Transmission ofPhytophthora infestans in cut potato seed

One or two cycles of late blight transmission may occur in cut potato seed between cutting and emergence. If seed are not planted immediately, mycelium from diseased pieces can infect across adjoining cut surfaces within 8 hr. Sporangia production begins on freshly cut diseased tubers in 1–2 days, on mycelium-infected seed pieces within 4–5 days, and on spore-infected seed pieces in about 1 wk. These spores are further dispersed during handling and planting, and may infect contaminated seed in the ground, regardless of suberization. Sporangia can be transferred to healthy tubers on cutting knives, although this appears infrequent. The risk and degree of secondary spread increase with the length of time cut seed are held before planting. Such secondarily infected seed are more likely to survive and produce infected stems than the original infected tubers. Treatment of seed with appropriate fungicides immediately after cutting substantially reduces or prevents transmission of late blight to healthy seed. Certain seed treatments withoutPhytophthora-specific components may increase the risk of tuber blight.