Brown spot of mandarins

Host and Symptoms. Alternaria citri is an ascomycete fungal plant pathogen that causes black rot in citrus plants. Specifically, certain lemon, lime, orange, mandarin and grapefruit species are susceptible hosts for this pathogen. The host is more susceptible to disease under ideal environmental conditions consisting of dry, warm summers and cool, moist winters. One symptom of the pathogen is the black rot that is produced. The black hyphae that forms on the surface of the plant is a sign of the actual pathogen. While healthy and uninfected fruits will display a particular hue, a plant infected by A. citri will possess atypical and usually more brightly colored fruits which signifies presence of the pathogen.
Disease Cycle. Little research on the specific disease cycle of Alernaria citri has been conducted because its life cycle is so similar to Alternaria alternata. The life cycle of Alternaria alternata can be used as a proxy for information on Alternaria citri. However, Alternaria citri does not produces external signs or symptoms on leaves and stems, like Alternaria alternata. Signs and symptoms of Alternaria citri do not develop until after the fruit is harvested.
Alternaria alternata has no known resting sexual stage. Instead, it overwinters in infected plant debris through asexual spores called conidia. Their production can begin in as few as ten days after the first symptoms appear, and can continue for to up to fifty days. Because of this, the life cycle is known as poly-cyclic. Alternaria alternata's conidia disperse via air currents, and their release from the lesions can be triggered by rainfall, or even just a sudden increase in humidity. When the conidium lands on a leaf, it will wait until the nighttime dew, and then germinate. It can either enter through the stomata, or penetrate directly through the top of the leaf, using its appressorium, infecting the leaf within 12 hours.