Submitted to: Journal of Florida State Horticulture Society
Publication Type: Proceedings
Publication Acceptance Date: 1/11/1996
Publication Date: N/A
Citation: Interpretive Summary: A fungal disease of citrus called greasy spot has become much more severe in recent years as the Florida citrus industry has been forced to move southward due to a series of hard freezes. This increase in severity has necessitated the use of additional fungicide sprays to control the disease and reduce damage caused by the fungus. A series of spore trapping experiments were conducted to determine the time of release of infectious spores by the causal fungus and to determine if alternative control measures would be appropriate to lessen disease impact and to reduce the number of fungicide sprays. Small citrus trap plants were also exposed on a bi-weekly time schedule to determine when maximum infection takes place. Some infection was found to take place nearly year round. The new irrigation practice, ie. microsprinklers, used by growers was also found to worsen the problem. As an outcome of this work, modification of irrigation practices will be tested in future studies to determine if disease severity can be lessened simply by modifying these irrigation practices.
Technical Abstract: Greasy spot, caused by Mycosphaerella citri, has been severe in southwest Florida frequently necessitating two fungicide applications for adequate control. Ascospore numbers were recorded throughout the 1992 and 1993 seasons in Lake Alfred and Immokalee to determine differences in abundance and time of release. Peak releases occurred in April and May at both locations. Ascospore production was greater in Immokalee than in Lake Alfred, especially in 1992, and more spores were liberated before and after the peak periods in Immokalee. Trap plants exposed for 2 wk periods in Lake Alfred had high levels of infection in 1992 with low to moderate infection in 1993 and 1994. Some infection occurred throughout the year. Conversion from overhead to under-canopy microsprinkler irrigation has apparently promoted major ascospore release prior to the summer rainy period which is most favorable for infection. Greasy spot severity is probably greater in south Florida because of more infection outside of the normal peak periods and warmer winter temperatures that speed disease development and defoliation.