Koenning S R, Abdel Alim F F, Grand L F, Phipps P M
Department of Plant Pathology, North Carolina State University, Campus Box 7616, Raleigh 27695-7616.
Tidewater Agricultural Research & Extension Center, Virginia Polytechnic Institute & State University, 6321 Holland Rd., Suffolk 23437.
Plant Dis. 2000 Nov;84(11):1251. doi: 10.1094/PDIS.2000.84.11.1251B.
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 × 10 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.
1999年6月14日至19日那周天气凉爽，持续有薄雾和雾气，6月18日收到了许多关于北卡罗来纳州东北部和弗吉尼亚州东南部棉花（陆地棉）上一种病因不明疾病的报告。接下来的一周，顾问和县级推广人员走访了多个棉田。在某些情况下，棉花茎部被环割，导致植株上部枯萎死亡。采集了表现出各种症状的棉花植株，包括死亡、枯萎、维管束系统出现条纹、茎部黑色凹陷病斑以及顶端坏死，用于检查和分离。在茎部和顶端组织中，真菌小茎点霉的分生孢子器和孢子大量存在。从受感染的茎组织中分离出该真菌，并在马铃薯葡萄糖琼脂（PDA）上进行培养。将含有2.5×10个小茎点霉孢子的悬浮液喷洒在棉花叶片上或注入茎中以确认致病性。对照组喷洒或注入蒸馏水。将植株置于100%湿度环境中72小时，此后在温室中养护。该实验重复了5次，并再次进行了一次。通常，维管束系统的条纹从茎接种点延伸1至5厘米。接种的棉花叶片上有类似于棉壳二孢引起的病斑。在所有处理中，均成功地从接种组织在马铃薯葡萄糖琼脂（PDA）上重新分离出小茎点霉真菌。克罗斯安（2）认为，从北卡罗来纳州的各种寄主上采集的许多壳二孢分离物，包括棉壳二孢，与菜豆壳二孢同义。菜豆壳二孢随后与小茎点霉同义（1）。壳二孢叶枯病（也称为灰白色斑点病或潮湿天气叶枯病[4]）通常是由小茎点霉（同义词：棉壳二孢）引起的轻微叶斑病，在北卡罗来纳州很常见。小茎点霉引起的茎溃疡病此前在北卡罗来纳州（3）或弗吉尼亚州尚未有报道。节部的凹陷溃疡是棉花茎溃疡病的最佳诊断症状。维管束组织中的深色条纹在溃疡病斑的上下延伸，但通常不像枯萎病那样延伸到根系。该病广泛分布，在北卡罗来纳州40号州际公路以北的大多数棉田以及弗吉尼亚州和95号州际公路以东的地区都有发现。作物顾问和县级推广人员估计，北卡罗来纳州个别棉田的发病率从不到1%到超过90%不等，弗吉尼亚州为6%至25%。发病率似乎不受棉花品种、耕作方式或轮作的影响。这种病原菌也是导致棉花在生长季节后期茎秆变脆、造成落铃的原因。正确识别致病生物对于制定管理策略至关重要，因为小茎点霉的寄主范围广泛，未来轮作不太可能有助于控制这种疾病。参考文献：（1）G.H.博勒马。菜豆壳二孢与小茎点霉同义。《荷兰植物病理学杂志》78:113 - 115，1972年。（2）D.F.克罗斯安。北卡罗来纳州出现的七种壳二孢的关系。《植物病理学》48:248 - 255，1958年。（3）L.F.格兰德，编。《北卡罗来纳州植物病害索引》。技术通报240，1985年。（4）G.M.沃特金斯。叶斑病。载于：《棉花病害简编》第1版，美国植物病理学会，明尼苏达州圣保罗，1981年，第28 - 30页。
