Ministry of Agriculture, Food and Rural Affairs

Bitter Rot Management in Apples

Bitter rot has been showing up in a few orchards recently and is becoming a serious disease in some Ontario apple orchards. The disease is caused by the fungal pathogen Colletotrichum fioriniae (formerly C. acutatum). Epidemics of this disease may occur when periods of warm or hot weather occurs together with intermittent rainy periods, much like the weather conditions experienced during the 2017 growing season. The spores of the pathogen can infect fruit at any time from as early as bloom right up until harvest. However, symptoms often show up mid - late season after a period of warm weather accompanied by rain or a thunderstorm.

Initial bitter rot infection appears as small grey, reddish 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. 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 are very diagnostic (Figure 4). These spores can then be rain splashed to other fruit resulting in further infections. 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 5). 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 2. Bitter rot develops into a circular sunken lesion.

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

Figure 4. Cream to salmon coloured spore masses develop in the sunken bitter rot lesion during humid conditions.

Figure 5. 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 cankers caused by other pathogens such as fire blight or black rot. There have been reports of 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 of an over-wintering 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.

Granuflo T / Thiram, 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 also provide some protection against infection from the bitter rot fungus. Fungicide applications should be made every 10-14 days during dry warm summer months, but 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 evidence that regular calcium chloride sprays may also help protect fruit. In a recent study conducted by OMAFRA during 2016, calcium chloride (CaCl₂) applied every 10 to 14 days suppressed the incidence of bitter rot and significantly reduced the number of lesions/fruit at harvest. CaCl₂ did not appear to cause damage to apple trees or increase Ca levels in fruit or leaf tissue at harvest.