Genetic Diversity Among Alternaria solani Isolates from Potatoes in South Africa

Abstract

Genetic diversity among isolates of Alternaria solani, the causal agent of early blight of potato, from various potato-growing regions in South Africa (SA), was determined using virulence assays, vegetative compatibility (VC) tests, and random amplified microsatellite (RAMS) primers. The virulence assays showed low virulence levels for the largest part of the population, but failed to otherwise characterize the population diversity. The VC tests revealed 19 VC groups (VCGs), indicating a relatively high level of diversity among the isolates. There was little correlation between geographic origin of isolates and VCGs. Analysis of RAMS profiles revealed 27% genetic diversity among 46 isolates. This value is relatively high for an asexually reproducing fungus, but is similar to values obtained previously by authors studying A. solani. Distance analysis of the RAMS profiles also provided no evidence for geographical clustering of isolates. VCG and RAMS profiles indicated that isolates are randomly spread across SA. This fact, together with the high diversity of A. solani in SA, indicates that the fungus has a high potential to adapt to resistant cultivars or fungicides. This information can aid in the breeding and deployment of A. solani-resistant potato varieties, and in early blight disease management in SA.

Tomato early blight (Alternaria solani): the pathogen, genetics, and breeding for resistance

“…Although A. solani appears to have only a nonsexual life cycle, it has a relatively high variation in morphology in vivo and in vitro, physiology, genetic makeup, and pathogenicity among isolates (Bonde 1929;Wellman 1943;Neergaard 1945;Henning and Alexander 1959;Rotem 1966;Weir and Huff 1998;Martinez et al 2004;van der Waals et al 2004). Bonde (1929) and Neergaard (1945) classified A. solani into conidial, mycelial, and intermediate types of isolates.…”

“…A high genetic diversity was detected among isolates of A. solani originating from the United States, South Africa, Cuba, Brazil, Turkey, Greece, Canada, China and Russia based on vegetative compatibility groups (VCG, van der Waals et al 2004) and molecular markers [isozymes, random amplified polymorphic DNA markers (RAPDs), random amplified microsatellite markers (RAMs), and amplified fragment length polymorphisms (AFLPs); Petrunak and Christ 1992;Weir and Huff 1998;Martinez et al 2004;van der Waals et al 2004]. In studies where isolates from several countries were compared using VCG assays, RAMs (van der Waals et al 2004), or RAPD markers (Weir and Huff 1998), A. solani isolates cluster according to country, indicating some degree of genetic isolation.…”

“…In studies where isolates from several countries were compared using VCG assays, RAMs (van der Waals et al 2004), or RAPD markers (Weir and Huff 1998), A. solani isolates cluster according to country, indicating some degree of genetic isolation. In contrast, isolates from the same country are not distinctly separated by geographical origin (Petrunak and Christ 1992;Weir and Huff 1998;Martinez et al 2004;van der Waals et al 2004). This can be ascribed to short-or medium-distance dispersal of the airborne spores and movement of plant material within the countries (Weir and Huff 1998;van der Waals et al 2004).…”

“…In contrast, isolates from the same country are not distinctly separated by geographical origin (Petrunak and Christ 1992;Weir and Huff 1998;Martinez et al 2004;van der Waals et al 2004). This can be ascribed to short-or medium-distance dispersal of the airborne spores and movement of plant material within the countries (Weir and Huff 1998;van der Waals et al 2004). In many cases, isolates originating from tomato and potato clustered according to their hosts based on RAPD (Weir and Huff 1998) and AFLP markers (Martinez et al 2004), suggesting host specialization.…”

“…For example, a recent study demonstrated that isolates in the midwestern United States have become less sensitive to a fungicide resulting in significant losses of disease in glasshouse cultures (Pasche et al 2004). The high genetic diversity and high degree of gene flow within countries could break down genetic resistance in the host; this possibility has been advanced as one of the reasons for the absence of potato cultivars with complete resistance to A. solani in South Africa (van der Waals et al 2004). …”