Epidemiological studies on Alternaria brown spot
Alternaria brown spot affects Minneola tangelos and some other citrus and causes abscission of immature fruit and blemishes on more mature fruit. The causal agent was originally designated as Alternaria citri, and later Alternaria alternata pv. citri, but the proper classification is still debated. For more details refer to EPPO RS 98/179 and 2000/063. It is noted that at least 2 genetically distinct strains exist: one effecting tangerines, and another affecting rough lemon (Citrus jambhiri) and Rangpur lime (C. limonia). Epidemiological studies were carried out to determine the effect of environmental factors (relative humidity, R/IR irradiation, rainfall, vibration) on the production, release and field populations of the fungus. Results showed that sporulation is greater on mature and moistened leaves, maintained near 100 % HR. It is supposed that a light rain or heavy dew is sufficient to induce sporulation (if leaves are sufficiently mature). Primary factors stimulating release of conidia were abrupt changes in RH or rainfall events. Vibration and R/IR irradiation did not induce conidial release. Field studies in Florida (US), indicated that conidia are present throughout the year with periodic large peaks, but this could not be related to disease severity. Further studies will be made to determine optimum temperatures and duration of leaf wetness.
Sources
Timmer, L.W.; Solel, Z.; Gottwald, T.R.; Ibañez, A.M.; Zitko, S.E. (1998) Environmental factors affecting production, release, and field populations of conidia of Alternaria alternata, the cause of brown spot of citrus.
Phytopathology, 88(11), 1218-1223.