Scaffolds 99 index

CALYX-END ROT OF APPLES

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Calyx-end rot (also known as blossom-end rot) is a sporadic problem in New York apple orchards. Infections occur around petal fall. Black or dark brown lesions appear at the calyx-ends of affected fruit during early to mid-June. The disease can be caused by any one of three different fungi: Sclerotinia sclerotiorum, Botrytis cinerea, or Botryosphaeria obtusa. Symptoms caused by these three fungi are similar, and distinguishing among them can be difficult even for experts.

Calyx-end rot is most common in years when extended wetting periods (2-3 days duration) occur between late bloom and first cover. The 11 days of rain that occurred last year near petal fall provided ideal conditions for infection and contributed to an unusual number of complaints about this disease in 1998. Recurrence of calyx-end rot as a significant disease in 1999 is unlikely unless an extended wetting period develops during late bloom, petal fall, or the immediate postbloom period. Losses to calyx-end rot can be reduced by including appropriate fungicides in the petal fall spray, but the economics of adding specific fungicides to control calyx-end rot are questionable because the disease occurs so infrequently.

Sclerotinia sclerotiorum, the most common cause of calyx-end rot in New York, grows on lower stems of infected weeds hosts (e.g., dandelion, wild clover) in the orchard ground cover. Ascospores are produced and released when the ground stays wet for 2-3 days. Apple fruit become infected when ascospores of S. sclerotiorum are blown from the infected plants in the groundcover to apple flower sepals or to the calyxes of small fruitlets. The infection usually spreads toward one side of the fruit, thereby creating an off-center lesion that is most evident when the fruit is viewed from the calyx end. The fruit surface at the edge of the lesion is sometimes bright red, especially if the lesion is still expanding. However, infections usually stop expanding and dry out by the time lesions reach 5 to 15 mm in diameter. Most affected fruit will ripen prematurely beginning in late July. Affected fruit usually drop from the tree before harvest. The incidence of infection rarely exceeds 5% of the total fruit load on a tree, and the disease will not spread from one infected fruit to another.

Applying Benlate or Topsin M at petal fall should control calyx-end rot caused by S. sclerotiorum. No one has conducted trials to verify the effectiveness of these fungicides, but research on other crops has shown that Benlate or Topsin M are effective for controlling S. sclerotiorum, whereas the other fungicides registered for apples are not very effective. However, the value of including Benlate or Topsin M in petal fall sprays is debatable. These fungicides are no longer effective for controlling apple scab because fungicide-resistant strains are present in many orchards, and their effectiveness against powdery mildew is suspect for the same reason. Thus, the only reason to include Benlate or Topsin M in petal fall sprays is for protection from black rot or from calyx-end rot caused by S. sclerotiorum. The sporadic nature of calyx-end rot means that in most years a specific spray to control S. sclerotiorum will not pay for itself.

Botryosphaeria obtusa, the same fungus that causes frogeye leaf spot and black rot fruit decay, occasionally causes a calyx-end rot that appears in early summer. We do not know why sepal infections with B. obtusa occasionally cause calyx-end rot symptoms, whereas they usually remain quiescent and cause a fruit decay only after fruit begin to ripen. Lesions of calyx-end rot caused by B. obtusa are usually dark brown to black and may completely surround the calyx or they may be offset to one side of the calyx. In orchards where inoculum levels are high and fungicide protection is lacking, B. obtusa can infect flower sepals and/or fruit calyxes anytime from green-tip through petal fall. All of the registered scab fungicides suppress B. obtusa, but the SI fungicides and low rates of mancozeb fungicides (1 lb/100 gal) are relatively weak. Captan, Topsin M, and Benlate provide the best protection against black rot infection and have been recommended at petal fall in orchards where black rot fruit decay has been a problem in previous years.

Botrytis cinerea, the third of the three fungi that can cause calyx-end rot, is the same fungus that causes gray mold decay of stored apples. (It also causes gray mold of strawberries and raspberries.) Apple fruit with calyx-end rot caused by B. cinerea usually have a light brown lesion that completely surrounds the fruit calyx. Sometimes infected fruit still have dried-out petals trapped in the calyx-end. The entrapped or retained petals probably provide a food source that enhances establishment of B. cinerea in the calyx. None of the scab fungicides have proven effective for controlling B. cinerea in apples, but incidence of calyx-end rot caused by B. cinerea remains low because green fruit are relatively resistant to this fungus.

5/10 Insects