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Use of quantitative traits to assess aggressivness of Phakopsora pachyrhizi isolates from Nigeria and the United States

Twizeyimana, T., Ojiambo, P. S., Bandyopadhyay, R., Hartman, G. L.
Plant disease 2014 v.98 no.9 pp. 1261
Phakopsora pachyrhizi, disease resistance, foliar diseases, genetic variation, genotype, leaves, lesions (plant), pathogenicity, pathogens, quantitative traits, soybean rust, soybeans, sporulation, Nigeria, United States
Soybean rust, caused by Phakopsora pachyrhizi, is one of the most important foliar diseases of soybean worldwide. The soybean-P. pachyrhizi interaction is often complex due to genetic variability in host and pathogen genotypes. In a compatible reaction, soybean genotypes produce tan colored lesions, whereas in an incompatible reaction, soybean genotypes produce an immune response (complete resistance) or reddish brown lesions (incomplete resistance). In this study, a total of 116 and 72 isolates of P. pachyrhizi from Nigeria and the U.S., respectively, were compared based on six quantitative traits to assess their aggressiveness on two soybean genotypes. All isolates produced reddish brown lesions on plant introduction (PI) 462312 and tan lesions on TGx 1485-1D. The number of days after inoculation to first appearance of lesions, uredinia, and sporulation, along with the number of lesions and sporulating uredinia per cm2 leaf tissue, and the number of uredinia per lesion, were significantly (P < 0.001) different between the two soybean genotypes for all isolates from each country. The number of days to first appearance of lesions, uredinia, and sporulation were greater on PI 462312 than on TGx 1485-1D for all the test isolates. Similarly, the number of lesions and sporulating uredinia per cm2, and the number of uredinia per lesion were lower on PI 462312 than on TGx 1485-1D. For both soybean genotypes, the number of sporulating uredinia per cm2 significantly (P = 0.0001) increased with an increase in the number of lesions per cm2. Although the slope of the regression of sporulating uredinia on number of lesions was greater (P < 0.0001) when TGx 1485-1D was inoculated with Nigerian isolates compared to U.S. isolates, slopes of the regression lines did not different significantly (P > 0.0675) when PI 46312 was inoculated with Nigerian and U.S. isolates. This is the first study that used a large number of isolates from two continents to assess aggressiveness of P. pachyrhizi using multiple traits in soybean genotypes with contrasting types of disease reaction.