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Blossom End Rot of Apples

Although apple scab was the major concern on apple fruit this year, harvest assessments also showed problems with blossom end rot, or calyx end rot. In the past, this damage has been reported occasionally, but recently has become a key factor in fruit out-of-grade for McIntosh and Empire in some orchards.

Blossom end rot, or calyx end rot, can be caused by three different fungi:

Sclerotinia sclerotiorum (the cause of white mold on beans)
Botrytis cinerea (the gray mold fungus)
Botryosphaeria obtusa (the black rot fungus).

Taking samples for lab diagnosis is the best way to determine the causal fungus, but characteristic symptoms are also helpful to determine the offending fungi.

When blossom end rot is caused by B. obtusa, symptoms usually develop only as fruit ripens in autumn, although sepal infections occurred earlier. Symptoms of blossom end rot caused by the other pathogens appear earlier during summer.

Blossom end rot, caused by B.cinerea, is sometimes called "dry eye rot" and is not common. The disease develops when B. cinerea colonizes dying petals, moves into sepals and eventually into fruit. Botrytis infections in sepals may remain quiescent until storage and then develop into fruit decays.

More commonly is calyx end rot caused by S. sclerotiorum. Calyx end rot lesions often die out at 1/4 to ½" diameter. Infected fruit appear normal at harvest, except for the dry lesion on the calyx. Sometimes, however, apples with calyx end rot color prematurely and drop from the tree before harvest.

Not much research has been done on the biology and management of S. sclerotiorum on apples, but we know about managing this fungus in other crops. The fungus survives in sclerotia (black pebble-like resting structures) in a wide range of broad-leaved plants. In spring, the sclerotia germinate and produce wind-dispersed ascospores that germinate and grow on senescing flower petals and then invade the plant tissues.

Reducing the presence of broad-leaved plants in the orchard ground cover with 2,4-D applications may help to reduce the inoculum. Leaving the orchard cover long in the early part of the season may also capture wind-dispersed spores before they reach the blossoms and newly formed fruit.
Little is know about how fungicide programs may affect calyx end rot, so the OMAFRA apple team will be investigating management strategies to try to reduce the increasing grade-out problem from blossom end rots.

Adapted from an article by Dr. David Rosenberger, Scaffolds, May 9, 2005.