Relative fitness of races 1 (0) and 3 (0, 1) of rust (Puccinia helianthi) in mixtures on susceptible sunflower (Helianthus annuus)

Race 1 (race 0 according to new international designation) of sunflower rust (Puccinia helianthi) remained dominant in the rust population for years after cultivars carrying the R1 gene conferring resistance were commonly grown, even though race 3 (0, 1), which is able to overcome R1, was present in nature when cultivars with R1 were first released. A mixture with originally equal proportions of races 1 and 3 was maintained for eight spore generations on line CM 303 susceptible to both races. Race 1 increased from 41% in the 1st generation to stabilize at 95% by the 5th. Factors that might contribute to the apparent greater fitness of race 1 under controlled conditions, including germination percentage of spores on the leaf surface, growth rate of germ tubes, formation of appressoria, size of mycelial colonies within leaves, size and density of pustules on leaves, and spore production, were compared. The critical factors appeared to be the higher germinability, more rapid germination, and more rapid formation of appressoria by spores of race 1.