Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/1/2006
Publication Date: 8/1/2006
Citation: Kaminski, J.E., Dernoeden, P.H., Oneill, N.R. 2006. Environmental influences on the release of Ophiosphaerella agrostis ascospores under controlled and field conditions. Phytopathology. 95:1356-1362. Interpretive Summary: A recently discovered disease affecting different species of turfgrasses is caused by a fungus that kills the grasses, causing dead and spotty areas. The disease is prevalent in the mid-Atlantic region of the United States during late June and mid-August The fungus produces spores which perpetuate the disease cycle throughout the growing season. We conducted studies to determine the environmental conditions for production and timing of release of spores. The influence of environmental factors such as humidity, temperature, light cycle, and precipitation, was determined under controlled conditions in environmental growth chambers as well as in the field. Spores were discharged from fruiting bodies on the surface of foliage and stem tissues whenever there was a sharp decrease in relative humidity, and discharge was not influenced by dark or day light cycle. In the field, spores were released primarily during early morning hours when relative humidity was increasing, or in the evening when relative humidity was low and dew began to form. Few spores were released when the turfgrass was dry. Spore release was also triggered within 10 hours after the beginning of a rain shower. New infections appeared in a cyclic pattern about every 12 days, and 3 to 10 days after a large spore release. Elucidation of these conditions provides scientists with a better understanding of the epidemiology of dead spot disease and the biology of the fungus. Knowledge about spore production and release will be useful to the turfgrass industry so that appropriate control measures can be implemented.
Technical Abstract: Ophiosphaerella agrostis, the causal agent of dead spot of creeping bentgrass (Agrostis stolonifera), can produce prodigious numbers of pseudothecia and ascospores throughout the summer. The environmental conditions and seasonal timings associated with O. agrostis ascospore release are unknown. The objectives of this research were to: 1) determine the influence of changes in light and relative humidity on ascospore release in a controlled environment; 2) document the seasonal and daily discharge patterns of ascospores in the field; and 3) elucidate environmental conditions that promote ascospore release under field conditions. In a growth chamber study, a sharp decrease (100% to ~50%) in relative humidity resulted in a rapid (1 to 3 h) discharge of ascospores, regardless of whether pseudothecia were incubated in constant light or dark. In the field ascospore release increased between 1900 and 2300 hours and again between 0700 and 1000 hours. The release of ascospores primarily occurred during the early morning hours when relative humidity was decreasing and the canopy began to dry or during evening hours when relative humidity was low and dew began to form. Few ascospores were released between 1100 and 1800 hours when the bentgrass canopy was dry. The release of ascospores also was triggered by precipitation. Eighty seven percent of the ascospores collected during precipitation events occurred within 10 h of the beginning of each event. Ascospore discharge and the appearance of new infection centers occurred in a cyclic pattern that peaked about every 12 days. New infection centers appeared 3 to 10 days after a large release of ascospores.