2010 Florida Citrus Pest Management Guide: Alternaria Brown Spot
Alternaria brown spot, caused by the fungus Alternaria alternata, affects Minneola tangelos, Dancy tangerines, Murcotts, and less frequently Orlando tangelos, Novas, Lees, and Sunburst. In rare cases, it may also infect grapefruit. Where severe, the disease results in extensive fruit drop and must be controlled on processing and fresh market fruit.
Spores of Alternaria are airborne. Most spores are produced by lesions on the mature leaves on the tree or recently fallen infected leaves on the grove floor. Many management practices are helpful in reducing the severity of Alternaria brown spot. 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 but should be inspected carefully to ensure that there are no unexpected trees with lesions. Even though spores are airborne, plantings of healthy trees will remain disease-free 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. When establishing new plantings, it is best to locate susceptible varieties in high areas where air drainage and ventilation are good and leaves dry more rapidly. Less vigorous rootstocks such as Cleopatra mandarin should be selected rather than vigorous stocks such as Carrizo citrange. Groves of Minneola tangelos in low, wet areas have conditions so favorable for the disease that Alternaria may be virtually uncontrollable. Susceptible trees should be spaced more widely than oranges to promote rapid canopy drying. 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 regularly rather than hedging severely but less frequently.