Phytophthora blight has become a very serious problem in peppers and cucurbits, particularly in areas with concentrated vegetable production. The pathogen is a water mold that thrives under conditions of high moisture and high temperature. It produces 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 is often seen first in low spots or other poorly drained areas of production fields, but the disease also occurs on well-drained, even sandy soils if the environmental conditions are right. An increase in intensive rainfall events that result in soil saturation and standing water in fields in the last decade or so is certainly a contributing factor to the uptick in problems with Phytophthora blight.
Effective management of Phytophthora blight in peppers requires an integrated approach:
Crop rotation. Phytophthora produces structures called oospores that can survive for a number of years in the soil. Plan to rotate out of peppers, cucurbits or green beans for 4-5 years if Phytophthora blight has been a problem.
Resistant varieties. A few pepper varieties are resistant to the root rot phase of the disease. In general, these varieties are susceptible to the crown rot phase, which affects foliage and fruits. Varieties with moderate to good resistance to Phytophthora blight are: Paladin, Aristotle, Declaration, Intruder, Vanguard (bell); Hechicero (jalapeño); and Sequioa (ancho).
Well-drained soil. Avoiding standing water is critical to limiting the movement of Phytophthora from plant to plant.
Avoid surface water for irrigation. We have found Phytophthora in irrigation ditches and ponds as early as late June in vegetable production-intensive areas in Ohio. Using surface water for irrigation is risky, especially if Phytophthora is present in fields near surface water sources.
Plant on raised beds. Prepared properly, raised beds will help prevent standing water near pepper plants. If possible beds should be domed, and there should be no depressions in the soil surrounding the plants.
Sanitation. Phytophthora can be moved from an infested field to a clean one on soil clinging to boots, equipment, etc. Power washing to remove soil is a good first step, followed by rinsing with a sanitizer.
Fungicides. There are a number of fungicides labeled for use on peppers to manage Phytophthora blight (see table below). The newest product, Orondis, has very good efficacy against this disease. It is available in the Midwest this year as a co-pack with either Revus (Orondis Ultra), Ridomil (Orondis Gold) or Bravo (Orondis Opti). Pre-mixes will be available in 2018. There are many restrictions on the use of Orondis – including the number of applications (no more than 1/3 of total applications for Phytophthora blight) and when it can be applied (to the soil or to the foliage but not both). Orondis Ultra and Orondis Gold can be applied in transplant water or through the drip, although Orondis does not move much in soil and emitters need to be right next to the plant. If the pepper variety is susceptible to Phytophthora blight, it may be a good idea to apply Orondis Gold or Orondis Ultra at planting, and follow up later with a program containing at least two of the fungicides with activity against Phytophthora (see table). Research conducted at the University of Illinois has shown that adding a copper-based fungicide to these foliar applications can improve their efficacy. If the pepper variety is resistant to Phytophthora, any of the three Orondis products can be used in a foliar fungicide program that includes other effective fungicides. The Bravo component of Orondis Opti will not help with Phytophthora blight, but will control anthracnose. Orondis Gold is considerably more expensive than Orondis Ultra and Orondis Opti, and resistance in Phytophthora to the Ridomil component of Orondis Gold has been found in numerous locations.