The only known stage in all Alternaria species is the asexual stage. Conidia and mycelium overwinter in the soil and on crop residues and cause primary infection of the leaves. Survival can be several years, in the case of black rot of carrots (Alternaria radicina) up to 8 years.
Life cycle and appearance of Black rot of carrots
Black rot of carrots (Alternaria radicina) is a seed-transferred carrot disease. The seeds are the main source of primary infection and this may lead to death of seedlings. From the primary infection of leaves, this type of carrot disease can spread to the flowers (especially dangerous in crops for seed production) and to the crown. Lesions of black rot of carrots (Alternaria radicina) may occasionally appear on the carrots at harvest but usually occur only during post-harvest storage.
Sporulation of Alternaria occurs at temperatures from 2-4 °C up to 28-30 °C (optimum temperatures lie between 15 and 28 °C) and at a relative humidity (RH) of over 90% or when the leaves are wet. Small differences in optimum temperatures occur between species, but in general, the risk of Alternaria infection is larger under quite warm and humid conditions. Infection may take place within a few hours, with lesions occurring as rapidly as two to three days later and sporulation within five days. Infection is both direct and through wounds and stomata. Spores are dispersed by wind and in some species through the splash of rain.
Black rot of carrots (Alternaria radicina) causes death of seedlings and dark brown lesions with a yellow halo on leaves, growing into crown rot. Infection of carrots during post-harvest storage causes dry, sunken, black lesions.
Typically, Alternaria causes spots on leaves, petals and stems. These spots can be distinguished by the dark and brown concentric rings that are caused by sporulation. Species may differ in specific symptoms.