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Ecology and Epidemiology

Infection, Colonization, and Uredospore Production on Wayne Soybean by Four Cultures of Phakopsora pachyrhizi, the Cause of Soybean Rust. J. Stanley Melching, Research plant pathologist, Plant Disease Research Laboratory, U.S. Department of Agriculture, Science and Education Administration, Agricultural Research, P.O. Box 1209, Frederick, MD 21701; K. R. Bromfield(2), and C. H. Kingsolver(3). (2)(3)Research plant pathologists, Plant Disease Research Laboratory, U.S. Department of Agriculture, Science and Education Administration, Agricultural Research, P.O. Box 1209, Frederick, MD 21701. Phytopathology 69:1262-1265. Accepted for publication 30 May 1979. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1979. DOI: 10.1094/Phyto-69-1262.

Under greenhouse conditions, cultures of Phakopsora pachyrhizi from Taiwan, India, Australia, and Indonesia were compared for quantitative characteristics related to their ability to colonize and reproduce on plants of soybean cultivar Wayne. 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 0.610.77 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 2.03.5 for the other three; 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 eight uredia per lesion for the Australian and 12.614 for the other cultures. Uredospores were collected daily from 1352 days after inoculation of plants on which numbers of lesions and leaf areas had been 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 μg and 2,028; Indian, 0.24 μg and 3,768; Indonesian, 0.40 μg and 6,268; and Taiwanese, 0.42 μg 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.

Additional keywords: Sporulation, pathogen aggressiveness, Glycine max.