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Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/23/2012 Publication Date: 10/1/2012 Citation: Copes, W.E. 2012. Rhizoctonia web blight development on azalea in relation to leaf wetness duration in the glasshouse. Journal of Phytopathology. 161:723-729. Interpretive Summary: In previous field studies, temperature but not relative humidity or leaf wetness durations was useful for predicting the buildup of Rhizoctonia web blight symptoms on azalea. Since periods of high moisture are needed for disease buildup, the effect of 0, 4, 8, 12, 16, and 20-hour cycles of 20 sec. mist at 30-min intervals on blight development was studied in individual open-topped, clear plastic chambers inside a greenhouse. The pathogen was placed on stems and blighted leaves were counted weekly for 6 weeks. Weather instruments recorded data every 30 min. Both the daily number of hours of leaf wetness and daily average relative humidity were significant in a regression equation that predicts web blight development. This information will be useful to research and extension scientists, and is being developed into a protocol that can be used by commercial ornamental plant producers to better time fungicide applications against web blight. Technical Abstract: In field trials done in nurseries, temperature was identified as the primary variable for predicting web blight development, caused by binucleate Rhizoctonia spp., on container-grown azaleas (Rhododendron spp.). Moisture, in the form of very low vapor pressure deficits, provided only a minor predictive contribution. To better define the influence of moisture on web blight development, plants were maintained in a greenhouse in open-topped, clear plastic chambers with 0, 4, 8, 12, 16, and 20-h cycles of 20 sec. mist at 30-min intervals. Binucleate Rhizoctonia AG-U infested barley was secured in netting at the base of terminal leaf clusters, and a proportional leaf blight incidence per stem recorded as a repeated measure over 6 weeks. Temperature, relative humidity (RH), and leaf wetness (LW) were recorded at 30 minute intervals in each chamber. A similar day (29°C) and night (22°C) temperature range occurred in all experiments. Both LW duration and assessments over time were always significant, with a significant interaction in 11 of 13 experiments using a mixed model analysis of variance procedure. Web blight incidence was significantly higher for longer LW durations. Disease incidence was highest mostly with 20 h LW, which sometimes was not different from 16 and 12 h of LW, but different than 0, 4 and 8 h LW. 8, 4 and 0 h LW often were not significantly different from each other. When data from all experiments were combined, maximum disease at 6 weeks (y) linearly increased in response to daily hours LW (x1) and average RH (x2) in the equation y = -0.36252 + 0.03040 x1 + 0.00526 x2 (adjusted R2 = 0.5470, p = <0.0001). LW duration was the main variable for explaining blight incidence. Blight incidence tended to be higher at RH = 75%. At LW = 6 h, RH was more influential on blight incidence than LW. Since the information was developed in a greenhouse environment, the equation still needs to be validated with disease development data collected in nurseries. |
