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Title: Survival of Sporangia of New Clonal Lineages of Phytophthora infestans in Soil under Semiarid Conditions

Author
item Porter, Lyndon
item JOHNSON, DENNIS - WASHINGTON STATE UNIV

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2007
Publication Date: 7/1/2007
Citation: Porter, L.D., Johnson, D.A. 2007. Survival of Sporangia of New Clonal Lineages of Phytophthora infestans in Soil under Semiarid Conditions. Plant Disease. 91: 835-841.

Interpretive Summary: Potato late blight caused by Phytophthora infestans is a major disease problem for potato growers in the Columbia Basin of Washington and throughout the world. Currently there is no evidence in the State of Washington that the pathogen is producing any survival spores that are capable of surviving in soil over the winter. However, the pathogen is capable of infecting and surviving in unharvested potatoes throughout the winter, and in the spring is capable of colonizing potato sprouts from these infected potatoes and releasing spores called sporangia that can infect other potato plants. Currently there is no information on the viability of these sporangia in soil of the new metalaxyl-resistant genotypes of P. infestans in the semiarid Columbia Basin of WA, and in potato growing regions throughout the world. Determining the length of survival of sporangia of P. infestans in soil is a key component to understanding and developing models to assess the potential spread of the late blight pathogen within and between potato fields and in predicting tuber infection both during the growing season and the risk of tuber infection at harvest. This study determined that sporangia of metalaxyl-resistant isolates of P. infestans survived a maximum of between 23 to 30 days in a Shano silt loam and a Quincy loamy fine sand, two of the most common soil types used in potato production in the state of Washington. Sporangia survival was significantly longer in soil that was shaded verses soil exposed to direct sunlight, and sporangia survived for longer periods of time in shaded conditions if the soil moisture was wet verses dry. Based on this study growers that wait three weeks after potato vines are killed before harvest can drastically reduce the risk of late blight tuber infections during harvest.

Technical Abstract: Currently there is no information on the viability of sporangia in soil of the new metalaxyl-resistant genotypes of P. infestans in the semiarid Columbia Basin of WA, and in potato growing regions throughout the world. Sporangia of metalaxyl-resistant US-8 and US-11 clonal lineages of P. infestans survived a maximum of between 23 to 30 days in a Shano silt loam and a Quincy loamy fine sand. There were no significant differences between soil types in area under the spore survival curve (AUSSC) in two trials, however sporangia of P. infestans in the Quincy sand did have a significantly greater mean maximum days of sporangia survival (MDSS) than did the Shano silt loam in one of two trials. AUSSC and MDSS were significantly greater (P < 0.05) for sporangia in wet soil than in dry soil under shaded conditions. Mean AUSSC and MDSS significantly decreased (P < 0.01) under non-shaded conditions verse shaded conditions. Three metalaxyl-resistant isolates (two US-8, one US-11) of P. infestans did not significantly differ (P < 0.05) in AUSSC and MDSS.