Alternaria Mali - Pathogenesis

Pathogenesis

A. mali can overwinter as mycelium on dead leaves on the ground, in mechanical injuries in twigs, or in dormant buds. Primary infection occurs about one month after petal fall the following year. The disease is favoured by temperatures between 77 and 86 °F (25–30 °C), and by wet conditions. Infection occurs at optimum temperatures with 5.5 hours of wetting, and an outbreak can become serious within two days of infection. The fungus attacks susceptible cultivars using a chemical toxin. Affected plants exhibit circular spots on the leaves that enlarge as the disease advances. Normally, hyphae cannot adhere to the surface of the host, but under moist conditions light-grey mycelium might be present on the surface. Normally the fungus will not attack fruits except in highly susceptible cultivars; fruit-spotting may occur on the tree or in storage, particularly on fruits with already damaged skin.