Center for Integrated Fungal Research, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh, NC 27695.
Department of Plant and Environmental Sciences, Clemson University, Coastal Research and Education Center, Charleston, SC 29414.
Phytopathology. 2022 Sep;112(9):1946-1955. doi: 10.1094/PHYTO-11-21-0467-R. Epub 2022 Aug 8.
In the United States, the cucurbit downy mildew pathogen, , has been shown to form oospores under laboratory conditions, but there are no reports on the formation of oospores in naturally infected cucurbit plants in the field. This study investigated the occurrence of oospores in naturally infected leaves from cucurbit fields in North Carolina and South Carolina from 2018 to 2020. Oospore viability and survival was also determined outdoors during the winter in North Carolina during this study period using soil containing leaves infested with oospores. About 5% of 1,658 naturally infected cucumber and cantaloupe leaves sampled during the study had oospores, with a mean density of 585 oospores per cm of infected leaf tissue. Absolute oospore viability, as assessed using the plasmolysis method, declined linearly (slope = -0.27; < 0.0001) over the 6-month exposure period from 67.8% in November to 19.3% in May. Other variables being equal, the decrease in oospore viability was significantly affected by soil temperature ( = -0.03 to -0.05; < 0.0001) and number of rainy days ( = 21.6 to 40.46; < 0.05), while the effects of soil moisture on oospore viability were less clear. About 20% of the oospores exposed to outdoor conditions at the end the study period were putatively viable and deemed potentially infective. However, these putatively viable oospores failed to germinate or initiate disease when inoculated onto cucumber or cantaloupe leaves. These results indicate that oospores might require some unrecognized stimuli or physiological factors to initiate germination and infection. Nonetheless, viability of oospores at the end of the winter season suggests that once exposed to the right conditions that stimulate germination, these oospores could potentially serve as a primary inoculum source in the southeastern United States where winter temperatures are cold enough to kill cucurbits plants.
在美国，已证实葫芦科霜霉病菌在实验室条件下能形成卵孢子，但尚未有田间自然感病葫芦科植物中卵孢子形成的报道。本研究调查了 2018 年至 2020 年期间北卡罗来纳州和南卡罗来纳州田间自然感病葫芦科叶片中卵孢子的发生情况。在本研究期间的冬季，还在北卡罗来纳州室外使用含有卵孢子侵染叶片的土壤来确定卵孢子的活力和存活情况。研究期间，约有 5%的自然感病黄瓜和哈密瓜叶片样本中检测到卵孢子，每平方厘米受侵染叶片组织的卵孢子密度为 585 个。使用质壁分离法评估的绝对卵孢子活力在 6 个月的暴露期内呈线性下降（斜率=-0.27；<0.0001），从 11 月的 67.8%降至 5 月的 19.3%。在其他变量相同的情况下，卵孢子活力的下降显著受到土壤温度（=-0.03 至-0.05；<0.0001）和雨天数量（=21.6 至 40.46；<0.05）的影响，而土壤水分对卵孢子活力的影响则不太明显。研究结束时，约有 20%暴露在户外条件下的卵孢子被认为具有活力，被认为具有潜在感染性。然而，当将这些假定有活力的卵孢子接种到黄瓜或哈密瓜叶片上时，它们未能发芽或引发疾病。这些结果表明，卵孢子可能需要一些未被识别的刺激物或生理因素来启动发芽和感染。尽管如此，冬季结束时卵孢子的活力表明，一旦暴露在能刺激发芽的适宜条件下，这些卵孢子可能成为美国东南部冬季气温足以杀死葫芦科植物的地区的主要初侵染源。
