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First Report of Ascochyta spp. on Soybean in South Africa

During February 2000 soybean fields over a wide area in South Africa were affected by a previously unreported disease. A typical target spot developed on leaves and many became blighted. Elongated dark brown to black lesions developed on the stems, often resulting in wilting of young shoots. Petioles and leaf axils were colonized, resulting in premature leaf drop. The most severe manifestation was on pedicels where infection suppressed podfill and plants remained green as ripening was delayed. Two distinct groups of isolates were obtained from lesions. In both groups an Ascochyta anamorph was present, while from some lesions collected in Kwazulu-Natal and Mpumalanga, a teliomorph (Mycosphaerella) was also present. Since dry beans are produced in these areas and M. phaseolorum is common on this crop, it was believed that the pathogen might have moved to soybean. In cross inoculations, both soybean and dry bean isolates were pathogenic to both hosts. However, M. phaseolorum isolates were more aggressive to both hosts than the Ascochyta sp. Morphologically the anamorphs of two types of isolates were indistinguishable. DNA was isolated from freeze dried mycelia using a modified version of the CTAB-method described by Graham et al. (1). The DNA concentration and purity were estimated by measuring absorbances at A₂₆₀ and A₂₈₀. Genetic difference between both isolates were determined using amplified fragment length polymorphism (AFLP) technique. The AFLP analysis was performed following the protocol described by Vos et al. (2) and the product manual supplied by Life Technologies Inc. (Gaithersburg, MD) with minor modifications. Five randomly selected primer pair combinations were tested for their ability to reveal polymorphisms between the isolates. The gel electrophoresis for AFLP products was as described by Vos et al. (2). AFLP gels were silver stained following the protocol described by silver sequence DNA sequencing system manual (Promega, Madison, WI). All five primer pairs revealed only polymorphisms between isolates. No corresponding bands between the two isolates were detected using these five primer pair combinations. It is concluded that both M. phaseolorum and an unidentified Ascochyta sp. were the cause of the epidemic. Ascochyta spp. have not previously been reported on soybean in South Africa.

References: (1) G. C. Graham et al. Biotechniques 16:48-50, 1994. (2) P. Vos et al. Nucleic Acids Res. 21:4407-4414, 1995.