Black Rot and Chemical Thinning

Black rot in apple orchards has been increasing in incidence and severity recently. The pathogen that causes Black rot can infect limbs, trunks, leaves and fruit, resulting in reduced productivity as well as reduced fruit quality. The Black rot fungus Botryosphaeria obtusa is an opportunist that enters wounds on limbs and trunks created by the cold injury. One reason for the higher incidence of this disease in Ontario orchards these past few years may be due to the fungus invading wounds caused by winter injury on limbs sustained in some apple orchards during the 2004-5 winter. Cankers that developed in orchards last year (in 2005) provided the inoculum for the high incidence of frog-eye leaf spot and Black rot on fruit observed this year in several orchards.

Figure 1. Black rot on fruit associated with mummified fruitlets

Black rot on fruit has often been associated with mummified fruit left in trees after chemical thinning Figure 1. Often chemical thinners are applied when king fruit is around 8 mm or even late at 14 mm, which optimizes fruit number and fruit size. Unfortunately, the abscission layer between the stem of the fruitlet and the spur does not form properly when thinners are applied at this fruit stage and the fruitlet remains attached to the spur throughout the entire season. Fruitlets that become infected with the Black rot fungus and mummified remain as a source of inoculum, which can infect nearby fruit.

Recently a study was initiated to see if the timing, rate and types of chemical thinners influenced the number of mummified fruitlets left in trees and the incidence and severity of Black rot infected fruit. A low and high rate of thinners were applied separately to trees either at petal fall, 8 mm king fruit or 14 mm king fruit. Results indicate that applying a thinner at petal fall, regardless of rate or type, resulted in a pretty good thinning response, good fruit weight and size, and a lower incidence and severity of Black rot on the fruit (Figure 2). Furthermore, a significantly lower number of mummified fruit occurred in trees that received an application at petal fall (Figure 3). Applying thinners earlier may not maximize yield as much as applying them later, however, applying thinners earlier to reduce the number of mummified fruitlets and subsequently reduce the incidence of Black rot on fruit may result in better quality of yield. Future research into the best rate and timing is required to ensure that results observed this year are consistent before earlier timings are recommended.

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