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Alternaria Brown Spot1
L.W. Timmer, P.D. Roberts, K.R. Chung and Alka Bhatia2Alternaria brown spot first appeared in Florida citrus groves about 30 years ago and has become a serious problem on some varieties in recent years. The disease was first reported in Australia in 1962, but no one knows how it arrived in Florida or whether it may have developed from other Alternarias locally. Currently, brown spot is also known to occur in South Africa, Turkey, Israel, Spain, and Colombia and probably occurs in other countries which grow susceptible varieties.
In Florida, Alternaria brown spot originally occurred mostly on Dancy tangerines, but the disease severely affects Minneola tangelos and also attacks Murcotts as well as Orlando, Nova and Lee tangelos, and Sunburst tangerines. This disease does not affect oranges, but may cause some spotting of grapefruit if they are adjacent to heavily infested tangerines or tangelos.
Symptoms
Alternaria brown spot attacks young fruit, leaves, and twigs producing small brown-to-black spots surrounded by a yellow halo (Figure 1). The halo is produced by a fungal toxin which rapidly kills citrus cells. The spots on young leaves (Figure 2) can appear as early as 36-48 hours after infection. Spots enlarge as the leaves mature (Figure 3) and if the disease is severe the leaves may drop or the entire shoot may die. If the spots are restricted, necrosis sometimes extends along the vein as the toxin spreads in vascular tissues.
Figure 1. Alternaria brown spot lesions on immature Dancy tangerine.
Figure 2. Alternaria brown spot lesions on young leaves of Dancy tangerine.
Figure 3. Alternaria brown spot lesions on mature leaves of Minneola tangelo. On fruit, severe attacks result in drop of the young fruitlets. Remaining fruit may have spots varying from small dots to large pock marks on the peel (Figures 4, 5). Fruit are susceptible for at least 3 months after petal fall. Even after that time, some fruit may fall as the result of earlier infections.
Figure 4. Alternaria brown spot lesions on mature Minneola tangelo.
Figure 5. Alternaria brown spot lesions and pock marks on mature Minneola tangelo. Tangerines are affected by one strain, or pathotype of the fungus currently designated Alternaria alternata pv. citri. Another pathotype attacks only rough lemon and Rangpur lime. This disease is seen commonly on rough lemon root sprouts (Figure 6) in old groves in Florida. Symptoms are due to host specific toxins which have been well characterized. Black rot of fruit, a postharvest disease, is caused by A. citri. Alternaria citri is currently separated from A. alternata as a different species but morphologically is very similar. Black rot isolates, however, do not cause brown spot on tangerines or rough lemon.
Figure 6. Alternaria brown spot lesions on rough lemon leaves. Disease Cycle
Spores of the fungus are thick walled, multicellular, and pigmented and thus tolerate adverse conditions well. They are produced primarily on old lesions on mature leaves. Spores are produced on leaves remaining on the tree as well as those which have fallen to the ground. Spores are air-borne and release is triggered by rainfall or by a sharp drop in relative humidity.The length of the wetting period required for infection is about 8-10 hours when the temperature is favorable (68-83°F (20-29°C)). Most of the infection probably follows rains, but dew is often sufficient for infection. In Israel, no rain occurs after petal fall, but they can still have significant infection as the result of heavy dews.
Disease Management
Fungicides are the primary means of controlling Alternaria brown spot. However, there are many management practices which are helpful in reducing disease severity.When new groves of susceptible varieties are planted, they should be established with disease-free nursery stock. Trees grown in greenhouses without overhead irrigation are usually free of Alternaria. Even though spores are air-borne, plantings of healthy trees remain so for long periods. If Alternaria is present from the outset, it builds to high populations during the period of vegetative growth on young trees and subsequently is difficult to control on fruit. In establishing new plantings, it is best to locate susceptible varieties in high areas where air drainage and ventilation is good and leaves dry more rapidly. Less vigorous rootstocks such as Cleopatra mandarin should be selected rather than vigorous ones like Carrizo citrange. Groves of Minneola tangelos in low, wet areas have conditions so favorable for disease that Alternaria may be virtually uncontrollable. Trees of susceptible varieties should be spaced more widely than oranges to promote rapid drying of the canopy.
In existing plantings, it is important not to promote excessive vegetative growth. Overwatering and excessive nitrogen fertilization should be avoided. Light hedging should be done frequently rather than hedging severely but less often.
For products registered for control of Alternaria brown spot see the Florida Citrus Pest Management Guide. The number of fungicide applications needed for control varies greatly with the susceptibility of the variety and the severity of the infestation. In the worst cases, the first spray should be applied when the new shoots are about 4-6 inches long to prevent buildup of Alternaria on the spring flush. The second application should be made at petal fall. Thereafter, applications may need to be made as often as every 10 days to achieve good control on fruit and foliage. For more precise timing see the Alter-Rater (http://edis.ifas.ufl.edu/CH183 ), a weather-based predictive model. Use of Abound should be limited due to the possibility that resistant strains may develop. When frequent applications of copper fungicides are being used, rates can be reduced to as little as one pound of metallic copper per acre. During dry periods which often occur in April and early May, spray intervals can be increased. Likewise, less susceptible varieties or less severely affected groves do not need such intense spray programs. Groves of susceptible varieties should be monitored frequently to determine the disease status. Spraying can be discontinued in late June since fruit usually become resistant to Alternaria in July as fruit growth slows. It is probably not economical to try to control the disease on late summer flushes of growth.
Footnotes
1. This document is Fact Sheet PP-152, a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: June 1992. Date revised: December 2001. Expiration date: December 2004.2. L.W. Timmer, Professor, Citrus Research & Education Center, Lake Alfred, Florida; P.D. Roberts, Assistant Professor, Southwest Florida Research & Education Center, Immokalee, Florida; K.R. Chung, Assistant Professor; and A. Bhatia, Senior Biological Scientist, Citrus Research & Education Center, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
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