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.
Gummy stem blight (GSB), caused by the fungus Didymella bryoniae, is considered the most widespread and destructive disease of watermelon in the southeastern United States. The quinone outside-inhibiting (QoI) fungicide azoxystrobin (AZO), which inhibits mitochondrial respiration by binding to the outer, quinone-oxidizing pocket of the cytochrome bc1 (cyt b) enzyme complex, was initially very effective in controlling GSB. However, resistance to AZO has been observed in D. bryoniae in many watermelon-producing regions. In this study, the DNA sequences of partial cyt b genes of four AZO-resistant (AZO-R) and four AZO-sensitive (AZO-S) isolates of D. bryoniae confirmed the amino acid substitution of glycine by alanine at the 143 codon (G143A) in the AZO-R isolates tested. Allele-specific primers were designed to detect the resistant or sensitive allele at codon 143 of the cyt b gene, which amplified a 165-bp polymerase chain reaction (PCR) product from genomic DNA of nine AZO-R and nine AZO-S isolates of D. bryoniae, respectively. The primer pairs did not amplify DNA from other pathogens tested in the study. The results indicated that the PCR assays developed in the study were specific in differentiating AZO-R and AZO-S isolates and could facilitate AZO resistance detection in D. bryoniae.
Black shank is an important disease of tobacco (Nicotiana tabacum) caused by the fungus-like organism, Phytophthora nicotianae. Three physiological races (0, 1, and 3) have been documented in the United States. Shifts in the pathogen population structure have become a concern due to the widespread use of cultivars possessing resistance to race 0 arising from a single gene (Php or Phl). A comprehensive statewide survey conducted throughout major tobacco-growing areas during summer 2006 and supplemented by additional isolates in 2007 and 2008 yielded 217 isolates from flue-cured, burley, and dark fire-cured tobacco fields. After determining species identity using a single-strand conformational polymorphism fingerprinting technique, the race identity of isolates was assessed via greenhouse tests using three differential cultivars (Hicks, L8, and NC1071). Approximately 76% of the isolates belonged to race 1, 21% to race 0, and the remaining 3% were race 3. This race structure was comparable with those in the other tobacco-producing states in the United States. Approximately 94% of isolates belonged to A2 mating type and merely 6% were A1. These data suggest that it is unlikely that sexual recombination serves as a major mechanism enhancing the genetic diversity of the pathogen in Virginia. All isolates were also evaluated against mefenoxam at 5 μg/ml. None were insensitive; 98% of isolates were either highly sensitive or sensitive and the remaining 2% were intermediately sensitive. These results indicate that mefenoxam remains effective for control of black shank in Virginia. The results of this study can assist breeders to develop cultivars possessing the most appropriate set of disease resistance traits, as well as extension specialists, county agents, and tobacco growers in their decision-making process to manage tobacco black shank in Virginia.
Irrigation water can harbour propagules of pathogenic oomycetes that may be a primary source of disease outbreak in crop plants. Irrigation ponds associated with vegetable production in southern Georgia, USA, were assessed in this study. Camellia and rhododendron leaves were used as baits for recovery of oomycetes that were further identified by morphological characteristics and analysis of the internal transcribed spacer rDNA regions. Pythium litorale was frequently isolated from all irrigation ponds sampled. Assessment of growth rates of P. litorale isolates at 5-45 • C indicated that the optimum and maximum temperatures were 30 and 40 • C, respectively. It appeared that these isolates were more tolerant to higher temperatures than previously described P. litorale isolates from Europe that had a maximum growth temperature of 35 • C. Pathogenicity assay with the P. litorale isolates showed that they caused fruit rot and seedling damping-off of yellow squash. Pythium litorale was first described as a new species in 2006 and the present study is the first report indicating it is pathogenic on plants. This study provides useful information for a more comprehensive understanding of the nature of P. litorale and its potential impact on vegetable production.Résumé: L'eau d'irrigation peut héberger des propagules d'oomycètes pathogènes qui peuvent être une source principale d'éclosion de maladies dans les cultures. Au cours de cette étude, les étangs d'irrigation utilisés pour la production de légumes dans le sud de la Géorgie, aux États-Unis, ont été évalués. Des feuilles de camélia et de rhododendron ont été utilisées comme appâts pour recueillir des oomycètes qui, par la suite, ont été identifiés en fonction de leurs caractères morphologiques et par analyse de l'espaceur transcrit interne de l'ADNr. Pythium litorale a été fréquemment isolé dans l'eau de tous les étangs échantillonnés. L'évaluation des taux de croissance des isolats de P. litorale à des températures variant de 5 à 45 • C a indiqué que les températures optimales et maximales étaient de 30 et 40 • C, respectivement. Trois isolats se sont par contre révélés plus tolérants aux températures élevées que ce qui avait été préalablement établi pour des isolats européens de P. litorale dont la température maximale de croissance était de 35 • C. Des tests de pathogénicité effectués sur des isolats de P. litorale ont montré qu'ils ont causé la pourriture du fruit et la fonte des semis chez la courge jaune. Pythium litorale a été décrit en tant que nouvelle espèce pour la première fois en 2006, et cette étude est la première mention du champignon en tant qu'agent pathogène. L'étude fournit des renseignements utiles contribuant à mieux comprendre la nature de P. litorale et ses conséquences possibles sur la production de légumes.
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