Infection, Colonization, and Uredospore Production on Wayne Soybean by Four Cultures ofPhakopsora pachyrhizi,the Cause of Soybean Rust

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Abstract
Under greenhouse conditions, cultures of P. pachyrhizi from Taiwan, India, Australia and Indonesia were compared for quantitative characteristics related to their ability to colonize and reproduce on ''Wayne'' soybean plants. All cultures required similar time periods from inoculation until lesion appearance (7 days) and initiation of secondary uredospore production (9 days). The Indian culture produced more lesions per unit leaf area per unit of inoculum than the other cultures. The mean lesion areas at 2 wk after inoculation on both upper and lower leaf surfaces were similar for the Indian, Taiwanese and Indonesian cultures (range 0.61-0.77 mm2), but for the Australian culture the mean lesion areas were smaller (0.30 mm2 on upper surface and 0.42 mm2 on lower surface). The mean number of uredia per lesion at 2 wk on the upper leaf surface was 1.0 for the Australian culture and 2.0-3.5 for the other 3; little increase with time occurred with any culture. On the lower surface, however, new uredia continued to form in lesions induced by all cultures. By 7 wk there were 8 uredia/lesion for the Australian and 12.6-14 for the other cultures. Uredospores were collected daily from 13-52 days after inoculation of plants on which numbers of lesions and leaf areas were determined. The mean mass (fresh weight) of spores produced per lesion each day and the calculated total number of spores produced over the life of the lesion were: Australian, 0.13 .mu.g and 2,028; Indian, 0.24 .mu.g and 3,768; Indonesian, 0.40 .mu.g and 6.268; and Taiwanese, 0.42 .mu.g and 6,600. Uredospores of all cultures were similar in length and width. No consistent differences in germination potential were found in uredospores tested from each culture at each harvest.

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