Heavy rains early in the planting season favor both Pythium root rot and Phytophthora blight. While Pythium root rot is caused by several different species of Pythium with different temperature optima – cool to hot, Phytophthora blight is only favored by hot weather. Periods of hot, rainy weather following a cool wet spring can be a predictor of future problems with these diseases.
Phytophthora and Pythium are soilborne oomycete pathogens, also called water molds, that thrive in rainy weather. They produce motile spores (zoospores) that are attracted to plants, then form a structure that allows them to infect, and aggressively attack any type of plant tissue. Zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight and Pythium root rot are often seen first in low spots or other poorly drained areas of production fields, but also occur on well-drained, even sandy soils if the environmental conditions are right. While Pythium root rot is not as explosive as Phytophthora blight, both must be managed preventatively. Pepper varieties partially resistant to Phytophthora blight are available and should be used in fields with a history of this disease. There are no varieties with identified resistance to Pythium root rot. Cultural practices including crop rotation, good drainage, raised beds, avoiding surface water for irrigation, and sanitation should be used – see details here.
During the growing season, fungicide application is the main option for management of Phytophthora blight. Andy Wyenandt (Rutgers University) published a really nice piece on Phytophthora and Pythium control in peppers in April (https://plant-pest-advisory.rutgers.edu/phytophthora-control-during-wet-weather-3/). Fungicides must be applied preventatively for maximum benefit. Keep in mind that:
- Orondis Gold premix contains oxathiapirolin, which is very effective against Phytophthora blight (but not Pythium) and mefenoxam, which is effective against both Phytophthora and Pythium. However, if mefenoxam (Ridomil Gold) or metalaxyl products have been used for a number of years in the same field, the Phytophthora population may be resistant. We have found mefenoxam/metalaxyl-resistant Phytophthora capsici in Ohio in recent years. Orondis Gold can be applied through drip and in transplant water.
- Ridomil Gold can be applied to peppers as a soil spray or via drip, but not in transplant water. Under some conditions peppers can be severely damaged and unlikely to recover.
- The active ingredient in Orondis, oxathiapiprolin, does not move well through the soil profile. Our research has not shown a benefit of using Orondis as a soil application vs. foliar sprays. I recommend “saving” Orondis Ultra for foliar application when the weather is continuously conducive for Phytophthora blight.
- Elumin is a newer product for Phytophthora blight and application through drip or soil spray at transplanting is labeled, as well as foliar sprays during the season. Pythium root rot is not on the label for peppers but is labeled for Pythium in potatoes and related crops.
- Like Elumin, Ranman and Presidio are labeled for Phytophthora blight management in pepper, and not Pythium root rot; however, they are labeled for Pythium management in other crops.
- For Previcur Flex, Pythium root rot is on the label for peppers, but Phytophthora blight is not.
- The phosphites like ProPhyt and others are labeled for both Phytophthora and Pythium and are systemic. The ProPhyt label allows drench application at transplanting although not in the transplant water per se. However, it can be drenched onto seedlings prior to transplanting. The phosphites are good supplemental products but will not control Phytophthora blight alone. They should be used in tank mixes or rotated with products listed below.
Growers have a lot of choices, but if wet conditions continue and both Pythium root rot and Phytophthora blight are a concern:
- If Ridomil or related products have been used routinely on the farm or Phytophthora is known to be resistant to mefanoxam/metalaxyl, peppers should be treated with a soil application at or near transplanting with Ranman, Elumin or Presidio, followed by foliar applications in a rotation that includes Orondis Ultra, Presidio, Elumin or Ranman. These may be tank-mixed with a phosphite product.
- Keep in mind that a number of products such as Orondis Gold, Orondis Ultra and Elumin have strict use limitations – e.g. two applications per season. Check the label.
- Always rotate fungicides with different modes of action (FRAC codes):
Ridomil Gold: 4
Orondis Gold: U15+4
Orondis Ultra: U15+40
Previcur Flex: 28
Phosphite products: 33