Infection Processes ofPuccinia reconditain Slow- and Fast-Rusting Wheat Cultivars

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Abstract
Development of P. recondita from spore germination to uredinia formation in 11 wheat cultivars was studied histologically to determine whether the long latent period and small uredinia in slow-rusting cultivars are due to reduced mycelial growth in leaf tissue. Six sequential samples of flag leaves (24-216 h after inoculation) were treated with an optical brightener (Uvitex BOPT) and observed with a fluorescence microscope. Percentages of urediniospore germination, germinated urediniospores forming appressoria, and appressoria forming substomatal vesicles were the same for fast-rusting cultivars (Morocco, Suwon 92, Monon and P72482), an intermediate slow-rusting cultivar (SW 210), and slow-rusting cultivars (Suwon 85, SW 72469, P6028, Cl13227 and 1.574-1), as well as for a hypersensitively resistant cultivar (P68130). However, at each sampling time the colonies in slow-rusting wheats were smaller and had fewer haustorial mother cells than colonies in fast-rusting wheats. Although the colony growth rates of P. recondita on fast-rusting and slow-rusting cultivars were different formation of uredinial beds, from which uredinia develop, in both groups of cultivars began when the average colony area was 0.12-0.14 mm2. The longer time required for colonies to reach this incipient stage of sporulation on slow-rusting cultivars would explain their long latent period.