First Report of Insensitivity to Cyazofamid among Isolates of Phytophthora capsici from the Southeastern United States

Authors: Kousik, Chandrasekar - Shaker; KEINATH, A - CLEMSON UNIVERSITY

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2008
Publication Date: 6/1/2008
Citation: Kousik, C.S., Keinath, A.P. 2008. First Report of Insensitivity to Cyazofamid among Isolates of Phytophthora capsici from the Southeastern United States. Plant Disease. 92:979.

Interpretive Summary: Vegetable crops in the United States are constantly under attack by many different disease causing agents. Phytophthora capsici is one such microorganism that causes fruit rots of cucurbits (plants in the cucumber and watermelon family) and is rapidly becoming an important problem in vegetable production in the southeastern United States. There is a continuous need to develop strategies to assist growers in combating these diseases. Growers have been using various fungicides (pesticides/chemicals that are used to control fungi) that can be effective in preventing manifestations of this disease in vegetable fields. Recently a new fungicide ‘cyazofamid’ was made available for management of P. capsici on cucurbits. The fungicide cyazofamid has been reported to be very effective in managing diseases caused by the microorganism Phytophthora capsici. However, recently we observed that this fungicide was not highly effective in our field trials, which led us to think that strains of the pathogen resistant (insensitive) to the fungicide might exist. We collected strains of this pathogen from fields in North Carolina, South Carolina, Georgia and Florida. We tested these strains in the laboratory on artificial growth media to which the fungicide was added. We also tested the ability of four of these strains to cause disease on cyazofamid treated and unsprayed cucumber and watermelon fruits. Based on these studies we identified few strains that were resistant to the fungicide cyazofamid. This finding suggests that growers should alternate application of the new fungicide cyazofamid with fungicides from other chemical classes to prevent extensive development and selection of resistant strains of the pathogen. The results from this study should be useful to growers and extension agents.

Technical Abstract: Phytophthora capsici is rapidly becoming an important limiting factor in vegetable production in the southeastern United States, particularly on cucurbits as fruit rots. One of the primary strategies used to manage diseases caused by P. capsici is the regular application of fungicides. Recently the new fungicide cyazofamid (trade name Ranman, FRAC Group 21, FMC Corporation, EPA Reg. No. 71512-3-279) was registered for management of P. capsici on cucurbits. Cyazofamid has been reported to be very effective against P. capsici on peppers. However, in a recent evaluation we observed that cyazofamid was not very effective on fruit rot of watermelon in a field artificially infested with P. capsici. Hence we decided to evaluate our collection of P. capsici isolates for sensitivity to cyazofamid. We confirmed all of our isolates as P. capsici based on morphology of colonies and sporangia and amplification of internal transcribes spacer (ITS) regions using specific PCR primers. Mycelial growth of 28 isolates from southeastern US including North Carolina (NC), South Carolina (SC), Georgia (GA), & Florida (FL) was evaluated on Ranman amended (0, 25, 100, 310, 518 & 1000 mg/L of the active ingredient cyazofamid) V8 juice agar. The EC50 values for mycelial growth ranged from 3.8 to 535 PPM. Thirteen isolates (8 GA, 3 SC, 1 NC, 1FL) had EC50 >100 mg/L. Similar results were obtained when technical grade cyazofamid was used. The same 28 isolates were evaluated on V8 juice agar amended with technical grade cyazofamid (0, 1, 10, & 100 PPM), and 100 mg/L salicylhydroymic acid (SHAM) which was added to inhibit the alternative oxidase enzyme and the EC50 ranged from <1 to >100 mg/L. Eight isolates (6 GA, 2 NC) had EC50 >100 mg/L. Three isolates, one sensitive and two insensitive to cyazofamid were used to inoculate cucumber (Cucumis sativus) fruits treated with Ranman at 0, 10, 100, 300 and 1000 mg/L of cyazofamid. Mycelial plugs (7-mm diameter) were placed on non-wounded fruits. The fruits were kept under high humidity at 25±1ºC in an incubator for 3 days. Two measurements of each lesion at right angles were averaged to get the lesion diameter. The EC50 for lesion diameter on fruits varied from 13 mg/L for the sensitive isolate to >233 mg/L for the insensitive isolates. EC50 for diameter of the lesion with sporulation ranged from 3 to 107 mg/L. Relative lesion diameters of the insensitive isolates at 100 mg/L treatment compared to non-sprayed check were 70-93% and at 300 mg/L it was 38-80%. Similarly in another experiment watermelon (Citrullus lanatus var. lanatus) fruits were sprayed with Ranman (0.716 µl/ml of water) plus Silwett L-77 (0.52 µl/ml) till runoff, and inoculated with four isolates. The insensitive isolates caused large lesions while the sensitive isolate was severely inhibited. The relative lesion diameter for the insensitive isolates on Ranman treated watermelon fruits were 76-100% of the non-sprayed fruits. All the experiments were conducted twice. To our knowledge these insensitive isolates were collected from fields that were never sprayed with Ranman. Because of the existence of cyazofamid insensitive P. capsici isolates, the fungicide should be rotated with fungicides from other chemical classes to prevent extensive selection of insensitive isolates.