Sources of inoculum and sites of infection of potato tubers by Phytophthora infestans in soil.

Abstract

The role of lesions of potato late blight on foliage and the possibility of lesions on below ground parts as sources of inoculum for the infection of tubers in soil were investigated. Infestation of the surface soil of the ridge by spores deposited from foliage lesions increased markedly within a few days after rain and then decreased rapidly. Tuber blight followed much rain when plants had foliage lesions, but not when foliage lesions were absent due to fungicide spray or death of haulm. If haulm was removed by cutting and the ridge watered much tuber infection occured only if the soil had been inoculated first. Blighted tubers buried in soil a week after inoculation were shown to produce spores. The infectivity of soil surrounding diseased tubers increased rapidly for a few days immediately after they were buried, reached maximum after about a week and then decresed. Increase in tissue resistance of the diseased tubers appeared to be the main reason for the decrease in sporulation. Only a few healthy tubers became diseased even when they were put in direct contact with diseased ones in the soil. It is concluded that tuber infection is more likely from haulm lesions than diseased tubers. Field observations showed that the site of tuber infection in soil was located more often on the bottom rather than the top surface of a tuber.