How to Manage Pests
UC Pest Management Guidelines
Alternaria rot is a fungal disease that affects mainly navel oranges and lemons. Fruit infected with Alternaria change color prematurely. The decay is softer on lemons than on oranges. Infections typically occur in the grove; disease often doesn't develop until after harvest, and most damage occurs during storage. On navel oranges, the disease is also called black rot, and results in dark brown to black, firm spots or areas at the stylar end or in the navel. If you cut the fruit in half, you can see the rot extending into the core.
There are many strains of the pathogen Alternaria citri. The strain that causes Alternaria rot is a nontoxin-producing strain. Toxin-producing strains that produce other diseases of citrus have not been reported in California. Strains on mandarin causing brown spot have been referred to as A. alternata pv. citri.
Healthy, good quality fruit are more resistant to Alternaria rot than stressed or damaged fruits, especially oranges with split navels. Preventing stress can reduce the incidence of splitting and Alternaria rot. Stylar-end infections generally occur on cultivars with poorly formed navels. Preharvest fungicide treatments are usually ineffective. Delaying harvest until infected fruit have fallen has been used as a strategy to prevent inadvertent inclusion of infected fruit in the harvested crop. However, unaffected fruit should be harvested at optimum maturity. Postharvest treatments with imazalil, 2,4-D, or both have provided some control. The growth regulator 2,4-D delays senescence and thereby restricts colonization of the host.
UC IPM Pest Management Guidelines: Citrus