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.

Severe damage to cucumbers by downy mildew. Photo by M. Mechling.

The OSU Vegetable Pathology lab confirmed downy mildew in cucumbers from Jefferson and Perry counties in eastern and central Ohio this week.  Given the confirmed distribution in Ohio counties and outbreaks in neighboring states, it is safe to assume that cucumbers are at risk of downy mildew throughout Ohio (see map). We have also reported downy mildew on cantaloupes in two of our sentinel plots in northern Ohio, but have not had any confirmed reports on pumpkin, squash, watermelons or other cucurbits in this state. Downy mildew has not been observed on any cucurbits in our sentinel plot on the OSU main campus in Columbus. This tracks with reporting from other states; cucumber is the main cucurbit crop affected so far. More information about downy mildew outbreaks and the interactive map can be found here. Management advice can be found in earlier blogs posts this summer.

Map of cucurbit downy mildew outbreaks by county. cdm.ipmpipe.org

Downy mildew on cantaloupe

Downy mildew has been reported in several Northern Ohio counties: on cucumbers in Huron, Medina, Sandusky, Seneca, and Wayne counties, and also on cantaloupe in Sandusky County. There have been also several reports of bacterial diseases in cucumbers (angular leaf spot) and pumpkins (bacterial spot) – these diseases can be mistaken for downy mildew. You can find information on cucurbit downy mildew management in my post here.

Downy mildew on basil

We have also found basil downy mildew in our sentinel plot in Huron County. Basil downy mildew management recommendations for gardeners and commercial growers can be found here.

Frogeye lesions on pepper leaves (photo by M. Netz)

Frogeye leaf spot, caused by the fungus Cercospora capsici, has distinctive symptoms on leaves, stems and peduncles of pepper and eggplant. Lesions are circular or oblong, tan in the center and surrounded with a necrotic border and often yellowing tissues. As the lesions expand, concentric rings may be present. The lesions resemble a frog’s eye, hence the name.

Several farms in Northwest Ohio have recently reported this disease in peppers.  It has not been common in this area, and likely appeared due to the unusually hot weather this growing season.  Frogeye leaf spot is favored by warm, wet conditions.  Cercospora produces spores in the lesions that are dispersed by air, rain, overhead irrigation and tools and equipment.

Frogeye lesions and chlorosis on pepper leaves (photo by M. Netz)

This disease is managed by a combination of cultural practices and fungicide applications. Cercospora survives at least a year on crop residue, so residues should be plowed under to hasten decomposition. Drip irrigation should be used if possible. Fungicides typically recommended are Quadris, Quadris Top, Aprovia Top, Cabrio and mancozeb. Fungicide applications should be alternated according to fungicide mode of action (FRAC code) to reduce the development of fungicide resistance.

White, diamond-shaped lesions of Plectosporium blight on pumpkin vine (photo by Francesca Rotondo)

We’ve had several reports this week of Plectosporium blight appearing in pumpkins in Ohio. Sometimes also called white speck, this is a disease of pumpkins and squash caused by the fungus Plectosporium tabacinum. The symptoms on vines, leaf petioles and veins on the back of the leaves are small, white and diamond-shaped lesions; on fruits the lesions are also small and white, generally

Advanced symptoms of Plectosporium blight on pumpkin vine (photo by Brad Bergefurd)

round. Under favorable environmental conditions (rainy, moderate temperatures) the lesions can coalesce, and the affected tissues appear white and become brittle. Plectosporium produces spores in the lesions that are dispersed long distances by the wind. The fungus survives in soil associated with plant debris for several years.

Plectosporium blight management requires an integrated approach that includes crop rotation, cultural practices and fungicide applications. Although pumpkin and squash varieties vary somewhat in susceptibility to this disease, none are resistant. In a study we conducted in 2018 at the OSU South Centers in Piketon, OH, the varieties ‘Hulk’, ‘Cronus’, ‘Warty Gnome’ and ‘Bayhorse Gold’ had less Plectosporium blight than other varieties in the trial.

Plectosporium blight lesions on pumpkin vine and handle (photo by Brad Bergefurd)

Plectosporium blight management:

1. Rotate out of cucurbits for 4-5 years.
2. Choose a site with good air circulation to allow plants to dry quickly.
3. Use drip irrigation instead of overhead irrigation.
4. Scout fields, looking for diamond-shaped lesions on leaf veins, vines and petioles within the canopy; if present, apply fungicides on a 7-day schedule.
   • Chlorothalanil (e.g. Bravo), the strobilurin fungicides such as Quadris, Quadris Top and Pristine, and Topsin M are the most effective fungicides against Plectosporium blight, but do not fully control the disease.  It is important to get good coverage of the fungicides well into the canopy.  Use high water volumes – at least 40 gal/acre. Strobilurin fungicides have translaminar activity and move through the leaves, improving coverage.

5. After harvest plow down the crop residue to encourage rapid decomposition.

Bacterial canker in fresh-market tomatoes.

Bacterial canker is a systemic disease of tomatoes caused by the bacterium Clavibacter michiganensis subsp. michiganensis. It can occur in fresh market and processing tomatoes, in open fields and in protected culture systems like greenhouses and high tunnels. Symptoms are stunting of whole plants, which never reach their full potential, plant death,

Bacterial canker on tomato leaves.

foliar lesions, “firing” on leaf margins and raised scabby lesions on fruit. Seeds are a major means of introducing the canker pathogen into a tomato crop, but the bacteria can survive in the field for several years, as well as on surfaces such as greenhouse walls or floors, tools, stakes, clips or ties, etc. Several cases of tomato canker have come into our diagnostic lab this summer; since the bacteria clog the plants’ water-conducting vessels, the stunting symptom may be more severe in the hot, dry weather we’ve experienced for much of this year’s growing season.

Bacterial canker symptoms inside a tomato stem.

Peppers are also susceptible to bacterial canker, but the disease is not systemic in peppers so the stunting symptom does not occur. However, firing of the leaf margins and leaf and fruit lesions do occur. Symptoms of bacterial canker on peppers are different than those on tomatoes (see figures). The bacteria that infect

Bacterial canker symptoms on pepper leaves.

tomatoes are the same as those infecting peppers, so infected peppers can be a source of bacterial inoculum for tomatoes and vice versa. Bacterial canker is relatively rare in peppers; if you suspect it please consider sending a sample to our diagnostic lab.  The service is free for Ohio vegetable growers.

Bacterial canker lesions on pepper fruit.

There are no bactericides or other products that control this disease once it is in the field or greenhouse. This disease is managed primarily through sanitation.

• Start with clean seed – For purchased seeds, buy certified, disease-free seed or sanitize seed with hot water (recommended), dilute bleach or hydrochloric acid. It is especially important to sanitize saved seeds, such as for heirloom varieties. Here is a link to the OSU fact sheet for Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens. In place of water baths for the hot water treatment, relatively inexpensive Sous Vide – type digital water heaters can be used too heat and maintain the water at the prescribed temperature.
• Keep transplants clean and healthy – Scout tomato and pepper plants daily and destroy plants with canker symptoms once a plant disease diagnostic laboratory has confirmed the disease. Apply one or two preventative copper fungicide applications and one application of streptomycin (conventional systems) to the plants before transplanting them into the field.
• Use clean equipment and tools – Clean and disinfect all tools and farm equipment prior to working with the transplants or plants. Good sanitation practices are critical to prevent contamination and cross contamination of plants by the bacterial canker pathogen. Quaternary ammonium products and 10% chlorine bleach are suitable disinfectants.
• Start with a clean field – The bacterial canker pathogen can survive in the field as long as there is infected crop debris present. Rotate with a non-host before re-planting the field with tomato. Ideally a 3-4 year out of crops in the same family as tomato (pepper, eggplant) should be implemented. Plant into a field free of weeds or volunteer tomato plants.
• Use best cultural practices – Use management strategies that maintain reduced-stress growing conditions. Provide plants with adequate but not excessive nitrogen, improve the organic matter content of the soil through the use of composted green or animal waste or cover crops, use well-drained soil and avoid overhead irrigation if possible.