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Pathogen biology
The causal pathogen of early blight is the fungus Alternaria solani. There is no known sexual stage, and, hence, it is classified as a Deuteromycete. The Alternaria genus is a large and important group of pathogenic fungi, which cause a significant number of important diseases on a wide range of agronomic and horticultural plants. The fungus is readily cultured on artificial media such as V-8 juice agar where it produces a deeply pigmented gray/black hairy colony. The mycelium is haploid and septate, becoming darkly pigmented with age. Sporulation in culture can be stimulated by exposure to fluorescent light. The asexual conidia are borne singly or in a chain of two on distinct conidiophores. The beaked conidia may possess 9-11 transverse septa as well as vertical septa (Figure 8). Morphological and pathogenic variability among isolates of A. solani has given rise to claims of the existence of races, although this remains unproven.
Figure 8
Copyright © 2002
by The American Phytopathological Society