The biology of Peronospora viciae on pea: laboratory experiments on the effects of temperature, relative humidity and light on the production, germination and infectivity of sporangia

SUMMARY Germination of Peronospora viciae sporangia washed off infected leaves varied from 20% to 60%. Sporangia shaken off in the dry state gave 11–19% germination. Most sporangia lost viability within 3 days after being shed, though a few survived at least 5 days. Infected leaves could produce sporangia up to 6 weeks after infection, and sporulating lesions carried viable sporangia for 3 weeks. Sporangia germinated over the range 1–24 °C, with an optimum between 4 and 8 °C. Light and no effct. The temperature limits for infection were the same as for germination, but with an optimum between 12 and 20 °C. A minimum leaf‐wetness period of 4h was required, and was independent of temperature over the range 4–24 °C. Maximum infectivity occurred after 6h leaf wetness at temperatures between 8 and 20 °C. Infection occurred equally in continuous light or in darkness. After an incubation period of 6–10 days sporangia were produced on infected leaves at temperatures between 4 and 24 °C, with an optimum of 12–20 °C. Exposure to temperatures of 20–24 °C for 10 days reduced subsequent sporulation. Sporangia produced at suboptimal temperatures were larger, and at 20 °C. smaller, than those produce at 12–16 °C. Viability was also reduced. No sporangia were produced in continuous light, or at relative humidities below 91%. For maximum sporulaiton an r.h. of 100% was required, following a lower r.h. during incubation. Oospores wre commonly formed in sporulating lesions, and also where conditons limited or prevented sporulation. The results are discussed briefly in relaiton to disease development under field conditions.