ARS | Publication request: Effect of actigard and other new fungicides on phytophthora fruit rot of watermelon

Submitted to: International Phytophthora Capsici Conference
Publication Type: Proceedings
Publication Acceptance Date: November 8, 2011
Publication Date: N/A

Technical Abstract: Fruit rot, caused by Phytophthora capsici is an emerging disease in most watermelon producing regions of Southeast U.S. Between 2003 and 2008, we observed many watermelon farms in Georgia, South Carolina, and North Carolina, where growers did not harvest the crop due to severe fruit rot. The National Watermelon Association (NWA) has considered Phytophthora fruit rot as a top research priority because of watermelon yield losses in these states. Our program is researching two approaches to manage Phytophthora fruit rot: a) identifying host plant resistance and, b) identifying effective fungicides. We have identified several sources of resistance to Phytophthora fruit rot in watermelon plant introduction (PI). However, it may take many years to move the resistance genes from wild type PI into cultivated watermelon. In order to find quick solutions to help growers manage this disease, we conducted field evaluations to identify fungicides which are effective for managing Phytophthora fruit rot. Based on three years of field testing (2006-2008) under severe disease pressure, we identified two relatively new fungicides Revus and Presidio that are effective in managing Phytophthora fruit rot and are labeled for use on watermelon. The objectives of the present study conducted in 2010 and 2011 were to determine if Actigard, a systemic resistance activator (SAR), could activate resistance in watermelon fruit against P. capsici, and to determine if rotating Actigard with Revus or Presidio would be effective in managing pre- and post-harvest development of Phytophthora fruit rot. Other new and established fungicides were also evaluated in these trials. The field trials were considered effective as significant fruit rot was observed on non-treated controls in both years (mean fruit rot incidence = 76%). The weekly treatments of Forum, Zampro, Prophyt + Kocide 2000, and the rotation of Revus with Presidio, all significantly (P=0.05) reduced fruit rot compared to the non-treated controls in both years. Application of Actigard every week reduced fruit rot (P=0.05) in 2010 but was not as effective in 2011. Rotation of Actigard with Revus or Revus rotated with Prophyt + Kocide 2000 reduced fruit rot (P=0.05) in both years. Ridomil Gold, the standard treatment was not significantly different from the non-treated check in 2010. In 2011, V-10208 was one of the best treatments and significantly reduced fruit rot. The weekly application of 1.0 oz/A actigard did not affect the number of fruits per plot compared to non-treated control and other fungicide treatments. Post-harvest development of fruit rot was evaluated by inoculation of symptomless fruit that were harvested four days after the last spray application. Each fruit was inoculated with a 7-mm agar plug of a mefenoxam insensitive strain of P. capsici. Inoculated fruit were maintained in a humid chamber (>95% RH) that favored development of fruit rot. Post-harvest development of Phytophthora fruit rot (disease lesion and sporulation) was significantly less (P=0.05) on fruit treated with Zampro, Forum, V-10208, rotation of Presidio with Revus and rotation of Actigard with Revus or Presidio compared to non-treated controls. Application of Actigard (1.0 oz/A) alone was effective in reducing post-harvest fruit rot in 2011 but not in 2010. Resistance in the pathogen population to fungicides such as Ridomil Gold and Ranman has been identified previously. Thus, there is always a possibility of P. capsici developing resistance to the above mentioned new fungicides. Therefore, these fungicides should be rotated or tank mixed with other available fungicides to prolong their usefulness in managing fruit rot. Fungicides should be part of an overall disease management strategy that also includes the use of well drained fields and proper irrigation practices.