White tip disease

White tip disease of leeks (Phytophthora porri) mainly attacks the crop from the end of July until September-December in humid conditions. Heavy rain, causing the soil to splash onto the foliage, can lead to serious infection. Onions are less often infected by the pathogen but seedling infection can occur.

Symptoms
Symptoms are a white die back of the leaf tips, white patches surrounded by a water-soaked zone appear on other parts of the leaf and stem, and stunted growth. These areas become dry and papery. Lesions are most often restricted to leaf tips, but may also occur on the margins, centre or base of leaves Infected leaves look watery, thin and papery, eventually they rot away.

Control
The pathogen survives as oospores in the soil and crop rotation is therefore the main method of control. Affected debris should not be returned to the land. Because the pathogen needs wet periods for establishment on the plant, crops should be grown in less humid areas and be irrigated only when really necessary. In many regions, the infection level rarely reaches the economic threshold, but yield losses up to 50% have been reported in some countries. Metalaxyl-resistant strains have been detected. Badly infected leeks should be removed and destroyed, and the remainder sprayed fortnightly with a fungicide until all the white patches disappear.

Chemical
Sprays: chlorothalonil, copper oxychloride, mancozeb, metalaxyl, propamocarb, thiram.