Pathogenicity of Pythium populations from corn-soybean rotation fields

Corn and soybean have been rotated for decades in the north central regions of the United States, but it is unknown how the use of long-term rotation affects the population of soilborne fungi pathogenic to both crops. Pythium populations were obtained from 73 commercial corn-soybean rotation fields in Iowa from 1993 to 1995. A total of 163 Pythium isolates were obtained from soil, diseased soybean seedlings, or diseased corn seedlings, and these isolates were pooled into six populations according to the source of samples. The isolates were evaluated for their aggressiveness on corn and soybean for each population. Regardless of the source of sampling, each population had many Pythium isolates that were highly aggressive on both crops. Seventy-one percent of the isolates from soil were pathogenic, and 29% were highly aggressive on both crops. Three populations of Pythium isolates were obtained from diseased soybean seedlings. Eighty-five percent of those isolates were pathogenic, and 49 to 64% were highly aggressive on corn and soybean. Of the isolates obtained from diseased corn seedlings, 87% were pathogenic and 43% were highly aggressive on corn and soybean. For each Pythium population, aggressiveness on soybean and corn (measured by disease index) were highly correlated. Further, 23 P. ultimum isolates were identified from diseased soybean seedlings and tested for pathogenicity on corn and soybean. Disease index of the isolates varied, but the correlation between disease indices on the two crops was significant (r = 0.75, P < 0.001). The study suggests that long-term corn-soybean rotation fields contain many Pythium isolates that are highly aggressive on seedlings of both crops.