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Submitted to: Pharmaceutical Crops
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/27/2013 Publication Date: 12/10/2013 Citation: Smith, B.J. 2013. Limited fungicide applications affect foliar and fruit disease severity and phytochemical content of muscadine grape (Vitis rotundifolia Michx.). Pharmaceutical Crops. 4:21-37. Interpretive Summary: Berry rot diseases cause significant reductions in yield and quality of muscadine grapes grown in the southeastern U.S., but these losses may be reduced significantly by fungicide applications. Current recommendations for control of berry diseases call for as many as 12 fungicide applications; however, fungicides are expensive to purchase and costly to apply. Four field studies were conducted to explore 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 but some treatments with fewer applications reduced fruit diseases to the same level as the full season treatment. Significant differences were found among fungicide treatments in pH, titratable acidity, sugar, acid, resveratrol and ellagic acid levels of berries. Berries from the least efficacious treatments for berry diseases had almost ten times as much resveratrol as those from the most effective treatments. Ellagic acid content was lower in berries from fungicide treatments than from the control treatment. Data from these four studies indicate that the number of fungicide applications required for control of muscadine grape diseases can be reduced without a significant 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 and had an effect on berry quality including lowering the content of the beneficial phytochemicals, resveratrol and ellagic acid. This information will be used by research scientists investigating the nutraceutical properties of muscadine grapes and other crops and by extension agents and industry personnel making disease control decisions. Technical Abstract: Berry rot diseases cause significant reductions in yield and quality of muscadine grapes, but these losses may be reduced significantly by fungicide applications. Four studies were conducted to explore the relationship between yield, disease control, berry quality, and phytochemical content following full season or early season application of fungicides. In each study foliar and fruit diseases were lower in the full season treatment compared to the control, 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 and lower total berry disease, bitter rot, and Macrophoma rot scores 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 treatment of 12 applications of 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 pre-harvest intervals of 56, 42, 28, 14, 7, 4, 2, 1, and 0 days. There were no significant differences in vine vigor, foliar diseases scores, or bitter rot scores among treatments indicating that fungicide applications could be stopped as much as six weeks before harvest without a significant increase of berry diseases thus saving the grower money, environment and resulting in higher levels of beneficial phytochemicals. In Study 4, there was not a significant difference between the full season treatment of eight applications and early season treatment of four applications in the bitter rot and total disease scores for six of the fungicide treatments. Studies 1 and 3 investigated the effect of the fungicide treatments on berry quality and phytochemcial content. In Study 1, there were no significant differences in berry size or soluble solid content due to fungicide treatment, but significant differences were found in pH, titratable acidity, fructose, glucose, tartaric acid, quinic acid, and resveratrol levels of berries. Berries receiving the full season, azoxystrobin, and control fungicide treatments had highest level of each of the sugars. Resveratrol content 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, the ellagic acid content was lower in berries from the spray treatment than from the control treatments. Resveratrol content was approximately four times higher in skins of berries that did not receive fungicide sprays than in those that did. The resveratrol levels in the skins of berries from fungicide treatments were very low even if the last fungicide application occurred 6 or 7 weeks before harvest. These data indicate that the number of fungicide applications can be reduced to as few as four without an increase in berry rot scores; however, when fruit diseases of muscadine grapes are controlled, levels of resveratrol, a phytochemical beneficial to humans, decrease. |
