Histopathology of Infection of Minneola Tangelo by Alternaria alternata pv. dtri and the Effect of Host and Environmental Factors on Lesion Development

After inoculation of Minneola leaves with conidia of Alternaria alternata pv. citri, germ lubes randomly grew over the leaf surfaces. Twenty-four h after inoculation, appressoria appeared over the epidermal cells; they formed infection hyphae that penetrated into the leaf, and developed intercellular branching hyphae which resulted in minute necrotic lesions. During the following 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: mature 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 detached or intact leaves, or between leaves incubated in darkness or under natural light. Fruit were highly susceptible throughout the whole season