Histopathology of infection of Minneola tangelo by Alternaria alternata pv. citri and the effect of host and environmental factors on lesion development

After inoculation of Minneola leaves with conidia of Alternaria alternata p v. citri, germ tubes randomly grew over the leaf surfaces. Twenty-four h af ter inoculation, appressoria appeared over the epidermal cells; they formed infection hyphae that penetrated into the leaf, and developed intercellula r branching hyphae which resulted in minute necrotic lesions. During the fo llowing 2-3 days, the lesions grew into large necrotic spots, subsequently accompanied by a dark discoloration of the veins extending from the lesions . Susceptibility of the leaves was negatively correlated with their age: ma ture leaves (approximately 2 months old) were very resistant to infection. Lesions developed faster when leaves were inoculated on the abaxial rather than the adaxial surface. There was no difference in disease severity on de tached or intact leaves, or between leaves incubated in darkness or under n atural light. Fruit were highly susceptible throughout the whole season.