Graduate Thesis Or Dissertation
 

Pathogenic specialization and heterothallism in Sphaerotheca pannosa var. rosae

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  • Studies were conducted on pathogenic specialization and heterothallism in Sphaerotheca pannosa var. rosae, causal agent of rose powdery mildew. Methods were developed for isolation and identification of Sphaerotheca pannosa var. rosae races on rose. Monoconidial isolates were initiated on detached leaves, increased on host plants, and inoculated to rose cultivars to obtain differential reactions. Virulence was evaluated by assessing the infection type of individual lesions and the percentage of leaf area covered with sporulating colonies. Five races of S. pannosa var. rosae were identified, and virulence formulae were developed to describe them on four differential cultivars. The susceptibility of detached rose leaves to S. pannosa var. rosae isolates of unknown and known virulence was assessed on physiologically similar tissue and compared with results on attached leaves. The correlation between inoculation to detached and attached leaves varied with the host and host-pathogen interaction. Detached leaves are useful in screening for differences in host specificity, but should not be used exclusively. Virulence patterns were studied in several conidial populations of S. pannosa var. rosae. Although pathogenic variation was greater in the field, repeated sampling of both field and green house locations demonstrated that rose powdery mildew populations are dynamic. Two populations consisted of more than one race; all others were racially homogenous. In one population, the presence of cleistothecia and ratio of pathotype occurrence suggested the meiotic recombination of two virulence factors. The sexuality of S. pannosa var. rosae was investigated. Nine monoconidial isolates were coinoculated to four rose hosts in 11 combinations. Heterothallism was demonstrated by the strong fruiting response which occurred when an isolate designated as RR1 was paired with two other isolates. In the nine combinations where cleistothecia did not form, the coinoculated isolates differed in host specificity and were reisolated when the experiment was terminated. The results indicated that the virulence of compatible strains on a mutual host was necessary for ascocarp formation, and that fruiting may require physiologic stimulation by the host.
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