Survival characteristics of oospore populations of Peronospora viciae f. sp. pisi in soil

Abstract

Survival and germinability of oospore populations of Peronospora viciae f.sp. pisi in soil were investigated. The percentage survival was assessed using the vital stain tetrazolium bromide. Germinability was defined as the percentage of oospores that germinated in water. Oospores 1-3 weeks old, embedded in plant tissue, were incorporated in a loamy sand or silt-loam soil and incubated at 3, 10 or 20°C or stored dry at 20C and 30% RH. The percentage of surviving oospores in soil decreased rapidly with or after decomposition of the surrounding plant tissue at 10 and 20°C. After 29 weeks less than 10% of the oospores had survived. At 3°C, survival was 25% or more after 29 weeks. Germinability of the oospores was 3% at time of incorporation and had increased to a maximum of 50% after 4 weeks in the loamy sand soil. Increase in germinability of the dry-stored oospores was significantly later than that of the soil-incubated oospores. In soil, the initial increase in germinability was followed by a decline after decomposition of the surrounding plant tissue. In order to investigate survival of oospores under natural conditions, ground tissue or pod pieces of pea plants containing oospores 7 or 5 months old, respectively, were incorporated in two loamy sand soils or a silt loam soil in field plots. The surrounding plant tissue decomposed within 4 weeks and the percentage of surviving oospores decreased to less than 6% after a year. Survival of oospore populations was generally well described by the lognormal model, indicating that the risk of oospore death initially increased before decreasing later. Germinability of the oospore populations from ground tissue and pod pieces, 61 and 62% at incorporation, decreased rapidly after incorporation in soil. Oospores extracted from the silt-loam soil germinated poorly in water but caused high disease incidences in a bioassay, suggesting that oospores had become dependent on the host plant for germination.