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.

Anthracnose of bell pepper

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.

2020 cucurbit downy mildew outbreaks as of July 8. cdm.ipmpipe.org

Downy mildew was confirmed today in a 3 acre cucumber field in Medina County, OH. Given the outbreaks reported in Michigan in June and an outbreak confirmed in Kent County, Ontario this week, this was expected. Although recent weather has been hot and dry, there have been localized intermittent rainstorms that favor downy mildew spread, and nighttime temperatures are usually cool enough for infection.

Growers in northern Ohio should protect cucumbers and melons with fungicides. Recommendations can be found here.

Thanks to Frank Becker, OSU Extension Wayne County IPM Program Coordinator, for bringing us the sample.

Downy mildew sporulation on underside of a cucumber leaf.

Outbreaks of cucumber downy mildew on two commercial farms in Monroe County, MI were detected on June 29. Monroe County is in southeast Michigan and borders Ohio’s Lucas County. In addition, spores of the downy mildew pathogen have been captured in spore traps in four Michigan counties, so downy mildew is ramping up and very likely to be in cucumbers in northern Ohio at this time. Although the sunny, dry, warm conditions of the past 5 days or so do not favor downy mildew epidemics, growers in northern Ohio should protect cucumbers, melons and giant pumpkins with appropriate fungicides as outlined here.  Squash and pumpkins are generally much less susceptible than cucumbers, melons and giant pumpkins to the group of isolates of the downy mildew pathogen that circulates early in the Great Lakes Region.  The second group affects all cucurbits and usually migrates from the southeastern states to Ohio in late July or August.

Although daytime temperatures are expected to be > 90°F for the next two weeks, cooler nighttime temperatures, high humidity and intermittent rainstorms may allow initiation and spread of downy mildew disease foci.

There are a number of downy mildew lookalikes, especially angular leaf spot, a bacterial disease, and anthracnose. If you aren’t sure and would like a lab confirmation, send leaf samples to the OSU Vegetable Pathology Lab for diagnosis. The diagnostic service is free for Ohio growers. Doing so will also help us track downy mildew on cucurbit crops across the state of Ohio.

Angular leaf spot of squash.

Anthracnose on cucumber – photo by M. Netz.