Effect of Urea, CaCO₃, and Dolomite on Pseudothecial Development and Ascospore Production of Mycosphaerella citri

Citrus greasy spot, caused by Mycosphaerella citri, produces lesions on leaves, followed by premature defoliation, and rind blotch on fruit. Ascospores produced in leaf litter represent the major source of inoculum. The effect of treatment of leaf litter with urea, CaCO₃, or dolomite on the development of pseudothecia and ascospore production was evaluated. In laboratory experiments, one urea application reduced production of pseudothecia and ascospores by up to 90%, but did not affect time of production of pseudothecia or ascospores or rate of leaf decomposition. Two applications of urea delayed leaf decomposition. As the rates of CaCO₃ or dolomite were increased, pseudothecial incidence, density, time to ascospore production, and total numbers of ascospores decreased and the rate of leaf compostion increased. Immature pseudothecia on leaves treated with urea or CaCO₃ degenerated and produced fewer ascospores per pseudothecium. The results observed in microplot studies in the field were similar to those observed in laboratory experiments. The number of days to pseudothecia and ascospore production and the pseudothecial incidence and density were negatively related to the rate of CaCO₃ or dolomite applied. Application of CaCO₃ dolomite, or urea to leaf litter can reduce inoculum and be useful in an integrated program of greasy spot management.