Leaf Spot of Citrus
Circular, mostly isolated spots appear on leaves and can reach up to 10 mm in diameter. They show a light-brown or grayish center, with reddish margin and a prominent yellow halo during dry weather. After the onset of rains, they become black and covered with spores, something that is particularly visible on the lower part of leaves. As the disease progresses, the spots coalesce and cause a generalized chlorosis of the leaf, sometimes followed by defoliation. Occasionally, the center of the spot falls out and a shot-hole effect occurs. On green fruits, spots can be circular to irregular, discrete or coalescent, often accompanied by a yellow halo. Severe infections result in black, raised, tumor-like growths, followed later by central necrosis and collapse. Lesions on mature fruit are variable in size but normally flat. They sometimes have a slightly sunken brown center. Occasionally lesions occur on stems, extending from the petiole. Several such lesions may result in dieback.
The symptoms are caused by the fungus Pseudocercospora angolensis. It most likely survives in dormant lesions on infected plant material until conditions are favorable for the production of spores. Its life cycle is favored by prolonged wet weather conditions followed by dry spells, and coupled with moderately cool temperatures of 22-26°C. Leaves are the primary source of infectious spreading, as their lesions produce more spores than similar lesions on fruit. Long-distance dispersal of the fungus is by windborne spores whereas local dispersal occurs primarily by rain splashes or raindrops. Human can also spread the disease by transporting infected material to other fields or locations.
Natural oils, extracted from fruits of rather resistant species of Citrus latifolia and Citrus limon, may reduce the growth of the pathogen. Lime leaf extract and oils of Citrus aurantifolia as well as bottlebrush plants (Callistemon citrinus and Callistemon rigidus) can inhibit the pathogen. These effects were only examined in controlled condition in laboratories so far. Copper-based fungicides can also be used.
Always consider an integrated approach with preventive measures together with biological treatments if available. Fungicides based on trifloxystrobin, mancozeb and benomyl, each combined with mineral spray oil showed good results in eracating the fungus. Fungicides based on chlorothalonil, copper and their mixtures are also effective. Spraying after rainfall is recommended, as rain stimulates spore production.