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Factors Affecting Pycnidium Production of Diaporthe citri on Detached Citrus Twigs

Melanose, caused by Diaporthe citri, produces black-to-reddish brown lesions on twigs, leaves, and fruit of citrus and reduces the external quality of fruit destined for the fresh market. Inoculum for infection is produced primarily in pycnidia formed on dead twigs, and conidia are dispersed by rainwater. In laboratory studies, the effect of moisture, temperature, twig size, and melanose severity on pycnidium production on detached twigs was investigated. Pycnidium production was greatest when twigs were soaked for 3 to 4 h on alternate days three times per week and the temperature was 28°C. Production was greatest on twigs 3 to 5 mm in diameter and less on thinner or thicker twigs. Pycnidium production was related linearly to melanose severity on the twigs, and almost no pycnidia were produced on asymptomatic twigs. In the field, pycnidium production was greatest on detached, melanose-affected twigs placed in the canopy monthly during January to April than it was on twigs placed in the canopy during other months. The largest number of pycnidia was produced from May to August when fruit is most susceptible. The number of pycnidia produced was related significantly to degree-days above 20°C and weakly related to cumulative rainfall. Knowledge of inoculum production peaks may assist in timing of pruning and fungicide sprays.