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United States Department of Agriculture

Agricultural Research Service


item Mondal, S. N.
item Gottwald, Timothy
item Timmer, L. W.

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2003
Publication Date: 8/1/2003
Citation: Mondal, S. N., Gottwald, T. R., Timmer, L. W. 2003. Environmental Factors Affecting the Release and Dispersal of Ascospores of Mycosphaerella Citri. Phytopathology.

Interpretive Summary: Greasy spot is a severe fungal disease of citrus that affects all citrus species and cultivar in the US, Caribbean and South America. The fungus forms fruiting bodies in decaying citrus leaves that have fallen from infected trees. These fruiting bodies produce spores known as ascospores that infect healthy citrus leaves causing new infections. This study was a study of the meteorological conditions that cause ascospores to be released and infect. A computer controlled microenviromental chamber was constructed for this purpose that controls rain, temperature, humidity, light, and wind speed. These factors were varied individually and in combination to determine under what conditions the ascospores are released. Understanding these conditions will help to control this disease. For instance if we can reproduce the conditions that will cause spores to be released at a time of the year or under conditions when infection cannot occur, then we have forced the disease causing spores to be wasted and they won't result in any new disease.

Technical Abstract: Greasy spot, caused by Mycosphaerella citri Whiteside, produces a leaf spot disease affecting all citrus species in Florida and the Caribbean Basin. M. citri produces pseudothecia and ascospores in decomposing leaves on the grove floor that are considered the principal source of inoculum. In studies using a computer-controlled environmental chamber (CCEC), a single rain event triggered release of most mature ascospores beginning 30-60 min after the rain event. Additional rain events did not bring about further release. High RH without rain triggered release of low numbers of ascospores, but vibration and R/IR irradiation had little or no affect on ascospore release. After 3 to 4 cycles of wetting and drying of leaves, all pseudothecia had matured and released their ascospores. In the field, ascospores were detectable starting about 2 h after the beginning of a rain or irrigation and most ascospores were released within 16 h. Ascospore release was greatest following rain events and somewhat less following irrigations, and low numbers of ascospores were detectable on days without precipitation. Ascospore numbers declined linearly with vertical and horizontal distance from the source. Low numbers of ascospores were detected 7.5 m above the ground and 90 m downwind from the grove. Ascospore release can be advanced by irrigating frequently during dry, nonconducive conditions to stimulate ascospore release when environmental conditions are unfavorable for infection, but the eventual effects on disease severity are uncertain.

Last Modified: 06/22/2017
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