Powdery mildew of tobacco (Erysiphe cichoracearum DC.)

Abstract

SUMMARYPercentage germination, and growth of hyphae from single conidia of Erysiphe cichoracearum DC., were measured on leaf discs from topped and intact tobacco plants, grown in aerated nutrient solutions consisting of basal medium plus large or small amounts of potassium. The effect of supplying sodium was also studied. Discs were incubated on water and on 10% sucrose solution. Changes in free amino nitrogen and carbohydrate in comparable uninfected leaf discs, before and after incubation, were also measured.Potassium deficiency resulted in more free amino nitrogen and soluble carbohydrate and less insoluble carbohydrate, per cm.2 of leaf. Spore germination was not greatly affected by treatments, though it was usually less on discs from potassium‐deficient leaves. The pathogen grew slower on potassium‐deficient leaf discs, whether they were incubated on water or on sucrose.Incubating discs from some leaves on sucrose, compared with water, gave greatly increased sugar content and less fungal growth; discs from other leaves had a much smaller increase in sugar, and hyphal length was similar to that on discs incubated on water.Sodium, when potassium was scarce, increased potassium deficiency symptoms, free amino nitrogen and sugar content, and resistance to powdery mildew.