Stem Canker on Cotton Caused by Phoma exigua in North Carolina and Virginia

Numerous reports about a disease of unknown etiology on cotton, Gossypium hirsutum L., in northeastern North Carolina and southeastern Virginia were received on 18 June 1999 following several days of cool weather with persistent mist and fog during the week of 14-19 June. Several fields were visited by consultants and county extension staff the following week. In some instances, the cotton stem was girdled, causing the upper portion of the plant to wilt and die. Cotton plants exhibiting various symptoms, including death, wilting, streaking of the vascular system, black sunken lesions on stems, and terminal necrosis were collected for examination and isolation. Pycnidia and spores of the fungus Phoma exigua were abundant in stem and terminal tissues. The fungus was isolated from infected stem tissue and cultured on PDA. A suspension containing 2.5 × 10⁸ spores of P. exigua was sprayed on cotton leaves or injected into the stems to confirm pathogenicity. Controls were sprayed or injected with distilled water. Plants were placed in 100% humidity for 72 h and maintained in the greenhouse thereafter. The experiment was replicated five times and repeated once. Typically, streaking of the vascular system extended 1 to 5 cm from the point of stem inoculation. Inoculated cotton leaves had lesions resembling those attributed to Ascochyta gossypii. Reisolation of the fungus P. exigua from inoculated tissue on potato-dextrose agar (PDA) was successful in all treatments. Crossan (2) considered many isolates of Ascochyta taken from various hosts in North Carolina, including A. gossypii, to be synonymous with Ascochyta phaseolorum. A. phaselorum was subsequently synonomyzed with P. exigua (1). Ascochyta blight (also called ashen spot, or wet weather blight [4]) is usually a minor leaf spot caused by P. exigua (syn. Ascochyta gossypi) and is common in North Carolina. Stem canker caused by P. exigua has not been reported previously in North Carolina (3) or Virginia. The sunken canker at a node is the best diagnostic symptom for cotton stem canker. Dark streaks in vascular tissue extend below and above the canker but do not usually extend to the root system, as with wilt diseases. The disease was widespread and found in most fields north of I-40 in North Carolina into Virginia and east of I-95. Crop consultants and county extension staff estimated disease incidence in individual fields from less than 1 to over 90% in North Carolina and 6 to 25% in Virginia. Disease incidence did not appear to be affected by cotton cultivar, tillage, or crop rotation. This pathogen was also responsible for brittle cotton stems late in the season, resulting in boll loss. Proper identification of the causal organism is essential in formulating management strategies, since P. exigua has an extensive host range and rotation is unlikely to aid in management of this disease in the future. References: (1) G. H. Boerema. Ascochyta phaseolorum synonymous with Phoma exigua. Neth. J. Plant Pathol. 78:113-115, 1972. (2) D. F. Crossan. The relationships of seven species of Ascochyta occurring in North Carolina. Phytopathology 48:248-255, 1958. (3) L. F. Grand, ed. North Carolina Plant Disease Index. Tech. Bul. 240, 1985. (4) G. M. Watkins. Leaf spots. Pages 28-30 in: Compendium of Cotton Diseases, 1st ed. American Phytopathological Society, St. Paul, MN, 1981.