Brown rot of citrus—Phytophthora spp.
Damage from brown rot caused by Phytophthora spp. can occur each winter in most citrus-growing regions of California. Trees may show only symptoms of brown rot of fruit, or brown rot may occur in combination other Phytophthora diseases such as Phytophthora gummosis, Phytophthora root rot, or both.
Identification
Brown rot of citrus is diagnosed by its symptoms as described below in Damage. Ripe or ripening citrus fruit on the lower part of the tree develop brown or tan, rounded lesions. Affected fruit develop a pungent odor.
Almond, stone fruits, and quince also develop a disease called brown rot. This disease is caused by Monilinia species of fungi that cause gum to ooze from cankered bark and growth of gray to tan spores on infected blossoms, fruit, nuts, and twigs. Brown rot of citrus produces no spores visible to the naked eye, but infected fruit may develop whitish mycelia.
Life cycle
Brown rot of citrus is caused by several Phytophthora species. It mainly damages fruit growing near or touching the ground because the pathogens persist in and spread from soil.
Brown rot disease develops when conditions are cool and wet and after Phytophthora spores from the soil are splashed onto the lower canopy during rain or sprinkler irrigation. At temperatures of 57º to 73°F spore-contaminated fruit can become infected after a continuous wet period of 3 hours.
Damage
Brown rot develops primarily on mature or nearly mature citrus fruit. Blossoms, leaves, and twigs are occasionally infected by brown rot and turn brown and die. Lesions on rinds are brown or tan and firm and leathery. However, if later colonized by secondary decay organisms, the diseased fruit soften. Infected fruit usually drop and have an unpleasant odor.
At high humidity, Phytophthora-infected fruit become covered with white mycelia. Phytophthora-infected fruit are also readily colonized by secondary pathogens, such as blue and green molds (Penicillium species); these cause the rind to soften and become covered with blue, greenish, or whitish mycelia. Fruit in the early stage of the brown rot disease or blue and green mold infection may go unnoticed at harvest and infect other fruit during storage.
Solutions
Periodically inspect fruit and pick and dispose of those that are diseased. Pick up and dispose of any fruit that has dropped on the ground. Do not let irrigation sprinklers wet foliage or fruit. Do not harvest fruit when it or conditions are wet. Do not store infected fruit with healthy fruit.
Where brown rot has been a problem, prune the lower canopy of citrus so branches and foliage are at least 2 feet from the ground. A preventive application of Bordeaux mixture, another copper fungicide, or fosetyl-al can be sprayed before the first fall rains. Thoroughly spray the branches, fruit, and leaves within 4 feet of the ground and the ground beneath the tree.
Adapted from Integrated Pest Management for Citrus and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
