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

Our 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

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.

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.

Downy mildew was confirmed today on cucumbers in our sentinel plot on the OSU North Central Agricultural Research Station in Fremont and a home garden in Clyde, both in Sandusky County.  This follows reports for cucumbers in Medina and Wayne Counties earlier this month. As I’ve indicated in previous posts, we believe that cucumber downy mildew has been present in northern Ohio counties for several weeks; growers should be protecting cucumbers with recommended fungicides.  We haven’t had reports of downy mildew on melons (cantaloupe) but melons are susceptible to the strain of the downy mildew pathogen circulating in northern Ohio, as are giant pumpkins. So these crops should also be protected now with fungicides.

Garden cucumbers with downy mildew

While downy mildew does not cause lesions on fruit, it does reduce yield significantly by damaging and eventually killing the foliage.  Growers who stop harvesting fields with severe downy mildew should destroy the plants as soon as possible to eliminate this source of inoculum. The pathogen does not survive in the soil.

Home gardeners who choose to treat cucumbers or melons with a fungicide should purchase a product containing chlorothalanil and start applications before the disease appears.  If the disease becomes severe gardeners should destroy the plants to reduce local inoculum.

Micrograph of a tape mount of spores and sporangiophores of the cucurbit downy mildew pathogen from cucumber leaves. Photo by Francesca Rotondo.

Downy mildew continues to spread in Ohio cucumbers despite the hot and mostly dry weather.  Frank Becker, OSU Extension Wayne County IPM Program Coordinator, brought cucumber leaves with downy mildew symptoms to to our Vegetable Pathology Lab on July 23 for confirmation. We do this by placing a piece of scotch tape on the underside of a leaf lesion then transferring to tape to a glass slide and looking for characteristic spores and sporangiophores (branched, threadlike structures that produce the spores) under a microscope. The samples came from commercial cucumber fields in Wooster and Apple Creek in Wayne County, and both were positive for downy mildew.

Cucurbit downy mildew as of July 24, 2020. cdm.ipmpipe.org.

Although we have confirmed reports in only Medina and Wayne counties, cucurbit downy mildew is likely present in most northern Ohio counties.  The map of downy mildew reports shows confirmed cases in Ontario, Michigan and western New York as well. All of these reports are from cucumbers; this clade, or strain of the pathogen affects cucumbers and cantaloupe, but not squash or pumpkins. We don’t expect downy mildew on squash and pumpkins until the other known clade, which has a broader host range, migrates to the Midwest from the Southeast.

Downy mildew in cucumber.

Fungicide recommendations are posted here.  If you suspect downy mildew in any cucurbit, please send us a sample.  This will help us track the disease and provide early warnings to growers to enable timely protection of cucurbit crops. Our diagnostic service is free to commercial growers in Ohio; gardeners may also send cucurbit downy mildew samples to us free of charge.  Instructions for sample submission are posted here.

Blossom end rot of bell pepper

The hot, dry weather of the last few weeks has been stressful for peppers, resulting in the appearance of blossom end rot, especially in early fruit sets. Blossom end rot is the result of plant stress brought on by periods of dry vs moist soil.  Calcium deficiency in the plant is the cause but applying calcium to the foliage won’t help.  Calcium is relatively insoluble and plants under stress can’t move it to flowers and developing fruit.  It is a vital component of plant cell walls and the matrix that holds the cells together. When fruits start to form without sufficient calcium the tissues soften and die.  Secondary molds often colonize the dead tissue.

Blossom end rot of bell pepper

Blossom end rot becomes less problematic with more consistent soil moisture and as the plants grow and develop their root systems.

Blossom end rot of bell pepper

Another fruit problem reported recently and related to hot, sunny weather is sunscald. Sunscald can appear similar to blossom end rot – it appears on the part of the fruit exposed to the sun.  Sunscald spots are tan in color, and eventually become dry and papery. There isn’t much that can be done about sunscald except to encourage good foliage coverage by appropriate fertilization.

Sunscald of bell pepper

Anthracnose also causes lesions on pepper fruit, but the disease is caused by a fungus dispersed by rainsplash (or overhead irrigation); it is less severe in dry than rainy weather.  This disease is managed by application of fungicides.

Thanks to Carri Jagger for the blossom end rot and sunscald photos.

The 2020 vegetable growing season has been relatively hot and dry in most of Ohio, resulting in fewer reported serious outbreaks of bacterial diseases. However, circumstances can change and bacterial diseases may need to be managed. Unfortunately, options for bacterial disease management at the field stage are limited.

Bacterial spot on tomato fruit

Copper-based products, often paired with mancozeb or related products, have been the mainstay for bacterial disease management in vegetables for decades. Copper treatment is only partially effective under rainy conditions that favor bacterial diseases, when disease pressure is moderate to high. Further, research conducted in Ohio and other states has shown that copper resistance is widespread in the Xanthomonas bacteria that cause bacterial spot in tomatoes and peppers, rendering these products mostly ineffective. We are no longer recommending copper treatments for bacterial spot management in tomatoes or peppers.  We have less information about other bacterial pathogens but copper resistance is possible in other Xanthomonas species as well as other pathogens such as Pseudomonas and Clavibacter.

There are a few other options for bacterial disease management (see table below) that are grouped into roughly three categories: 1) plant resistance activators/inducers, 2) antimicrobials, and 3) bacteriophage. Keep in mind that none of these products fully control bacterial diseases under moderately to highly conducive conditions. In our research with bacterial spot of tomatoes, Actigard applications consistently reduced bacterial spot damage to foliage, although the incidence of fruit lesions was less consistently reduced and yield not improved compared to the non-treated control in small plot trials.  Actigard is labeled for bacterial disease management in brassicas, cucurbits, tomatoes and non-bell peppers. The other resistance inducers in this group have been shown to suppress bacterial diseases but there is inconsistency and lack of control under highly conducive conditions.

Products in the antimicrobials group also have been shown to suppress bacterial diseases, but again, results vary among trials and these products are not effective under highly conducive conditions. In our experience, under low to moderate disease pressure, disease severity is often significantly reduced compared to non-treated plants.  As a rule of thumb, the reduction in symptom severity ranges from about 25-40%. If these products are going to be used, they should definitely be applied preventatively to keep bacterial pathogen populations low.

Finally, AgriPhage is a product that contains antibacterial viruses (phage) that infect and kill specific bacterial pathogens. There are different mixtures of phage for different pathogens.  This product also must be applied early in an epidemic.

Given the inadequacies of these “rescue” treatments for bacterial disease management, proactive approaches should be undertaken:

• Start with seeds tested for bacterial diseases; if not possible, treat seeds with hot water or dilute Clorox.
• Create conditions during transplant production that discourage bacterial pathogen multiplication on plants – dry growing, good air circulation, low relative humidity.
• Apply labeled antimicrobials (see Table) to seedlings in the greenhouse.
• Sanitize transplant houses after seedlings are moved to the field.
• Sanitize vehicles and equipment prior to transporting and transplanting seedlings.
• Use new or sanitized stakes each season.
• Sanitize pruning tools after each plant (tomatoes).
• If possible remove and destroy diseased plants.
• Practice regular crop rotation.