Graduate Thesis Or Dissertation
 

Verticillium wilt of hops : pathogenicity, host range, and temperature relations of the causal agents

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  • Verticillium wilt of hops in Oregon, caused by Verticillium dahliae Kleb., was first reported in 1956. Later, V. albo-atrum Reinke & Berth. was also isolated from infected hop plants, and thus both species of the fungus were shown to be the causal agents of the disease in Oregon. The purpose of this research was to study temperature relations and host range of hop isolates of Verticillium existing in Oregon, and also to determine infectivity and pathogenicity of these isolates to different varieties of hops. The temperature relations of two V. dahliae and two V. alboatrum isolates, recovered from infected hops grown throughout the Willamette Valley of Oregon, were studied by dry weight measurements. Two additional V. dahliae isolates, one from peppermint and one from potato, were included in the experiments, The isolates were grown in Czapek. Dox broth plus yeast extract at ten temperatures between 5° and 32°C. The dry weight was affected by the interactions of isolate, temperature, and period of incubation. At temperatures up to 17°C, the dry weights of all isolates were increased by longer incubation periods with no autolysis of the mycelial mass observed in 17 days. Above 17°, however, the incubation periods required for maximum dry weight production and the onset of autolysis varied, depending on the temperature and the isolate. A faster initial growth rate, caused by increasing temperature, hastened the onset of autolysis, and thus did not result in greater amounts of dry weight production in longer periods. On the other hand, a slow initial growth rate, caused by too high a temperature, resulted in delay of autolysis and production of greater dry weights in longer incubation periods. V. albo-atrum isolates did not grow at 30°C and evidence was obtained showing that failure to grow was due to thermal death of germinated and ungerminated spore inoculum rather than to inhibition of germination. All V. dahliae isolates grew fairly well at 30°C but failed to grow at 32°C. The latter temperature was lethal to the spore inocula of V. dahliae isolates after 7 to 12 days of incubation. These results point out a significant physiological difference between V. albo-atrum and V. dahliae and thus support the validity of considering V. dahliae as a distinct species. In host range studies, one V. albo-atrum isolate (No. 138), originally obtained from Fuggle hops, was found to infect and proliferate greatly in stems of potato, and to a lesser extent in tomato and eggplant. Among V. dahliae isolates, one originally recovered from Bullion hops (No. 148), proliferated poorly in potato, eggplant, and strawberry. The other (No. 150), obtained from Fuggle hops, proliferated greatly in stems of peppermint and was highly pathogenic to that host. The conclusion was reached that the hop isolates of Verticillium existing in Oregon, are primarily pathogens of other plants, with host ranges that include hops to some extent. Verticillium isolates from infected hops, peppermint and potato, infected a low percentage of plants among 14 hop varieties grown in infested soil. Differently prepared inoculum (laboratory infested soils and straw culture) or different levels of inoculum, did not affect the percentage of infected plants appreciably. All Verticillium isolates, regardless of their origin or species, proliferated poorly in stems of infected hop plants and no actual wilting or death of the infected plants was observed. It was concluded that the strains of Verticillium affecting hops in Oregon are not virulent pathogens of this crop and that they are primarily adapted to plants other than hops.
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