Pathogen

Abstract

species pathogenic on cruciferous crops are, A. brassicae, A. brassicicola and A. raphani. Among the four species of Alternaria, A. brassicae is more common on oil-yielding crops, while other three species are major pathogens of vegetable crops. Phylogeny, taxonomy, morphology, classification, infection process, pathogenesis, disease cycle and identification characteristics along with synonymous of all four species have been well documented. Identification, cloning and sequencing of virulence genes of Alternaria infecting crucifers have resolved some doubts about their relationship with cruciferous hosts. Pathogenicity factor and transcription factor Amr 1 have been identified in A. brassicicola. A non-ribosomal peptide synthase gene (AbNPs2) is important for cell wall integrity, conidial viability and virulence of aged spores of A. brassicicola. More than 100 genes have been functionally analysed through various techniques like gene knockout and overexpressions making A. brassicicola the species of choice for functional genomics research. For minimum, maximum and optimum growth, and sporulation of Alternaria species pathogenic on crucifers, in vitro studies have been carried out. The suitable culture media, temperature, relative humidity, pH, nutrient sources and light and darkness conditions for growth, sporulation and spore germination have been determined. The pathogen survives through infected seed, crop residue, cruciferous hosts, weeds, microsclerotia and chlamydospores and all act as source of primary inoculum. Secondary infection during the crop season is through conidia produced on infected host plants (under high humidity), which spread through rain splashes and wind to complete the disease cycle. Major identification characteristics of Alternaria species infecting rapeseed–mustard crops are given in Table 3.1.