Septoria spot of citrus—Septoria citri
The Septoria fungus causes spotting of the rinds of Valencia oranges, navel oranges left on the tree until spring, and occasionally on grapefruit and lemons. The disease develops during and after cool, wet weather.
Identification
The early symptoms of Septoria spot are small, light tan to reddish brown pits about 1/25 to 1/12 inch (1–2 mm) in diameter on fruit. The lesions generally do not extend deeper than the oil-bearing tissue, the colored, outer portion of rinds. Sometimes pits become deeper and larger, about 1/6 to 1/4 inch in diameter, and extend into the white portion of the rind. Dark brown to black fruiting bodies (pycnidia) often develop in the small depressions.
Septoria spot lesions generally occur after fruit develop their mature color and winter rains occur. As disease progresses, such as on fruit in storage, lesions can develop into a tearstained or tearstreaked pattern or merge into larger, dark, sunken blotches.
Septoria citri also causes spotting or lesions on leaves or twigs that are injured or weakened by frost damage or pests. On leaves, lesions 1/25 to 1/6 inch (1–4 mm) in diameter develop as raised blisterlike black spots surrounded by a yellow halo. The halo turns pale brown in older infections.
Septoria commonly occurs together with anthracnose. Both fungi and possibly certain adverse environmental conditions can cause tearstain or tearstreak discoloration on citrus rinds.
Life cycle
The Septoria pathogen can be naturally present on citrus leaves and twigs throughout the growing season. Septoria infection occurs primarily on fruit injured by cold weather and after high rainfall. Spores are spread throughout the tree and onto fruit by splashing irrigation water or rain. Disease then develops slowly over several weeks and may initially cause infections without visible symptoms. Infected fruit may not develop obvious disease until after harvest and storage.
Solutions
The disease occurs sporadically depending on the weather. Generally, no management actions are recommended on home fruit trees. Septoria spot on home citrus trees is mostly an aesthetic malady marring the rinds.
Adapted from Integrated Pest Management for Citrus and Pest Management Guidelines: Citrus, University of California Statewide Integrated Pest Management Program (UC IPM).
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