Publications - Faculty - Recently Submitted Faculty Publications 2010
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Dewdney - Plant Disease
First report of citrus black spot caused by Guignardia citricarpa on sweet orange [Citrus sinensis (L.) Osbeck] in North America
T.S. Schubert, M.M. Dewdney, N. A. Peres, M.E. Palm, A. Jeyaprakash, B. Sutton, S.N. Mondal, N.-Y. Wang and J. Rascoe
Abstract.In March 2010, citrus black spot symptoms were observed on sweet orange trees in a commercial grove near Immokalee, FL. Several symptom types were observed on fruit in the field including hard spot, cracked spot and early virulent spot. Hard spot lesions were small (up to 5 mm) depressed (Fig. 1) with a chocolate margin and a necrotic, tan center, often with black pycnidia present (140-200 µm dia.). Cracked spot lesions were large, dark brown (1.5 cm dia.), with diffuse margins and raised cracks and, in some cases, hard spots formed in the center. Early virulent spot lesions were small, bright red, irregularly shaped, indented, and often with many pycnidia. In addition, small, elliptical, 2-3 mm dia., reddish brown lesions with depressed tan centers and yellow halos that developed as they aged were observed on some leaves of trees with symptomatic fruit (Fig. 2). Leaves had single lesions, but rarely had up to four lesions/leaf. Tissue from hard spot and early virulent spot were aseptically placed on PDA, oatmeal agar or carrot agar and maintained in 12 h light/dark at 24ºC. Fourteen single-spore cultures were obtained from these isolates that grew slower than the G. mangiferae cultures grown for comparison, although pycnidia formed more rapidly in the G. mangiferae cultures (1). No sexual structures were observed. Cultures on half-PDA were black and ropey with irregular margins and numerous pycnidia, often with white cirrhi after 14 days. Conidia (7.1-7.8 x 10.3-11.8 µm) were hyaline, aseptate, multiguttulate, ovoid with a flattened base surrounded by a hyaline matrix (0.4-0.6 µm) and a hyaline appendage on the rounded apex, corresponding to published descriptions of G. citricarpa (1). A yellow pigment was seen in oatmeal agar surrounding G. citricarpa, but not G. mangiferae, colonies, as previously reported (1,2). DNA was extracted from lesions and cultures and amplified with primers from Peres et al. (2) as was G. mangiferae DNA, healthy citrus fruit tissue and water as controls. The G. citricarpa-specific primers (2) produced a 300 bp band from fruit lesions and pure cultures. G. mangiferae-specific primers (2) produced 290 bp bands with DNA from G. mangiferae cultures. The internally transcribed spacer, translation-elongation factor and actin gene regions were sequenced from G. citricarpa isolates and ITS sequences were deposited in Genbank (accessions JQ231197-JQ231200; CBS 131864; BPI 882720). They were found to have 100% homology to G. citricarpa ITS sequences from South Africa and Brazil in the GenBank database (accessions AF374364 and FJ538312). Pathogenicity tests with G. citricarpa are very difficult to conduct because the organism infects immature fruit and has an incubation period of at least 6 months (3). In addition, quarantine restrictions limit work with the organism outside a contained laboratory. To our knowledge, this is the first report of black spot in the northern hemisphere. The area of infestation was approximately 57 km2. The disease is of great importance to the Florida citrus industry because it causes serious blemishes and significant yield reduction, especially on the most commonly grown ‘Valencia’ sweet orange. Additionally the presence of the disease in Florida may result in restricted market access because black spot is considered a quarantine disease in the USA and internationally.
