Lenticel Infections And Bitter Rot Of Apples

By Growing Produce Staff|September 4, 2012

The hot and dry weather conditions during 2012 have not only reduced crops in Michigan, but caused somewhat uncommon diseases to occur when trees are under these stresses. One type of disease that has shown up on apples this season appears as numerous small, dark spots, arranged somewhat uniformly on the skin surface. Initially, the spots are quite shallow, but over time they often become wider and deeper into the fruit. These infections are more common on the side of the fruit receiving direct sunlight. Isolations from these tissues show a range of fungi such as Cladosporiumand Alternaria; all of these fungi are generally considered to be weak.

We hypothesize that the combined heat and sunlight weakened the lenticels of the skin, allowing the weak pathogens to colonize the area. Lenticels can also be weakened by an extended dry period followed by rain, a situation that will also cause peach skin to split. Lenticels that “split” appear as enlarged and roughened lenticels on fruit at harvest.

No spray program is particularly effective against lenticel spot, although routine calcium sprays help to strengthen fruit tissue and Surround (kaolin clay) applications have been shown to help reduce direct sunlight damage to apples.

Bitter rot, caused by the closely related fungi Colletotricum gloeosporiodes and Colletotrichum acutatum, can also show up in hot years. Although bitter rot is well known in warmer, humid apple growing regions such as the southeast U.S., the disease is rare in Michigan. The symptoms are more common on light or bicolored fruit such as Empire, Mclntosh, Sunrise, Paulared, and Jonagold. Bitter rot spots usually appear on the side of the apple directly exposed to the sun. Lesions are light to dark brown, flattened, and may have a pink, slight orange to light tan mass of spores.