From Clemson Plant Pathologist Tony Keinath and Extension Agent Andy Rollins.
After 3.28 inches of rain in the Upstate from May 23 to 26, symptoms of Phytophthora blight showed up in one or more fields of pepper and tomato after June 1. The heavy downpour and the volume of rain saturated the soil, providing the ideal environment for oospores of the water mold Phytophthora capsici to germinate and release zoospores, the swimming spores that infect roots.
Growers are encouraged to avoid planting low spots in infested fields because in the long run, this saves money by 1) eliminating wasted input costs—the cost of plastic, drip, fertilizer, transplants, and labor spent on a part of the field that might not yield anything—plus 2) lowering the risk of Phytophthora spreading to the rest of the field.
After transplanting, fungicides are the main method for managing Phytophthora blight. Copper is the only certified organic fungicide, and it is only partially effective. See Land-Grant Press 1014 for other management options for future crops.
Conventional growers should choose among the fungicides in Table 1. The three in bold type are the most effective, based on trial results and observations from the southeastern U. S. Based on NCSU recommendations, the first application should be a drench right after transplanting—to concentrate the fungicide in the seedling root zone—followed by drip applications as the roots spread. If the disease progresses from root and crown symptoms to foliar blight or fruit rot, either switch to foliar applications or alternate foliar and drip applications. Once disease is found in a field, apply fungicides on a weekly schedule for the rest of the season. Make one application per week, not more.
There are several important label restrictions that must be followed:
- Do not use Orondis Gold and Orondis Ultra on the same crop. Choose one or the other product.
- Do not apply Orondis Ultra, Revus, or Zampro back-to-back because they have the same FRAC Code.
Resistance to Ridomil, the “Gold” component in Orondis Gold, is common in Phytophthora capsici. Do not use Orondis Gold if you know you have a Ridomil-resistant population. These applications will increase the risk of resistance to Orondis since it is essentially being applied as a single active ingredient, a very risky use. Growers who have seen resistance to Ridomil, or want to be cautious, should choose Zampro or Elumin for drip applications instead.