Cucurbit Downy Mildew Spreads to Squash, Pumpkins and Watermelons in Ohio

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.

Cucurbit Downy Mildew Update

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

Notes on Downy Mildews – Cucurbits and Basil

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.

Cercospora “Frogeye” Leaf Spot of Pepper

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.

Plectosporium Blight of Pumpkins, Squash

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 2-3 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.
  1. After harvest plow down the crop residue to encourage rapid decomposition.

 

Bacterial Canker Showing Up in Tomatoes this Summer

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.

 

 

 

 

 

 

 

Cucumber Downy Mildew Confirmed in Sandusky County in Ohio

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.

Cucurbit Downy Mildew is Spreading in Ohio Despite Hot Weather

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 Peppers

 

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.