Alternative hosts of Asian soybean rust (Phakopsora pachyrhizi) in South Africa

Abstract

Phakopsora pachyrhizi Syd., the causal organism of Asian soybean rust (ASR) is an obligate parasite and cannot survive independently of its hosts or on debris. Thirteen legume species were tested for susceptibility to P. pachyrhizi. The abaxial leaf surface of the V3 growth stage of test plants was uniformly inoculated with a 5.5 x 10⁵ urediniospores ml⁻¹ suspension in water containing Tween 20. Host reaction was recorded 21 days post-inoculation. Cajanus cajan (pigeon pea), Glycine max (vegetable soyabean), Lablab purpureus (lablab), Lupinus angustifolius (lupin), Phaseolus vulgaris (common bean), Pueraria lobata (kudzu vine) and Vigna unguiculata (cowpea) showed typical sporulating uredinia of ASR. To test the viability of these spores, a suspension of 2.5 x 10⁵ urediniospores ml⁻¹ was inoculated onto G. max (Prima 2000), an ASR susceptible cultivar. Urediniospores from pustules on C. cajan, G. max, L. purpureus, L. angustifolius, P. vulgaris, P. lobata and V. unguiculata produced viable spores on Prima 2000. No ASR symptoms were found on Canavalia ensiformis, Coronilla varia, Lespedeza cuneata, Medicago sativa, Mucuna pruriens and Trifolium repens.