The role of water in the spread of <i>Phytophthora infestans</i> in the potato crop

Abstract

SUMMARY During falls of rain before 5% of the haulm was destroyed by blight, most rain water was deposited on soil at the bottom of the ridge, and least at the top in crops of King Edward, Majestic, and Up‐to‐Date potatoes. Afterwards less water was trapped half way up the side of the ridge than above or below. Water was also channelled down the stems and, while these were upright, was deposited in the channel often formed around the stem bases by wind movement. Fewer than 500 sporangia per ml. were seen in water collected when blight was present in the crop. Growing tubers of Ulster Ensign and King Edward, inoculated with P. infestans , infected healthy tubers less than 1–3 cm. away, when the soil water content was greater than 20%, and sporulation on the inoculated tubers was maximal. Similarly inoculated Up‐to‐Date and Majestic tubers sporulated less abundantly and failed to infect healthy tubers. Sporulation also occurred on inoculated seed tubers, although infection of the haulm from these tubers could not be confirmed. Water movement from the seed tuber region to the furrows may occur under suitable conditions.