Studies on resistance and overwintering in hop powdery mildew (Sphaerotheca humuli (DC) Burr.)

Abstract

The development of S. humuli on hop leaves was analysed with eight isolates, collected from various regions, and with several cultivara/ seedling hops containing either no resistance genes or one or more of three known major genes for resistance. Three mechanisms of resistance were discovered. (1) Controlled by gene I1, in which epidermal cells reacted hypersensitively immediately after penetration; haustoria were degenerated allowing no colony development. Originally this reaction occurred under natural conditions at Wye College but selection then resulted in the appearance of a new strain to which the ^ genotype was susceptible. (2) Controlled by gene B which causes a bare -blisters to appear on an infected leaf in the glasshouse. Here, hypersensitivity was relatively delayed until after the formation of haustoria, only a proportion of which remained functional; limited hyphal growth and sparse sporulation resulted. (3) Controlled by gene I, where an immediate hypersensitivity resulted as a response to attempted penetration; no haustoria were observed and there was no hyphal development. Among the isolates, four strains were recognised by their behaviour on the host genotypes. One of them behaved like the strain prevalent at Wye College before 1968 and to which the 1^ gene was effective. Experiments showed that S. humuli can overwinter as cleistocarps in hop debris and as mycelium in dormant rootstock buds. Among cleistocarps overwintered in a hop garden, a few had dehisced naturally, and others were induced to release ascospores in the laboratory, only after April each year. Ascospores germinated in the laboratory but failed to infect leaves. However, leaf infections in the field were shown to have originated from ascospores. Buds bearing external mycelium in autumn and overwintered naturally produced either infected or healthy shoots in spring, or died. Infected shoots originated from mycelium established between the bud scales. Of over 11O species of hop garden weeds inoculated with S. humuli none were infected.