Cucurbit downy mildew occurrences in the eastern US. https://cdm.ipmpipe.org.

Downy mildew was reported on pickling cucumbers in Seneca County today. The field was heavily damaged, with about 90% of the plants showing symptoms. We also found one confirmed downy mildew lesion in our sentinel plot in Wooster (Wayne County). These are the first reports for 2021 of cucurbit downy mildew in Ohio, following an outbreak in Chatham-Kent, Ontario reported on June 17. Growers in northern Ohio should protect cucumbers  and melons with effective downy mildew fungicides such as Orondis Opti, Ranman, Omega, Elumin, Previcur Flex or Zing!. Remember to alternate products in different FRAC groups and tank mix with a chlorothalanil product or other protectant fungicide – except for Zing!, which contains chlorothalanil in the premix.  Check the labels carefully for use instructions and restrictions. Fungicides must  be applied preventatively – they  are far less effective if applied after  infection.

Growers in central Ohio should intensify scouting of cucumbers and melons and apply a protectant fungicide. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and fuzzy grey/brown growth on the undersides of the lesions. With a good hand lens or a smartphone camera with high magnification you may be able to see small dark brown/purple spots within the fuzzy growth. These are the spores of the downy mildew pathogen.

Smartphone image of a downy mildew lesion with the pathogen sporulating on the underside of a cucumber leaf.

If you suspect downy mildew in cucumber or melon please text or email pictures to Sally Miller (330-466-5249; miller.769@osu.edu) of both sides of lesions, with the underside in the highest possible magnification. I can often confirm downy mildew from photos, but if not will ask you to send a sample to the OSU Vegetable Disease Diagnostic Lab for confirmation. Instructions for sample submission are here. Thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program, there is no fee for this service for Ohio vegetable growers.

Thanks to Jim Jasinski for providing more info on fungicides for powdery mildew that he has tested on cucurbits. The following table is amended from my post on July 6, 2021 to include three additional effective fungicides: Miravis Prime, Luna Experience and Luna Sensation. Please read the July 6 post for more details on how and when to use these products.

Downy mildew of cucumber.

Downy mildew and powdery mildew are very different diseases of cucurbits but they  have in common that they are obligate pathogens that require a living plant for survival and thus don’t overwinter in Ohio. Downy mildew appears from late June to August in northern Ohio (but not observed yet!) and August to September for central  and southern Ohio. Powdery mildew appears without fail in the first half of July throughout Ohio. Both mildews are managed by resistant varieties when available and fungicides. For gardeners, fungicide options are limited.  For downy mildew the best option is a chlorothalanil product applied preventatively. You  can follow me on Twitter @OhioVeggieDoc for alerts on downy mildew appearance in Ohio, or check this blog or the national cucurbit downy mildew forecasting site: https://cdm.ipmpipe.org.

Advanced powdery mildew on pumpkin.

Powdery mildew will show up on pumpkins and other cucurbits soon if not already here. Fungicides should be applied as soon as powdery mildew is observed. Sulfur products are most effective of those listed. Potassium bicarbonate and Serenade Garden also have some efficacy against powdery mildew.  Note these are all protectants and only work on the leaf surface. Therefore full coverage is needed throughout the canopy (top and bottom of leaves, petioles).

Powdery mildew colonies on upper leaf surface.

Powdery mildew normally appears on pumpkins and other cucurbits in Ohio in early July. Signs of infection are small circular powdery growths on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves. Stems and leaf petioles are also susceptible, but the disease is not observed on fruit. In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens. It is time to start scouting cucurbits for powdery mildew.

Powdery mildew is managed using disease-resistant varieties and fungicides. Pumpkin and squash varieties vary in resistance to powdery mildew; in general, the more susceptible the variety, the more fungicide needed. The choice of fungicide is important because insensitivity to overused fungicides is common. It is critical that a fungicide resistance management

Powdery mildew colonies on lower leaf surface.

program is followed. Alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. Fungicide applications should begin when the disease first appears and incidence is low. Fungicides that are labeled for use against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings and FRAC codes are on page 11.

Our evaluations of efficacy of powdery mildew fungicides in Ohio in 2020 indicated that Aprovia Top, Inspire Super, Gatten, Vivando and Procure provided very good control of powdery mildew on pumpkins (see table below).  Microthiol Disperss was not tested in 2020 but historically has provided good control. Quintec and Fontelis provided moderate (53-65%) control and Bravo Weather Stik, Merivon Xemium,  Pristine, Torino and Rally provided poor control. The bioassay full report can be found here (pp. 15-16).

A list of products for powdery mildew management in organic cucurbits prepared by Dr. Meg McGrath of Cornell University can be found here.

Numerous intense rainstorms that have moved across Ohio during the 2021 growing season so far have created conditions for the establishment and spread of bacterial diseases. Unseasonably cool weather in early June plus rain favored leaf spots and blights caused by Pseudomonas spp. such as bacterial leaf spot of parsley and peppery spot of crucifers. In late June we are seeing bacterial diseases favored by rain and warmer weather, caused by Xanthomonas spp., such as

Cabbage black rot.

black rot in crucifers and leaf spot of radish. These bacteria enter vegetable crops through infested seeds and seedlings, overwintered crop debris or weeds. Epidemics tend to be more severe when started at the seed or seedling stage. Other bacterial diseases to watch for this summer are bacterial canker of tomato and pepper (Clavibacter), bacterial wilt of cucurbits (Erwinia), yellow vine decline of cucurbits (Serratia), and various soft rots (Pectobacterium).

Microscopic image of bacteria streaming from a leaf lesion.

Bacterial disease symptoms can be similar to those caused by certain fungi or oomycetes, so an accurate diagnosis should be obtained before applying fungicides or bactericides. When in doubt, growers may send samples to us for diagnosis. A simple microscopic evaluation for bacterial streaming from diseased tissue allows us to separate bacterial diseases from others (see image, right). Thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and  Small Fruit Research and Development Program, there is no fee for this service for Ohio commercial vegetable growers.

Unfortunately, bacterial diseases are very difficult to manage in susceptible varieties at the field stage if rainy conditions persist. The go-to treatment for bacterial diseases in the field has been a fixed copper product with or without mancozeb. However, copper application has never been highly effective due to its inability to reach the inside of plants, relatively low residuals and sensitivity to pH.  It is not uncommon for bacterial populations in a leaf with lesions to exceed 100 million bacterial cells per gram of leaf tissue.  Copper compounds in currently available products are too large to enter lesions and thus high levels of inoculum are protected. In addition, our research and studies conducted in other states have shown high levels of resistance to copper in Xanthomonas spp. causing bacterial spot of tomato and pepper. Other bacterial species also develop copper resistance.

So what can be done if a bacterial disease is found in an open-field vegetable crop? If it is early in the epidemic and few plants with symptoms are observed during scouting, various non-copper-based products can be applied preventatively. These products are listed in my July 18, 2020 post. Many of these products, falling in three categories of 1) plant resistance inducers, 2) antimicrobials and 3) antibacterial viruses have been shown to be partially effective under low- to moderate-disease pressure. If disease pressure is high, and rainy, humid conditions persist, these products are not likely to control a bacterial disease epidemic.

Now that we know cucumber downy mildew has appeared in the Great Lakes Region (Ontario report – see my post on 6/17/2021), cucumber and melon growers should prepare for fungicide applications. There are a number of products labeled for downy mildew management in cucurbits, but not all are consistently effective due to development of fungicide insensitivity in downy mildew pathogen populations.

The OSU Vegetable Pathology Lab conducts bioassays each summer to assess the efficacy of fungicides commonly used to manage cucurbit downy mildew. We treat potted young cucumber plants with fungicides individually at the labelled rate or with water alone, and the next morning place the plants in a cucumber field with active downy mildew. Depending on the year, tests were conducted in Wayne, Sandusky and Huron counties, all in northern Ohio. After 48 hrs of exposure the plants are returned to the greenhouse and rated for symptom severity 7 days later.  The results for 2018, 2019 and 2020 are shown in the table.

Click to enlarge.

The fungicides Previcur Flex, Ranman, Omega and Orondis were highly effective against cucumber downy mildew for all three years. Zing! and Elumin were only tested in 2018 and 2019/2020, respectively, and both were highly effective. One or more of these fungicides should be part of a downy mildew management program.

Curzate, Forum, Presidio, Quadris and Zampro were inconsistence across years and location and were ineffective in a least one location in one of three years. Revus was consistently ineffective.

Spray Program Recommendations for Northern Ohio Cucumber and Melon Growers

1. Apply a protectant fungicide such a chlorothalanil (Bravo, Equus, etc.) on a 7-10 day schedule now.
2. When downy mildew is reported on cucumbers or melons in northern OH, SE MI or western PA add effective fungicides shown in green to the spray program. Tank mix with a protectant fungicide, alternate fungicides with different modes of action (FRAC codes) and follow the label restrictions and requirements. Shorten the application interval to 7 days under favorable weather for downy mildew: rain, overcast skies, high humidity, and cooler day and/or nighttime temperatures. Follow this blog, Twitter @OhioVeggieDoc or the Cucurbit Downy Mildew IPM PIPE for downy mildew reports.
3. Forum, Presidio, Quadris, Zampro and Revus are not recommended. Curzate may be moderately effective in some locations.

Downy mildew was reported on cucumbers in Ontario today. This is the first report for 2021 of cucurbit downy mildew in the Great Lakes region. There have been no reports of cucurbit downy mildew in Ohio or Michigan, although the Hausbeck lab at Michigan State University has detected spores of the pathogen in their spore traps for several weeks now. The storm system currently in the upper Midwest is moving in an east/southeast direction, and as it moves through Ontario may pick up spores and drop them further south. Historically cucumbers in the northern approx. 1/3 of Ohio have been the first in the state to show symptoms of downy mildew. Growers should intensify scouting of cucumbers and melons and apply a protectant

Top left: early downy mildew lesion on cucumber; top right: advanced cucumber downy mildew; lower left: underside of cucumber leaf with spores of the pathogen; lower right: microscopic image of downy mildew spores.

fungicide such as chlorothalanil. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and sparse fuzzy growth on the undersides of the lesions. With a good hand lens or a smartphone camera with high magnification you may be able to see small dark brown/purple spots within the fuzzy growth. These are the spores of the downy mildew pathogen.

If you suspect downy mildew in cucumber or melon please text or email pictures to Sally Miller (330-466-5249; miller.769@osu.edu) of both sides of lesions, with the underside in the highest possible magnification. I can often confirm downy mildew from photos, but if not will ask you to send a sample to the OSU Vegetable Disease Diagnostic Lab for confirmation. Instructions for sample submission are here. Thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program, there is no fee for this service for Ohio vegetable growers.

We often get questions from growers about reusing wooden or other stakes for tomato and pepper production, particularly if diseases like Phytophthora blight, Pythium root rot or bacterial canker were present where the stakes were used the previous year. The pathogens that cause these and other diseases can survive over the winter in soil and plant debris on stakes. We recommend power-washing or brushing stakes to remove all of the soil and plant sap, followed by disinfecting. This may be a big logistical headache for growers with a lot of stakes. In the Midwest Vegetable Production Guide for Commercial Growers (pages 80-81), we recommend soaking stakes in 10% bleach or quaternary ammonium disinfectants, followed by rinsing and drying.  

There is more information on disinfecting stakes in the Mid-Atlantic Commercial Vegetable Recommendations (page 12): “The preferred (and most expensive) method of stake disinfestation is heat treatment. Pathogens are completely eliminated from wooden stakes with exposure to ≥ 220°F for ≥ 15 minutes. This can be accomplished in a large capacity autoclave, or seed dryer. It is unlikely that most growers will have access to such equipment. Alternatively, therefore, stakes may be exposed to disinfectants such as commercial chlorine solutions (sodium hypochlorite) or Oxidate® (hydrogen dioxide; see below). Research has shown that a 20-minute soak in a solution made of 5 to 20 parts by volume sodium hypochlorite (commercial bleach) to 80 to 95 parts by volume water is effective in eliminating pathogens only from the surface of wooden stakes. It is crucial to maintain the pH of the bleach solution within the 6.0 to 6.5 range, as effectiveness decreases at lower and higher pH levels.”

“Studies on stakes treated with bleach solutions show that pathogens may still be present beneath the surface at depths ≥ 1/16th inch. Pathogens embedded within the stake may be able to migrate back to the surface and re-infest plants, although this has not yet been demonstrated. To improve the effectiveness of procedures for removing microbial pathogens from stakes, consider the following: Add a non-ionic surfactant to the disinfesting solution; increase the soaking time to ≥ 1 h; apply a vacuum during the stake soak; use a higher concentration or more potent source of hypochlorite (such as “heavy duty” or swimming pool grade chlorine); or use stakes comprised of nonabsorbent stake materials (such as plastic or metal). Many growers have successfully used the commercial product Oxidate® or chlorine dioxide to disinfest stakes. Oxidate® is OMRI certified and had been demonstrated to be an effective control agent for several important plant pathogens. However, data on the efficacy of this treatment as compared to using heat or commercial chlorine solutions are not available.”

All disinfectants are quickly inactivated by organic matter, so getting as much of the soil off the stakes as possible before sanitizing would improve the results.  

Ohio pepper growers are taking advantage of some dry, warm days to set transplants in the field. If Pythium root rot or Phytophthora blight has been a problem in these fields in the past, or high levels of rainfall are expected in the coming weeks, growers may want to consider preventive fungicide applications. The following is an update of my blog on this subject in June 2020.

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. Transplanting peppers into wet soil followed by high temperatures 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 2020 (https://plant-pest-advisory.rutgers.edu/phytophthora-control-during-wet-weather-3/). 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

Downy mildew lesions on pumpkin. Photo by Francesca Rotondo.

We are coming to the end of the cucurbit growing season, and until this week had only seen downy mildew on cucumbers and a few cantaloupes in this state.  However, this Sunday my colleague Francesca Rotondo found a “fresh” infection (it had not been visible a few days before) of downy mildew in our pumpkin research plot at the OARDC in Wooster. Today we confirmed downy mildew in acorn squash and watermelons in our sentinel plot nearby.

At this time of year, the choice to apply downy mildew-active fungicides depends on the progress of the crop. If pumpkins or squash are already mature and foliage is not needed to finish fruit ripening, it is possible to forego fungicide application since downy mildew affects only leaves, not stems and fruit. However, keep in mind that defoliation can lead to sunburn of fruits if we get some hot sunny days. Also note that allowing downy mildew to run rampant through a cucurbit field contributes overall to the inoculum density and puts other cucurbits on the farm or on neighboring farms at risk.

Cucurbit downy mildew fungicide recommendations can be found here.  Phytophthora fruit rot can also be a problem so fungicides that help suppress Phytophthora should be used in fields where this pathogen is or has been present in recent years. Downy mildew fungicides that have activity against Phytophthora are Orondis Opti, Orondis Ultra, Ranman, Elumin and Zampro. The fungicide product ratings for cucurbit diseases can be found on page 130 of the Midwest Vegetable Production Guide.