Leaf mold is a disease problem found generally in protected culture due to the high humidity requirements of the pathogen, Cladosporium fulvum. Older leaves show the yellowish light-green blotches first, followed by a purplish or olive green mold on the lower side of the leaves coupled with a yellowing on the upper leaf surface. Lower leaves of infected plants will eventually turn yellow and drop off. Spread of the fungus is gen-erallyvia the air or transportbyworkers or irrigation water (Watterson 1986). Langford (1937) discovered the first resistance gene, Cf-1. Twenty-four additional genes have been proposed by Kerr and coworkers in
Canada (Kanwar et al. 1980a, b). Haanstra (2000) revised this number to 19 due to identical specificities of five of the genes, but added two new genes for a total of 21 Cf genes. Virulent races of the pathogen appear quickly, thus Lindhout et al. (1989) have suggested at least two genes not overcome by current isolates be used in developing varieties. To date, 18 races have been reported (Lukyanenko 1991), but the nomenclature system is not uniform (Scott and Gardner 2007). Chromosome 5 has Cf-2 and Cf-5 with both genes being either allelic or tightly linked (Jones et al. 1993). Balint-Kurti etal. (1994) reported the two genes on chromosome 1 Cf-4 and Cf-9 to be allelic. Haanstra et al. (1999) and Haanstra (2000) reported additional loci, Cf-ECP2 and Cf-ECP5 also on chromosome 1 and linked to Cf-4. Currentlybreeders are using Cf-2, Cf-4, Cf-5 and an unknown gene (Lukyanenko 1991; Scott and Gardner 2007).
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