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Title: Sensitivity of Phytophthora capsici isolates from the southeast US to Fluopicolide and Cyazofamid.

Author
item Kousik, Chandrasekar - Shaker
item KEINATH, A - CLEMSON UNIVERSITY

Submitted to: International Phytophthora Capsici Conference
Publication Type: Proceedings
Publication Acceptance Date: 10/22/2007
Publication Date: 11/27/2007
Citation: Kousik, C.S., Keinath, A.P. 2007. Sensitivity of Phytophthora capsici isolates from the southeast US to Fluopicolide and Cyazofamid. International Phytophthora Capsici Conference, Islamorada, FL, 2007. p. 20.

Interpretive Summary:

Technical Abstract: The plant pathogen Phytophthora capsici is rapidly becoming an important limiting factor in vegetable production in the southeastern United States particularly on cucurbits and peppers. The diseases caused by P. capsici are known by various names such as Phytophthora blight and crown rot on peppers, fruit rot on cucurbits, and buckeye fruit rot on tomatoes. One of the primary strategies used to manage diseases caused by P. capsici is the regular application of fungicides. However, development of resistance to such fungicides is not new. For example there are many reports describing resistance to the fungicide mefenoxam in southeastern US and other parts of the world. Thus there is a constant need to develop new and effective fungicides or other means of managing diseases caused by P. capsici. Several new fungicides are currently being tested and registered to manage P. capsici on vegetables. One such fungicide is fluopicolide (trade name ‘Presidio’, Valent Corp.). We evaluated over 100 P. capsici isolates collected from various cucurbits and pepper in the southeast US on V8 juice agar plates amended with technical grade fluopicolide dissolved in DMSO. At 300 ppm which is about the recommended rate for field applications, none of the isolates grew. We selected 28 isolates to determine EC50 values for mycelial growth. Five isolates were from North Carolina, 8 from South Carolina, 9 from Georgia, and 6 from Florida. The EC50 values for mycelial growth on amended media ranged from concentrations of 0.098 ppm to 0.239 ppm of fluopicolide. Most of the isolates tested did not grow at a concentration of 3 ppm fluopicolide. On the same amended plates we also assayed for levels of sporulation and the EC50 values for sporulation ranged from <0.03 ppm to 0.062 ppm. Similar to the above experiments we tested >100 P. capsici isolates on V8 juice agar plates amended with technical grade cyazofamid (trade name ‘Ranman,’ ISK Biosciences) dissolved in acetone. At 300 ppm which is approximately the recommended rate for field applications, 22% of the isolates had a relative colony diameter (RCD) of less than 33%, 61% of the isolates had a RCD between 34-90% and 22% of the isolates had RCD >90% of the check plates, which were amended with only acetone. We also tested some select isolates using V8 juice agar amended with the commercial cyazofamid product, and the EC50 values for mycelial growth ranged from 10 ppm to >300 ppm. These isolates were obtained from fields that had never been sprayed with cyazofamid. In addition to the above mentioned fungicides we will also present some data on other newly registered fungicides which we are currently testing.