Infection Process, Pathogenesis and Disease Cycle

Depending upon the growing season, species in crucifers cause primary infection through the windborne spores produced from over summered or overwintered primary inoculum in infected seeds, plant debris, collateral hosts and weeds. Under congenial environmental conditions, spores (conidia) germinate readily in the presence of moisture by producing a germ tube, which penetrates the hosts directly or through stomata. Further, the process of infection and pathogenesis is governed by a number of external and internal factors of a specific host–pathogen interaction. Induction of cell wall-degrading enzymes, secretion of toxins, enforcement of fungal cell wall architecture and detoxification of host defence molecules are essential functions for successful infection and pathogenesis of Alternaria species. Genes important for each of these functions have been identified in various fungal species. A transcription factor gene, AbVF19, has been discovered from A. brassicicola, which positively regulates hydrolytic enzyme-coding gene. Another transcription factor gene, Amr1, negatively regulates a subset of these genes during the late stage of pathogenesis and positively regulates melanin biosynthesis during conidiogenesis. It seems that the suppressive functions of Amr 1 gene contribute to the specialized adaptation of A. brassicicola as an efficient and successful facultative parasites of cruciferous plants.