Since tangerines and grapefruit they are eaten fresh, a blemish-free peel is highly desired. Two diseases that frequently mar the peels of tangerines, and occasionally grapefruit, are Alternaria brown spot (ABS) and citrus scab. Both diseases cause symptoms on leaves and fruit.
ALTERNARIA BROWN SPOT
The early foliar symptoms of ABS are small yellow flecks that quickly become larger with dark brown centers. The lesions have large yellow halos. Oftentimes, dark streaks along the veins can be observed. Eventually the centers will die, and leaves may fall off early, defoliating trees.
On the fruit, early lesions resemble those on leaves. If the fruitlets do not fall from the infection, lesions develop a corky, raised appearance. Lesions can blemish over half the surface of a fruit. As fruit approach maturity, the corky centers can fall from the lesions, leaving tan depressions.
The most obvious scab symptoms occur on the fruit. Fruit lesions start as large, raised, wart-like lesions on young green fruitlets with a pink-tan color. The color changes over time from pink to yellow to tan. Depending on cultivar susceptibility, the lesions can remain very raised or flatten to resemble wind scar. Lesions on grapefruit often become very flat with a silver sheen.
Leaf lesions are more easily seen on some tangerines. They start on the young flush as small tan spots. If a leaf is particularly susceptible, it becomes distorted and develops pinnacles with the lesion at the tip. When turned over, there is a corresponding divot on the opposite side of the leaf. Lesions can occur on either side of the leaf.
Management of these diseases starts with the young, expanding flush to reduce inoculum for fruit and defoliation from ABS. A fungicide application, starting at one quarter of full leaf expansion will help protect the leaves from both diseases. If ABS severity was high previously or control was insufficient from the first spray, it may be necessary to apply again near full expansion as the leaves remain susceptible until hardened.
At petal fall, fungicide application to protect young fruitlets will be needed for both diseases. For scab, the final spray is approximately three weeks after petal fall. For ABS, the fruit will need further protection, particularly for badly affected groves.
The frequency of application will depend on rainfall in April and May and disease severity in the grove. The sprays may be as frequent as every 10 days to as few as once per month. By June, a regular rainfall pattern is established but fruit growth has slowed. On average, two applications in June suffice. Fruit usually become resistant to ABS by mid-July, but it is possible to see continued fruit drop from the disease.
For fungicide recommendations and further information about these diseases, consult the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Florida Citrus Production Guide.
Megan Dewdney is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.