Another teaser for the 2013 Muscadine Field Day — see the abstract below provided by Dr. Barbara Smith, USDA-ARS Plant Pathologist.
Fungicides can significantly reduce losses due to disease in the yield and quality of muscadine grapes. In four studies fungicides were applied individually or as part of a full season schedule from early bloom until harvest to three muscadine grape cultivars. The objective was to compare the effect of a full season treatment of 9-12 fungicide applications applied on a 10-day interval to fewer applications of individual fungicides on disease incidence, yield, and berry quality. Foliar and berry diseases were rated on visual scales. Sugars, acids, ellagic acid, and resveratrol content were determined by HPLC.
Four studies explored the relationship between disease control, berry quality, and phytochemical content following full season or early season application fungicides. In each study foliar and fruit diseases were lower in the full season treatment compared to the control, each fungicide was effective in reducing at least one disease, and some treatments with fewer applications reduced fruit diseases to the same level as the full season treatment. In Study 1, the full season treatment of 9 applications applied at 10-days intervals and the azoxystrobin treatment of 3 applications applied at 30-day intervals resulted in significantly higher yields, lower fruit disease scores, and more asymptomatic berries than the control treatment. In Study 2, four applications of the azoxystrobin, myclobutanil, and the combination fungicide, cyprodinil plus fludioxonil applied at 30-day intervals were as effective in reducing total berry diseases as the full season schedule of 12 applications (three fungicides alternated at 10-day intervals). In Study 3, three fungicides were applied on an alternating schedule every 10 days beginning at bloom and stopping at various pre-harvest intervals. There were no significant differences in vine vigor, foliar diseases scores, percentage of asymptomatic berries, or bitter rot scores due pre-harvest interval. In Study 4, there was not a significant difference in the percentage of asymptomatic berries between the full season treatment of eight applications and early season treatment of four applications or in the bitter rot and total disease scores for five of the fungicide treatments. Data indicate that fungicide applications can be stopped as early as six weeks before harvest without significant effects on berry diseases.
Studies 1 and 3 also investigated the effect of fungicide treatments on berry quality and phytochemcial content. In Study 1, significant differences were found in pH, TA, fructose, glucose, tartaric acid, and resveratrol levels. Total resveratrol was lower in the skins of berries from the full season and azoxystrobin treatments than from the control and other fungicide treatments. Berries from the least efficacious treatments for berry diseases had almost ten times as much resveratrol as those from the full season and azoxystrobin treatments. In Study 3, ellagic acid content was lower in berries from the spray treatment than from the not sprayed and control treatments. Total resveratrol content was approximately four times higher in skins of berries that did not receive fungicide sprays than in those that did. Resveratrol levels in the skins of berries from fungicide treatments were very low even if the last fungicide application was 8 weeks before harvest.
Data from these four studies indicate that the number of fungicide applications required for control of muscadine grape diseases can be reduced without an increase in berry rot disease severity. The most effective fungicides reduced berry diseases with as few as four applications compared to 12 applications in the full season schedules. Fungicides that controlled berry disease had an effect on berry quality including lowering the content of the beneficial phytoalexin, resveratrol.