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Title: Effectiveness of Fungicides in Managing Phytophthora Fruit Rot of Watermelon in South Carolina

item Kousik, Chandrasekar - Shaker
item Thies, Judy
item HASSELL, RICHARD - Clemson University

Submitted to: International Phytophthora Capsici Conference
Publication Type: Proceedings
Publication Acceptance Date: 10/21/2009
Publication Date: 12/1/2009
Citation: Kousik, C.S., Thies, J.A., Hassell, R. 2009. Effectiveness of fungicides in managing Phytophthora fruit rot of watermelon in South Carolina. International Phytophthora Capsici Conference. pp 19-20.

Interpretive Summary:

Technical Abstract: Watermelon crops grown in most commercial production areas in the Southeast US are vulnerable to Phytophthora fruit rot caused by Phytophthora capsici. Phytophthora fruit rot is considered as an important problem and a top research priority by the National Watermelon Association (NWA). Managing diseases caused by P. capsici is known to be difficult especially when weather conditions are favorable for disease development. Managing Phytophthora fruit rot poses a significant challenge because of the long duration of contact between the soil and the fruit, and the difficulty in getting fungicides through the plant canopy to the belly of the fruit. We have been evaluating several fungicides to manage Phytophthora fruit rot in South Carolina for the past few years. These trials have helped us identify three relatively new fungicides with varying levels of effectiveness that have been labeled for use on cucurbits. Over three years (2006-2008) and under severe fruit rot pressure in our fields in Charleston, SC, the fungicide Revus (a.i. mandipropamid, Syngenta) provided 25-67% reduction of fruit rot (mean=52%) compared to untreated check. Presidio (a.i. fluopicolide, Valent) reduced fruit rot by 30-65% (mean=48%) and Ranman (a.i. cyazofamid, FMC) reduced fruit rot by 28-47% (mean=36%). Ridomil Gold, the current standard reduced fruit rot by 7-28% in these tests (mean=20%). Another new fungicide, SA-110201 (a.i unknown, Sipcam), was also effective in managing Phytophthora fruit rot (29-59% reduction), but is not yet labeled for use on cucurbits. In these trials (2006-2008) the actual fruit rot incidence in the untreated checks ranged from 60-79% with a mean of 68%. We conducted similar trials in 2009 where the plants were sprayed in the field. However, in this trial fruit rot did not occur in the field. Therefore the fruits were harvested 5 days after the last spray treatment and inoculated with P. capsici in a humid chamber. Revus, and Presidio were as effective in 2009 as in the previous three years. Forum (a.i. dimethomorph, BASF) also significantly reduced disease development compared to the untreated check in 2009 trials. Actigard at 0.5 oz/A did numerically lower disease levels compared to the untreated check however, it was not significantly different. Resistance in P. capsici populations to Ridomil Gold is well known and recently resistance to Ranman was identified. Because there is always a possibility of P. capsici developing resistance to any fungicide, they should be rotated or tank mixed along with some of the available older fungicides to prolong their usefulness. However, Forum should not be rotated or tank mixed with Revus as they have similar mode of action. Fortunately, Revus, Presidio & Ranman have a different mode of action and therefore can also be rotated among themselves.