped rot that progresses towards the core.

Figure 2. Bitter rot lesions develop a diagnostic V-shaped rot that progresses towards the core.

The pathogen can overwinter in infected mummified fruit left on trees or on the orchard floor, as well as in colonized cankers caused by other pathogens such as fire blight or black rot. There have been some reports of more bitter rot appearing in orchards that also had fire blight possibly due to the pathogen surviving in the fire blight cankers and spreading to fruit later in the growing season.

Management of bitter rot is through good orchard sanitation. Removing old cankers, as well as mummified fruit left in the trees from chemical thinning will remove the potential primary source of this disease from the orchards. Mulching or removing infected fruit on the orchard floor will also reduce inoculum and the potential of spreading the disease. Some fungicides registered for the management of apple scab and other summer diseases may provide some protection against infection from the bitter rot fungus. However, there is still more to learn about managing bitter rot in Ontario. More research into the efficacy of new fungicides, timing of fungicide application, protection of fruit going into storage and other potential solutions for managing bitter rot is required.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca