Related wild species for breeding of tomato resistant to early blight disease (Alternaria solani)

Tomato ( Solanum lycopersicum L.) is susceptible to many fungal diseases, including early blight of foliage caused by the necrotroph fungus Alternaria solani . Frequent application of fungicide is the major component to keep the disease low. Resistant tomato cultivar is the most desirable as it can reduce the cost of disease control significantly. So far, sources of resistance to early blight can only be found in wild relative species of tomato, and few of them have been used in traditional breeding. Unfortunately, tomato lines bred from wild donor parent still exhibit poor horticultural performances like low yield, and late maturity, and indeterminate plant habit, which hinders the release of these lines directly as cultivars. The quantitative expression and polygenic control of the early blight resistance trait, as well as the influence of plant developmental stages and environmental conditions, complicate phenotypic selection in traditional breeding. Genotypic selection by using closely linked-markers to the resistance loci is thus preferred, but mapping of early blight resistance QTL in interspecific crosses of tomato have not obtained markers which are useful for marker-assisted breeding. This review presents sources of early blight resistance in wild species of tomato and efforts in dissecting early blight resistance QTL via linkage analysis with molecular markers. Strategies to obtain closely-linked markers and genomics-assisted breeding to facilitate the introgression of useful resistance genes to cultivated tomato are discussed.