| Summary: | The virulence of eight isolates of Sphaerotheca humuli (DC.) Burr, was examined on nine varieties of hops (Humulus lupulus L.). Genotypes determined for varieties with race specific resistance generally agreed with previous reports. Although major genes for resistance usually conferred immunity from infection the effectiveness of the gene was shown to be reduced at low temperatures, thereby allowing weak sporulation to develop. The leaf blistering response previously considered to be a consequence of Rg gene determined resistance was shown to be unrelated to the expression of this gene. Quantitative microscopical examination of isolate/variety combinations exhibiting race specific resistance revealed that incompatible fungal sporelings were restricted in the early stages of their development. The expression of the Rg, and R^ resistance genes was associated with a reduction in the frequency with which germinated spores formed haustorial initials. Histochemical and ultrastructural studies showed that the hypersensitive reaction(cell death seen as granulation) in response to epidermal cell penetration was a feature common to most interactions involving major genes for resistance. Adjacent palisade mesophyll cells 'frequently also showed signs of reaction which in severe cases resulted in cellular browning. A 1,3—glucan (probably callose) was deposited in the paramural space and lignin-like compounds appeared to accumulate in the walls and cytoplasmic contents of most reacting mesophyll cells. Callose deposition and lignification were also observed in responding epidermal cells. Infection development on partially resistant breeding lines was studied under laboratory, glasshouse and field conditions. Laboratory experiments revealed that partial resistance to S. humuli was expressed as a reduction in the number of fungal colonies established, an increase in the incubation period and reductions in both the extent and intensity of sporulation. Cellular necrosis was less obvious than in race specific resistance. Comparative studies showed that plants severely infected as seedlings in a glasshouse screen were also severely infected when adult in the hop garden. However, in overall terms there was a relatively poor correlation between the levels of sporulation on seedlings and adult plants of partially resistant breeding lines.
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