Submitted to: Acta Horticulturae
Publication Type: Abstract Only
Publication Acceptance Date: July 1, 2004
Publication Date: August 1, 2004
Citation: Jackson, J., Hummer, K.E., Mahaffee, W.F. 2004. High temperature-dependent resistance to powdery mildew in hops [abstract]. Acta Horticulturae. p. 26. Interpretive Summary: Recently powdery mildew has become widespread in the Pacific Northwestern United States. The cost of hop production has greatly increased to control this disease. Most American hop varieties are susceptible to this disease. One variety, 'Symphony' is usually very susceptible. However, in hot temperatures, this plant develops much less mildew and seems to have 'high temperature dependent-resistance'. We wanted to find out if the same effect occurred in other hop plants. We also wanted to observe the mildew growth at different temperatures. We confirmed that this temperature dependent-resistance occurred in 'Symphony' and other hop plants. We examined samples under the microscope and saw that mildew grew much slower on plants grown at high temperatures. In some resistant hops the mildew spore would germinate but not grow any further. We are continuing to research potential causes for slowing or stopping the growth of powdery mildew on the hop leaves at high temperature. We will examine the thickness of waxes on the leaf surfaces to see if they slow the mildew growth.
Technical Abstract: Decreased susceptibility to Podosphaera macularis Braun & Takamatus has been observed on Humulus lupulus L. cv. Symphony at high temperatures. Our objectives were to determine if this effect occurs in other hops and to search for changes in fungal infectivity and hop morphology in genotypes having this resistance. One- to two-month old, potted H. lupulus cv. Nugget(resistant), Symphony(susceptible) and a wild Kazakhstani selection (hypersensitive ' tolerant) were grown for 7 to 10 days in growth chambers at 29, 32 and 35C. Plants were inoculated with a spore suspension of 20,000 conidia/ml, air-dried and placed into growth chambers at 18C. After 7 to 10 days, depending on the rate of fungal development, the infection frequency (lesions/cm2 of leaf area) was determined for the abaxial surfaces of the four most apical, fully-expanded leaves of three replicates. The infection frequency at temperatures less that 29C was not significantly different than at 18C. High temperature-dependant resistance was confirmed on 'Symphony' and observed on the Kazakhstani. 'Nugget' showed no infection at any temperature. Spores germinated on each of the hops within 15 to 24 hours of inoculation. On 'Symphony' at 18C,fungal spore germination occurred normally with 4 to 5 germ tubes developing within 2 to 3 days. On the Kazakhstani growing at 18C,germ tube growth was similar but delayed. Epidermal leaf cells then collapsed, formed a lesion, and seemed to cause hyphal death. On 'Nugget' at any temperature, fungal spores germinated but did not develop past the initial germ tube before dying; no further fungal development or sporulation occurred. Temperatures of 32 and 35C significantly slowed the hyphal growth rate, and delayed sporulation on 'Symphony' and the Kazakhstani selection.