Managing Phytophthora Blight and Pythium Root Rot in Peppers – Fungicide Update

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.

Young pepper plants killed by Phytophthora blight

Pepper plants (background) stunted by Pythium root rot

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 ( Fungicides must be applied preventatively for maximum benefit. Keep in mind that:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. For Previcur Flex, Pythium root rot is on the label for peppers, but Phytophthora blight is not.
  7. 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:

  1. 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.
  2. 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.
  3. Always rotate fungicides with different modes of action (FRAC codes):

Ridomil Gold: 4

Orondis Gold: U15+4

Orondis Ultra: U15+40

Elumin: 22

Presidio: 43

Ranman: 21

Previcur Flex: 28

Phosphite products: 33



Wayne County IPM Notes (Week of June 1- June 5)

These observations are from Frank Becker, Dept. of Extension Wayne County.

Vegetable Pests

The Colorado Potato Beetle is being seen feeding in both potato and eggplant. When approaching plants to look for them, be cautious. When the beetle is startled, they drop to the ground and may be difficult to see. They do significant damage to the foliage and can cause significant reduction in yield. The Colorado Potato Beetle also has a history of developing resistance to insecticides being used as control measures. This has limited our choices for treatment options. The best way to prevent further resistance is to avoid using the same insecticide repeatedly. At the current plant stage for potato, the threshold is approximately 1 beetle per plant. For eggplant, it is 25 beetles per 50 plants.

Another pesky insect this time of year is the flea beetle. Their damage may seem insignificant at first, however, their populations can rapidly increase and can quickly overwhelm young plants. Flea beetle damage is occurring primarily on potato, eggplant, cole crops and sweet corn. Sweet corn is of particular concern due to Stewart’s Wilt disease which is vectored by the flea beetle. Susceptible sweet corn varieties have a threshold of 6 beetles per 100 plants, while tolerant varieties have a threshold of 2 beetles per plant. On cole crops, the threshold is 5 or more beetles per plant. For potato, you will need to count the “shot holes” in the leaves caused by the beetle. The threshold is 15 shot holes per leaflet. Eggplants have a threshold of 8 beetles per plant.

In sweet corn, there is light slug damage occurring as well as some light damage being done by the European corn borer larva. Young sweet corn is also a target of black cutworm. The cutworm will cut plants at the soil line. If you find a cut plant, dig up some soil around the plant to see if you can find the cutworm.

Vegetable Diseases

In high humidity this time of year, greenhouse tomato crops become especially susceptible to infection from Botrytis. This can initially present itself on the fruit as “ghost spot” which appear as pale or white rings on the fruit. It can then progress into Botrytis gray mold and the fruit will begin to rot. It is important to increase airflow in the tunnel as well as between plants. It would also be beneficial to reduce the humidity within the tunnel.

Blossom end rot is also prevalent this time of year in crops such squash and tomatoes. Although this is not necessarily a pathogen, secondary infections commonly compound the issue. To manage blossom end rot, it is important to limit moisture stress on a plant, from either too much or not enough moisture. Being consistent in watering and monitoring soil moisture conditions will help to prevent exposing the plant to moisture stress. Proper moisture will also provide conducive conditions for adequate nutrient uptake, given that the nutrients are present at appropriate levels in the soil.

Fruit Pests

Strawberry producers typically are facing several insect pests this time of year. One of these pests is the eastern flower thrips. This small insect feeds on and damages the strawberry blossom. As the berry begins to develop, this damage results in cat-facing on the berry or a russeting/bronzed appearance. When you notice these symptoms on the developing berry, the damage has already been done and there are no treatment options. To look for thrips in the blossoms, take a white piece of paper or a plate and shake the blossoms onto the plate and watch for any small, slender yellow thrips to be moving around. Once you have reached 2 or more thrips per blossom, you should move forward with a treatment. Consider the pollinators before applying an insecticide, considering the target of your application is primarily associated with the blossoms. Preventative sprays can also be used in successive plantings.

Another pest of strawberries and small fruits is the spotted wing Drosophila. The SWD is a small fruit fly that can lay its eggs in ripening fruit while it is still on the plant. As you are picking, do not discard unwanted fruit on the ground right next to the plant. The rotting fruit on the ground will attract SWD. Instead bring a bucket to discard unwanted fruit in and either bury it a foot or so deep in soil or seal the fruit in a clear plastic bag exposed to the sun for about a week to kill any larvae. If culls are discarded in the trash or compost pile, they might attract SWD flies and allow for more generations to be produced. This is also the time to put traps out in your bramble and blueberry patch but if you have June bearing strawberries, they likely won’t be affected by this pest. More details about how to set up traps can be in the  OSU IPM YouTube page under the SWD playlist at and on Celeste Welty’s page.

Orchard traps are now out in Wayne County and we will be monitoring Codling Moth and Oriental Fruit Moth numbers closely.

Fruit Diseases

Now is the time to be managing early season diseases in apples. Scab, rust and powdery mildew are the three main diseases of concern at this point in the season.

Strawberry leaf diseases may appear unsightly right now, however, now is not the time to be managing these leaf diseases. Once harvest is done and during patch renovation it is recommended that you address these concerns, either with a fungicide or with resistant plant varieties. This is also a critical time to be watching for fruit rots such as Botrytis.

Grapes are currently around the pre-bloom stage. This stage is the most critical stage of development for controlling diseases. Fungicide applications for black rot, powdery mildew and downy mildew are highly recommended during this time.

Grafted Plants, Suppliers, and Experiments

Growers typically convert to using grafted plants (e.g., tomato, watermelon) primarily because they can be much more productive when specific soilborne diseases are present (and the correct rootstock is used). In addition, however, grafted plants are often more vigorous than ungrafted ones of the same scion (fruiting variety). Grafted plants may also use water, fertilizer, and other inputs more efficiently. Therefore, it is necessary to optimize cultural and fertility practices for grafted plant-based production. Two experiments will be completed in this 1+ acre parcel in 2020. One experiment tests alternative fertilizer rates and the second experiment tests in-row spacings (plant populations/acre). All grafted plants are supplied by Tri-Hishtil in Mills River, NC.

Tri-Hishtil ( is one among a constantly-lengthening list of commercial grafted plant suppliers. Others include Banner Greenhouses (, Grafted Growers (, and Re-Divined ( in the eastern U.S. and others based in the west. Local suppliers are also operating in Ohio and some farmers are preparing their own grafted plants. Commercial suppliers continue to ramp-up their capacity to meet the needs of vegetable growers, regardless of the size, location, or type of their operation (field and/or high tunnel; conventional and/or organic). Also, grafted plant costs are increasingly competitive. Overall, access to grafted plants is strong and increasing and no longer a reason for being unable to test the performance of grafted plants on your farm. The Vegetable Production Systems Lab at OARDC can also assist, if needed; we teach people how to graft and, in 2021, we hope to resume preparing small numbers of plants by request.

One empty and three filled cells of a 128-cell tray holding grafted watermelon plants prepared by Tri-Hishtil. Note roots are visible on the surface as healthy white ‘threads’ with smaller root hairs near the tip, creating a bottle-brush appearance. Slotted cells (as shown at the bottom-left) contribute to this root condition and morphology.

Hand-grafted watermelon plants from Tri-Hishtil in Mills River, NC.
Plant at left is Jade Star and plant at right is Fascination, both grafted to Carnivor rootstock.
Clear and green clips show the location of the graft union and supports (white sticks) will be removed at planting (scheduled for 6/8/20).
Root systems are well-developed, stems are sturdy, and the plants pull easily. Roots are not spiraling, partly due to the larger size and special shape of the cells.
Clips can be removed at planting or allowed to be forced off naturally by stem growth.

Contact Matt Kleinhenz (330.263.3810; and see updates at this blog for more information.