First Report of Target Spot Caused by Corynespora cassiicola on Cotton in Georgia

Abstract

In 2005, crop consultants in southwestern Georgia reported an unusual occurrence of leaf spot in cotton (Gossypium hirsutum L.). Initial symptoms first developed as brick red dots that led to the formation of irregular to circular lesions with tan-to-light brown centers. Lesions further enlarged and often demonstrated a targetlike appearance formed from concentric rings within the spot. Observations included estimates of premature defoliation up to 70%, abundant characteristic spots on the leaves and bracts, and losses of several hundred kg of lint/ha. When symptomatic leaves were submitted to the University of Georgia Tifton Plant Disease Clinic in Tifton, GA, for identification in 2008, the causal agent was tentatively diagnosed as Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei on the basis of similar symptoms and signs previously reported on cotton (3). In September 2011, symptomatic leaves were obtained from diseased cotton within a field (var. DP 1048B2RF) near Attapulgus, GA. Symptomatic tissue from diseased leaves was surface disinfested in 0.5% sodium hypochlorite for 1 min and plated on potato dextrose agar (PDA). Ten isolates were incubated at 21.1°C for 2 weeks with a 12/12 h light/dark cycle using fluorescent light located approximately 70 cm above the cultures. After 1 week, two isolates were transferred to quarter strength PDA for enhanced sporulation and were grown under the same conditions. Conidiophores from the isolated fungus were simple, erect, intermittently branching and septate, and gave rise to single, subhyaline conidia. Conidia had 4 to 17 pseudosepta and were 50 to 197 μm long and 7 to 16 μm wide, straight to curved, and obclavate to cylindrical. Pathogenicity tests were conducted by spraying 10 cotton seedlings (DP 555BR and DP 1048B2RF, two to four true leaf stage) until runoff with a blended suspension from a 2-week-old pure culture of the fungus diluted with 100 mL of sterile water. Five plants were sprayed with sterile water as noninoculated controls. Cotton seedlings were then incubated in a moist chamber at 21.1°C for 48 h. Within 1 week, all inoculated plants showed symptoms similar to those of diseased field plants. Symptoms were not observed on noninoculated control plants. The fungus was reisolated five times from symptomatic leaves and grown in pure culture. Conidia and conidiophores were identical to the morphology of the original isolates, and were similar to descriptions of C. cassiicola (2). To confirm the identity of the pathogen, DNA was extracted from a week-old culture and amplified with specific primers for loci “ga4” and “rDNA ITS” (1). DNA sequences obtained with the Applied Biosystems 3730xl 96-capillary DNA Analyzer showed 99% identity to C. cassiicola from BLAST analysis in GenBank. The resulting sequence was deposited into GenBank (Accession No. JQ717069). To our knowledge, this is the first report of this pathogen in Georgia. Given the increasing prevalence of this disease in southwestern Georgia, its confirmation is a significant step toward management recommendations for growers. Because foliar diseases caused by C. cassiicola are commonly referred to as “target spot” in other crops (e.g., soybeans), it is proposed that Corynespora leaf spot of cotton be known as “target spot of cotton.” References: (1) L. J. Dixon et al. Phytopathology 99:1015, 2009. (2) M. B. Ellis and P. Holliday. CMI Description of Pathogenic Fungi and Bacteria, 303, 1971. (3) J. P. Jones. Phytopathology 51:305, 1961.

Corynespora Leaf Blight of Cotton in Brazil and Its Management

“…Corynespora leaf blight occurs on cotton in several countries and was reported for the first time in 1959 in the State of Alabama, USA [1]. In recent years it is reported to be a serious problem in the USA [2] [3].…”

First Report of Target Spot Caused by Corynespora cassiicola in Louisiana Cotton

“…KP759986, KM458795, JQ801302, and GU46130). Target spot of cotton has been reported in Alabama and Georgia causing problems annually (Campbell et al 2012;Fulmer et al 2012). To our knowledge, this is the first report of target spot of cotton occurring in Louisiana.…”

“…Colonies that originated from surface-sterilized diseased tissue margins were selected for isolation via hyphal tip transfer and subsequent analysis. Conidiophores were erect, simple, straight or slightly curved, smooth, septate, with up to 8 cylindrical proliferations; while conidia were variable, cylindrical, straight to curved, hyaline to slightly brown, smooth, 6 to 15 pseudoseptate, 42 to 150 μm long by 11 to 18 μm wide (Ellis and Holliday 1971;Fulmer et al 2012;Jones 1961). An isolate of C. cassiicola was grown on V8 agar for 14 days at 25°C.…”

Biodiversity, Distribution and Taxonomy of Conidial Fungus Corynespora (Corynesporascaceae) Associated With Malvaceae

“…Mycelium internal. [12,13]; Gossypium barbadense L. from Barbados [14]; Gossypium hirsutum L. from Alabama [15], Georgia [16], India [17,18]; Hibiscus cannabinus L. from Papua New Guinea [19]; Abelmoschus esculentus (L.) Moench (=Hibiscus esculentus L.) from Cuba [20], Ghana [21], Sierra Leone [22]; Hibiscus mutabilis L. from Korea [23,24]; Hibiscus sabdariffa L. from Sierra Leone [22]; Hibiscus sp. from Cuba [20]; Hibiscus syriacus L. from Korea [25]; Pavonia sp.…”