Bitter rot symptoms and management in apples during 2015

Bitter rot has recently become a serious disease of apple fruit in some orchards in Ontario. The disease is caused by the fungal pathogen Colletotrichum fioriniae (formerly C. acutatum). Prior to 2010, this disease had 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. However, something has changed 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, much like the 2015 growing season we are experiencing in Ontario. The spores of 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 (Figure 1) that quickly enlarge into circular sunken light to dark brown rots on infected fruit (Figure 2). As the fruit lesions enlarge, a diagnostic V-shaped rot may progress towards the core (Figure 3), but this does not always occur and may not be the most reliable symptom for disease diagnosis. Warm temperatures hasten the rotting process until the fruit becomes shriveled and completely rotten (Figure 4). Severely rotten fruit often fall from the tree prematurely. During humid conditions, cream to salmon-coloured masses of spores are produced on the surface of the rotting fruit which is very diagnostic (Figure 5). 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 (Figure 6). Often a few small black spore producing structures develop within the lesions during cold storage. These small lesions will begin to enlarge within a few days after the fruit is removed from storage.

Figure 1. Bitter rot initially appears as small grey, reddish or tan brown spot on fruit.

Figure 1. Bitter rot initially appears as small grey, reddish or tan brown spot on fruit.

Figure 2. Bitter rot develop into a circular sunken lesion.

Figure 2. Bitter rot develop into a circular sunken lesion.

Figure 3. Bitter rot lesions develop a diagnostic V-shaped rot that progresses towards the core.

Figure 3. Bitter rot lesions develop a diagnostic V-shaped rot that progresses towards the core.

Figure 4. Advanced bitter rot lesion completely rots fruit which fall prematurely.

Figure 4. Advanced bitter rot lesion completely rots fruit which fall prematurely.

Figure 5. Cream to salmon coloured spore masses develop in the sunken bitter rot lesion during humid conditions.

Figure 5. Cream to salmon coloured spore masses develop in the sunken bitter rot lesion during humid conditions.

Figure 6. Small grey bitter rot lesions with small black fruiting bodies can develop slowly in cold storage.

Figure 6. Small grey bitter rot lesions with small black fruiting bodies can develop slowly in cold storage.

The pathogen can overwinter in infected mummified fruit left on trees or on the orchard floor. It can also colonize in cankers caused by other pathogens such as fire blight or black rot. Recently, there have been 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 fire blight, black rot and other cankers, will reduce the potential primary source of this disease in the orchards. Mulching or removing infected fruit on the orchard floor will also help reduce inoculum and the potential of spreading the disease. Allegro and Pristine fungicides are registered for the control of bitter rot in Ontario apple orchards. However, some fungicides registered for the management of apple scab and other summer diseases may provide some protection against infection from the bitter rot fungus. Fungicide applications should be made every 10-14 days during the warm summer months, however, spray intervals should be shortened to 7 days if frequent rain is experienced. If possible, timing an effective fungicide application just prior to a rain event or thunderstorm will protect fruit from rain splashed spores. Always rotate products to reduce the potential for resistance developing. There is some evidence that calcium sprays may also help protect fruit, however, the efficacy, timing, rate and frequency of calcium applications is still under investigation.


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