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 Peppers

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.