Managing Phytophthora Blight in Peppers

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.

What Are the Most Worrisome Diseases in Your Vegetable Crops?

Many specialty crop growers are aware of the USDA IR-4 Minor Use Program, which works to promote registration of products for  pest, disease and weed management in “minor use” crops – including vegetables, fruits, herbs and spices, among others.  The US IR-4 Program will participate in a Global Minor Use Summit in Canada later this year, and is asking for help in identifying the most important priorities among growers worldwide.  This is a good opportunity to voice your opinion about the diseases, pests and weeds that are your greatest concern and for which labeled products are needed to manage the disease.  In some cases this is a matter of expanding labels that include other crops, while in others it may involve research to find a solution to a problem.

For example, bacterial diseases such as bacterial spot and bacterial canker in tomatoes, are very difficult to manage in the field, and therefore can cause significant losses.  There are no highly effective products available, so research is needed.  

Please take the time in the next ten days to let us know what you think are the most important crop/disease, pest or weed priorities on your farm or community.  Which are the ones that keep you awake at night? You can send me an email at or reply in the Comment section below.

Basil Growers – Be on the Watch for Downy Mildew

Basil plants in a nursery in Ohio were confirmed to have downy mildew by Nancy Taylor of OSU’s Plant and Pest Diagnostic Clinic (Fig. 1).  The pathogen that causes basil downy mildew, Peronospora belbahrii, is seed borne and favored by cool, humid/rainy conditions.  Overhead irrigation in the greenhouse/nursery promotes the development and spread of the pathogen, as does rainfall outdoors.  Symptoms on leaves start with diffuse yellowing and browning that can look like sunburn.  Leaf lesions later become black (Fig. 2).  A diagnosis of downy mildew is made when spores of the pathogen are observed on the underside of lesions (Fig. 3); the best time to look for these dark purple-black structures clustered together is early in the morning.

Fig. 1. Downy mildew symptoms on basil seedlings

Fig. 2. Black lesions on upper surface of leaf of basil plant.








Fig. 3. Spores (sporangia) of the basil downy mildew pathogen on the lower surface of a basil leaf lesion.

There are differences in susceptibility of basil varieties and types to downy mildew, with sweet basil generally the most susceptible.  Fungicides are available for downy mildew management in basil – see Dr. Meg McGrath’s article for recommendations for both conventional and organic systems. However, as is the case for the closely related cucurbit downy mildew, fungicides work best when applied preventatively – before symptoms are observed.  Many, but not all of the fungicides are allowed for use in greenhouses or other protected culture systems such as high tunnels.  See page 45 of the 2017 Midwest Vegetable Production Guide for Commercial Growers for a listing of allowed uses of fungicides for protected culture.

If you suspect downy mildew in your basil crop, you may send or drop off samples to the OSU Plant and Pest Diagnostic Clinic in Reynoldsburg or the OSU Vegetable Pathology Lab in Wooster (call, text or email Sally Miller ( or Francesca Rotondo (