Quantitative Analysis of the Early Powdery Mildew Infection Stages on Resistant Barley Genotypes

A classification system was developed, that allowed quantification of the leaf surface development of the barley powdery mildew fungus on barley. An experiment with Manchuria and Pallas as susceptible controls and 4 resistance gene each represented by three lines with different gene backgrounds showed two types of gene background effects. First a general effect comprised of different distributions of the elongating secondary hyphae (ESH) in the stomatal and interstomatal region between Manchuria and, Pallas, and a higher number of lobes per appressorium on Pallas than on Manchuria. These effects also applied to isolines of Manchuria and Pallas possessing each of the four genes investigated. Second a specific effect was noticed on the ESH frequency on Ml-k lines. The ESH frequency varied significantly at the 5% level between the highest and lowest value. An experiment with Pallas as susceptible control and Pallas isolines with 11 different resistance genes, showed that powdery mildew development was unaffected by host genotype until after the formation of an appressorium. The first host effect observed was on the number of lobes on the appressorium, which reflects the number of penetration attempts. This number increased as the degree of resistance increased, i.e. the ESH frequency decreased. The penetration stage also invariably proved to be the limiting stage, where the largest proportion of fungal propagules was stopped.