First Report of Black Leaf Spot of Banana Caused by Deightoniella torulosa in Georgia.

Abstract

Black spots were observed on the leaves of bananas (Musa spp.) grown at the University of Georgia Bamboo Farm and Coastal Gardens in Savannah, GA in November 2007. Symptoms occurred on more than 60 plants, representing 16 of 34 cultivars of bananas investigated. Most lesions were less than 10 mm in diameter and tan to black. However, larger oval lesions more than 20 mm across with black borders and yellow halos also occurred. Lesions were more prevalent on older leaves. On young leaves, lesions first appeared along the leaf margin near the tip of the leaf on one side of the central vein. Lesions expanded to the entire leaf as the disease progressed, but were more prevalent along leaf margins. Thirty-two diseased leaf samples, two from each cultivar, were incubated at 25°C in the dark and conidia were produced on the lesions 2 days after incubation. Pure cultures of the fungus were obtained from five leaf samples by single-spore culturing on potato dextrose agar medium and identified on the basis of morphological characteristics. Conidia on V8 agar are straight or slightly curved, obpyriform to obclavate, and olive to brown with 3 to 13 septa. Conidiophores are brown and swollen at the apex. The fungus was identified as Deightoniella torulosa (Syd.) Ellis on the basis of morphological characteristics described previously (1,2). Pathogenicity studies to fulfill Koch's postulates were conducted on banana cvs. Dwarf Namwah and Dwarf Nino under greenhouse conditions (25 to 27°C). Six plants of each cultivar were used in one experiment and the experiment was repeated one more time. Banana leaves were inoculated by spraying with a suspension of conidia from a pure culture. Symptoms developed as small black lesions on the leaves of both cultivars within 1 week of inoculation. As the disease progressed, some of the small lesions expanded to form larger oval lesions. Symptoms were identical to those on the field samples and were identified as the black spot disease as described on abaca and banana (2). The fungus was reisolated from symptomatic leaves and the identity was confirmed. No symptoms were observed on noninoculated control plants. The black spot disease has been reported in Florida attacking banana and plantain (3). To our knowledge, this is the first description of the presence of the disease on field-grown banana in Georgia. In recent years, increasing efforts have been made in Georgia in the search of banana cultivars suitable of commercial production in the coastal and southern areas of the state. Black spot of banana is an important disease and its occurrence deserves consideration in evaluating banana cultivars and developing disease management approaches for banana production in Georgia.