Effect of calcium chloride on bitter rot in apples

Bitter rot was a serious disease in many Ontario apple orchards this past season. The disease is favoured by very warm or hot weather with occasional rainy periods similar to the conditions experienced in 2016. Initial infections can occur as early as late bloom or petal fall but the pathogen can infect fruit at any time during the growing season. Early symptoms appear 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). Symptoms can appear as early as mid-July but often the disease is not noticed in Ontario orchards until early August or later depending on environmental conditions. 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 sticky spores are produced on the surface of the sunken lesion which is very diagnostic (Figure 3). The sticky spores are rain splashed or carried by insects to other fruit resulting in further spread and more rotten apples (Figure 4).

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

Figure 1. Bitter rot initially appears as a 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. Cream to salmon -coloured spore masses develop in the sunken bitter rot lesion during humid conditions.

Figure 3. Cream to salmon -coloured spore masses develop in the sunken bitter rot lesion during humid conditions.

Figure 4. Insects that visit bitter rot lesions can pick up the sticky spores and carry them to other fruit.

Figure 4. Insects that visit bitter rot lesions can pick up the sticky spores and carry them to other fruit.

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. Research at the University of Guelph has shown that the pathogen can also survive and produce a few spores on the surface of weed leaves for several weeks without infecting or causing disease of the weed.

Bitter rot management starts with good orchard sanitation. Removing old fire blight, black rot and other cankers reduces the overwintering source of this disease in the orchards. Mulching or removing infected fruit from the orchard floor will also help reduce inoculum and the potential of spreading the disease. Allegro, Granuflo T and Pristine fungicides are registered for the control of bitter rot in Ontario apple orchards. However, other fungicides registered for the management of apple scab and other summer diseases may provide some protection against infection from the bitter rot fungus.

The application of calcium salts and particularly calcium chloride (CaCl2) have been shown to reduce bitter rot incidence and severity in apple trials conducted in West Virginia during the late 1990's. In 2016, OMAFRA set up a trial in a block of Empire apple trees to further investigate the potential of CaCl2 for controlling bitter rot under Ontario conditions. The trees were sprayed with either CaCl2, Pristine WG or Allegro 500F on a 10-14 day schedule throughout the season. Untreated trees were left for comparison.

Both Pristine WG and Allegro 500F significantly reduced the incidence of bitter rot and number of lesions/fruit at harvest. CaCl2 applied at 3.9 kg/ha (1.4 kg Ca/ha) suppressed the incidence of bitter rot and significantly reduced the number of lesions/fruit. CaCl2 did not appear to cause damage to the Empire trees or increase Ca levels in fruit or leaf tissue at harvest. Results of this trial will be presented in a poster at the 2017 Ontario Fruit and Vegetable Convention in Niagara Falls, Ontario on February 22-23, 2017.


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