Transmission of Phytophthora infestans from Infected Potato Seed Tubers to Emerged Shoots.

Transmission of Phytophthora infestans from infected seed tubers to emerged potato sprouts, infectivity of sporangia deposited on whole tubers before burial in soil, and infectivity of sporangia in a loamy fine sand to leaflets were investigated in the greenhouse under simulated spring planting conditions of the Columbia Basin. Incidence of late-blight-infected shoots from infected seed tubers was significantly greater when foliage was exposed to wet periods in mist chambers (mist for 45 s every 15 min) for either 24 or 48 h than when not exposed to a wet period. Proportion of infected shoots from infected tubers was 0.210 to 0.261 in a moist environment versus 0.013 to 0.052 in a nonmist environment. Development of chlorosis, necrosis, and sporangia occurred on shoots that emerged from infected, symptomatic tubers buried in soil. However, approximately 20% of the infected shoots produced sporangia before stems had visible discoloration of late-blight symptoms. Sporulation was sparse and formed near the soil line on some of the shoots after 24 h in the moist environment. The latent period or time from inoculation to sporulation on young stems of Russet Burbank was 5 to 6 days, which is too long to account for an infection from either sporangia or zoospores at the soil level of shoots during the wet period in this study. Sporangia were infective when placed directly on eyes of whole tubers before planting. Leaflets touching a loamy fine sand infested with sporangia developed typical late-blight lesions beginning at the leaflet tip within 7 days after a 24-h wet period and the infested loamy fine sand was infective when splashed on leaflets.