THE LEAF SCORCH DISEASE OF STRAWBERRY (DIPLOCARPON EARLIANA) AND THE NATURE OF RESISTANCE TO IT

Abstract

Conidial germination required free water, was independent of an external supply of nutrients, and was not influenced by water leachates of resistant or susceptible strawberry leaves. Twenty and 25 °C were optimal for conidial germination, growth and sporulation in vitro and for infection. Chlorogenic acid was demonstrated in strawberry leaves and inhibited conidial germination at 100 p.p.m., retarded growth and sporulation at 1000 p.p.m. and inhibited them at 2000 p.p.m.Thiamine was found to be essential for in vitro growth and sporulation. Sodium nitrate and ammonium sulphate were poorly utilized as compared with casein hydrolysate. Among the amino compounds and sugars found in strawberry leaves, aspartic acid, asparagine, glutamic acid, glutamine, γ-aminobutyric acid, glucose, and sucrose promoted growth or sporulation; serine, alanine, and fructose were less suitable, while threonine and xylose did not support any growth.The young leaves of a selected number of strawberry varieties, considered to be resistant or susceptible to Diplocarpon earliana in the field, were all highly resistant in the greenhouse to isolates of D. earliana used; in contrast, mature leaves of all but Catskill were susceptible.In two sets of experiments, analyses of strawberry leaves for free amino compounds and sugars showed detectable amounts of aspartic, glutamic, and γ-aminobutyric acids, serine, asparagine, glutamine, threonine, alanine, glucose, fructose, sucrose, and xylose. Generally the highly resistant young leaves were lower in aspartic and glutamic acids, glucose, and sucrose. Mature leaves of the resistant Catskill were generally characterized by lower levels of one or more of aspartic, glutamic, and γ-aminobutyric acids, glucose, and sucrose than those of susceptible varieties.In the growth chambers, a short time before the appearance of symptoms, infected mature leaves of the susceptible Jerseybelle and Pocahontas had more γ-aminobutyric acid, glucose, or sucrose than those of the resistant Catskill. In the greenhouse during summer, mature leaves of the susceptible Sparkle in an advanced stage of infection had a great deal more glutamic and γ-aminobutyric acids, glutamine, alanine, glucose, fructose, and sucrose than comparable leaves of Catskill.