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 in 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.

Anthracnose of bell pepper

Bacterial Disease Management in Vegetable Crops without Copper?

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.

Product

Type Manufacturer Crops labeled

Plant Resistance Inducers

Actigard Plant activator Syngenta Bacterial diseases of brassicas, cucurbits, tomato, non-bell pepper
Regalia Plant (Reynoutria) extract, plant resistance inducer Marrone Bio Innovations Most vegetables
Taegro 2 Bacillus subtilis var. amyloliquefaciens FZB24 Novozymes BioAg Inc. Bacterial diseases of fruiting vegetables
Vacciplant Laminarin, plant defense stimulant UPL Fruiting vegetables, brassicas, leafy vegetables
Antimicrobials
Double Nickel Bacillus amyloliquefaciens D747 Certis USA Most vegetables
LifeGard WG Bacillus mycoides J Certis USA Most vegetables
Serifel

Serifel NG

Bacillus amyloliquefaciens MBI600 BASF Most vegetables
Serenade ASO

Serenade Opti

Bacillus subtilis QST-713 Bayer CropScience Most vegetables
Stargus Bacillus amyloliquefaciens F727 Marrone Bio Innovations Most vegetables
Antibacterial Viruses
AgriPhage Antibacterial phage (type of virus) Certis USA Most vegetables

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.

 

 

Cucumber Downy Mildew Confirmed in Medina County, OH

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.

Another Cucumber Downy Mildew Outbreak in MI – and Closer to Home

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.

Managing Cucurbit Powdery Mildew

Powdery mildew colonies on lower surface of leaf.

Powdery mildew has begun to appear on pumpkins and other cucurbits in Ohio. Signs of infection are small circular powdery growths on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves. Stems and leaf petioles are also susceptible, but the disease is not observed on fruit. In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens. It is time to start scouting cucurbits for powdery mildew.

Powdery mildew is managed using disease-resistant varieties and fungicides. Pumpkin and squash varieties vary in resistance to powdery mildew; in general, the more susceptible the variety, the more fungicide needed. The choice of fungicide is important because insensitivity to overused fungicides is common. It is critical that a fungicide resistance management program is followed. Alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. Fungicide applications should begin when the disease first appears and incidence is low. Fungicides that are labeled for use against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings and FRAC codes are on page 129. Vivando (U8), Quintec and fungicides containing FRAC 3 group active ingredients (Aprovia Top, Inspire Super, Luna Experience, Procure, Rally) have fewer reported failures due to fungicide resistance than others listed in the Guide and are recommended for Ohio (see table below – click too enlarge). These products should be tank-mixed with a protectant fungicide such as chlorothalanil (Bravo and similar products), copper- or sulfur-based products.

Our evaluations of efficacy of powdery mildew fungicides in Ohio in 2018 indicated that Inspire Super, Procure, Rally, Aprovia Top and Quintec provided very good control of powdery mildew on pumpkins in three locations.  Bravo Weather Stik and Fontelis provided moderate control and Pristine, Merivon Xemium and Torino provided poor control.

A list of products for powdery mildew management in organic cucurbits prepared by Dr. Meg McGrath of Cornell University can be found here.

Cucumber Downy Mildew Confirmed in Michigan

Cucumber downy mildew was found today in Berrien County in southwest Michigan.  This is the first report of cucurbit downy mildew this year in the Great Lakes region. The pathogen was detected in spore traps in Berrien County June 8, 10, and 13, and in Muskegon County on June 8. Dr. Mary Hausbeck has provided details of the outbreak and a link to the spore trap data here.

This is relatively early for cucumber downy mildew in Michigan and Ohio – we often see it around the 4th of July and last year it appeared weeks later. Humid, rainy, cool to warm weather favors this disease. It is likely that downy mildew will be in Ohio soon, especially the northern counties, if it is not already here. Cucumber growers in the northern third of the state should begin a downy mildew fungicide program immediately.  Dr. Hausbeck recommends a rotation of the following fungicides tanked mixed with chlorothalanil or mancozeb: Ranman, Elumin, Zampro, Previcur Flex, or Orondis Opti (no need to tank mix Orondis Opti since it is a premix with chlorothalanil). Make sure to check the labels for use restrictions and preharvest intervals (PHIs).

The Michigan recommendations are based on 2019 field evaluations of these fungicides.  Our Ohio bioassay evaluations in 2019 had similar results, although we did not test Previcur Flex.

You can follow reporting of cucurbit downy mildew outbreaks in the US on the CDM ipmPIPE website.  This website was revamped this year and if you want to receive alerts of downy mildew outbreaks you will need to sign up, even if you had signed up on the previous website.

If you think you have downy mildew in cucumbers or other cucurbits on your farm or in your home garden, you can send samples to the OSU Vegetable Pathology Lab for a free diagnosis.