Journal
PHYTOPATHOLOGY
Volume 112, Issue 9, Pages 1946-1955Publisher
AMER PHYTOPATHOLOGICAL SOC
DOI: 10.1094/PHYTO-11-21-0467-R
Keywords
cucurbits; dormancy; ecology; germination; oomycetes; oospore formation; oospore viability; pathogen survival
Categories
Funding
- Hatch Funds from the North Carolina Agriculture Experiment Station [02693]
- U.S. Department of Agriculture (USDA)-National Institute of Food and Agriculture [2016-68004-24931]
- USDA-Animal and Plant Health Inspection Service [10025-PPQST00-19-0253]
- Hatch Funds from the South Carolina Agriculture Experiment Station [SC-1700583]
- USDA-AMS Multi-State Specialty Crop Grant [6061-CU-PDA-7029]
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This study investigated the occurrence of oospores in naturally infected cucurbit leaves in North Carolina and South Carolina, and determined the viability and survival of oospores in outdoor conditions. The study found that oospore viability decreased over time, with factors like soil temperature and number of rainy days affecting the decline.
In the United States, the cucurbit downy mildew pathogen, Pseudoperonospora cubensis, 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(2) of infected leaf tissue. Absolute oospore viability, as assessed using the plasmolysis method, declined linearly (slope = -0.27; P < 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 (b = -0.03 to -0.05; P < 0.0001) and number of rainy days (b = 21.6 to 40.46; P < 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.
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