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.

Septoria Leaf Spot is Here on Tomatoes

Septoria leaf spot has been found in Ohio this week, perhaps not surprisingly due to frequent heavy rains in many areas. Dr. Francesca Rotondo, vegetable disease diagnostician and research associate in the OSU Vegetable Pathology Lab, texted me this photo taken today in her home garden. This excellent photo is diagnostic for Septoria leaf spot: round tan to brown spots on the leaves and leaf yellowing. In the more mature spots, margins are dark brown and small round black dots can be seen in them through a hand lens or the lens of your smartphone camera. The tiny black dots are called pycnidia, the fruiting bodies of this fungus. Pycnidia are flask-shaped with a small hole at the top, and partially submerged in leaf tissue. Pycnidia contain large numbers of spores held in a gelatinous matrix; when humidity is high or free water is on the leaf surface, the spores ooze out of the pycnidia like toothpaste being squeezed out of a tube. The spores are dispersed by rain or irrigation water to other leaves on the same plant and to other nearby tomato plants.

Septoria lycopersici is seedborne and also survives at least 1-2 years in soil. Septoria leaf spot is favored by moderate temperatures, high humidity and rain or overhead irrigation. While Septoria does not cause spots on tomato fruit, it can rapidly defoliate the plant. If this happens early the plant is likely to die. Later on, defoliation leads to small fruit, poor ripening and problems with sunscald. Even large, previously healthy, vigorous plants can be completely defoliated.

Commercial growers can manage Septoria leaf spot by including a strobilurin fungicide such as Quadris or Cabrio in a fungicide program that also includes a protectant such as chlorothalanil or mancozeb. A list of labeled fungicides to manage Septoria leaf spot can be found in the Midwest Vegetable Production Guide for Commercial Growers. Cultural practices to minimize Septoria leaf spot should be undertaken, including crop rotation of 3 years or more, planting tomatoes on raised beds, using adequate spacing, using drip irrigation, pruning foliage to allow good air movement through the canopy, and removing diseased plants from the field. Septoria leaf spot is rarely a problem in high tunnels, greenhouses and other structures that protect plants from rain.

Organic growers need to follow the cultural practices described above and may consider growing tomatoes in protected culture. Copper-based fungicides formulated for organic production can suppress disease development if applied soon after initial symptoms appear.

Home gardeners should adopt the cultural practices described above and should also remove and destroy leaves with symptoms. This is really only effective when symptoms first appear. Always avoid the foliage when watering plants. Fungicides containing chlorothalanil or copper can be applied to slow disease spread.

 

 

 

 

 

Managing Phytophthora Blight and Pythium Root Rot in Peppers – Fungicide Update

Heavy rains early in the planting season favor both Pythium root rot and Phytophthora blight. While Pythium root rot is caused by several different species of Pythium with different temperature optima – cool to hot, Phytophthora blight is only favored by hot weather. Periods of hot, rainy weather following a cool wet spring can be a predictor of future problems with these diseases.

Young pepper plants killed by Phytophthora blight

Pepper plants (background) stunted by Pythium root rot

Phytophthora and Pythium are soilborne oomycete pathogens, also called water molds, that thrive in rainy weather. They produce motile spores (zoospores) that are attracted to plants, then form a structure that allows them to infect, and aggressively attack any type of plant tissue. Zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight and Pythium root rot are often seen first in low spots or other poorly drained areas of production fields, but also occur on well-drained, even sandy soils if the environmental conditions are right. While Pythium root rot is not as explosive as Phytophthora blight, both must be managed preventatively.  Pepper varieties partially resistant to Phytophthora blight are available and should be used in fields with a history of this disease. There are no varieties with identified resistance to Pythium root rot. Cultural practices including crop rotation, good drainage, raised beds, avoiding surface water for irrigation, and sanitation should be used – see details here.

During the growing season, fungicide application is the main option for management of Phytophthora blight. Andy Wyenandt (Rutgers University) published a really nice piece on Phytophthora and Pythium control in peppers in April (https://plant-pest-advisory.rutgers.edu/phytophthora-control-during-wet-weather-3/). Fungicides must be applied preventatively for maximum benefit. Keep in mind that:

  1. Orondis Gold premix contains oxathiapirolin, which is very effective against Phytophthora blight (but not Pythium) and mefenoxam, which is effective against both Phytophthora and Pythium.  However, if mefenoxam (Ridomil Gold) or metalaxyl products have been used for a number of years in the same field, the Phytophthora population may be resistant.  We have found mefenoxam/metalaxyl-resistant Phytophthora capsici in Ohio in recent years. Orondis Gold can be applied through drip and in transplant water.
  2. Ridomil Gold can be applied to peppers as a soil spray or via drip, but not in transplant water. Under some conditions peppers can be severely damaged and unlikely to recover.
  3. The active ingredient in Orondis, oxathiapiprolin, does not move well through the soil profile. Our research has not shown a benefit of using Orondis as a soil application vs. foliar sprays. I recommend “saving” Orondis Ultra for foliar application when the weather is continuously conducive for Phytophthora blight.
  4. Elumin is a newer product for Phytophthora blight and application through drip or soil spray at transplanting is labeled, as well as foliar sprays during the season. Pythium root rot is not on the label for peppers but is labeled for Pythium in potatoes and related crops.
  5. Like Elumin, Ranman and Presidio are labeled for Phytophthora blight management in pepper, and not Pythium root rot; however, they are labeled for Pythium management in other crops.
  6. For Previcur Flex, Pythium root rot is on the label for peppers, but Phytophthora blight is not.
  7. The phosphites like ProPhyt and others are labeled for both Phytophthora and Pythium and are systemic.  The ProPhyt label allows drench application at transplanting although not in the transplant water per se.  However, it can be drenched onto seedlings prior to transplanting. The phosphites are good supplemental products but will not control Phytophthora blight alone. They should be used in tank mixes or rotated with products listed below.

Growers have a lot of choices, but if wet conditions continue and both Pythium root rot and Phytophthora blight are a concern:

  1. If Ridomil or related products have been used routinely on the farm or Phytophthora is known to be resistant to mefanoxam/metalaxyl, peppers should be treated with a soil application at or near transplanting with Ranman, Elumin or Presidio, followed by foliar applications in a rotation that includes Orondis Ultra, Presidio, Elumin or Ranman. These may be tank-mixed with a phosphite product.
  2. Keep in mind that a number of products such as Orondis Gold, Orondis Ultra and Elumin have strict use limitations – e.g. two applications per season. Check the label.
  3. Always rotate fungicides with different modes of action (FRAC codes):

Ridomil Gold: 4

Orondis Gold: U15+4

Orondis Ultra: U15+40

Elumin: 22

Presidio: 43

Ranman: 21

Previcur Flex: 28

Phosphite products: 33

 

 

OSU Plant Disease Diagnostic Services for Commercial Vegetables Will Resume April 13

Septoria leaf spot on tomato transplants

The OSU Vegetable Pathology Lab will resume plant disease diagnostic services in Wooster on Monday April 13 on a limited basis (see below). This service is provided at no cost to Ohio commercial vegetable growers. Dr. Francesca Rotondo will carry out the usual diagnostic procedures during the limited time she is in the lab. If possible, please contact us with digital samples before sending physical samples:

  1. Send photos or short videos that represent the problem via text or email to Dr. Sally Miller (330-466-5249/ miller.769@osu.edu) AND Dr. Francesca Rotondo (865-640-5621/rotondo.11@osu.edu).  If we can’t diagnose the problem from the digital samples we will ask you to send a physical sample.  Growers who can’t access/don’t use digital technology should call Dr. Miller or Dr. Rotondo.
  2. No drop-off samples can be accepted. Ship samples Monday through Thursday via overnight delivery (USPS, FedEx, UPS or other courier) to the OSU Vegetable Pathology Lab, c/o Dr. Francesca Rotondo, 234 Selby Hall, 1680 Madison Ave., Wooster, OH 44691 (330-466-5249).

Please see the Vegetable Pathology Lab website before sending samples, for more information and to access the Plant Sample Submission Form.

2019 OSU Vegetable Pathology Field Research Results Available

2019 has come and almost gone, and the results of our vegetable disease management field trials can be downloaded here: OSU Vegetable Disease Management Research Reports 2019. It was a tough year for research, with flooding in some research plots and a surprising lack of disease development in some others. I will summarize some of the more interesting trials in future posts.  These are the report titles:

Evaluation of biological and chemical products for the control of bacterial spot of processing tomatoes, 2019

Evaluation of biorational products for the control of bacterial spot of processing tomatoes, 2019

Evaluation of resistance of pepper cultivars to bacterial canker, 2019

Evaluation of resistance of pepper cultivars to Pseudomonas leaf spot, 2019

Integrated management of Phytophthora blight using grafted peppers and fungicides, 2019

Evaluation of resistance of cucumber varieties to downy mildew and Phytophthora fruit rot, 2019

Bioassay for sensitivity of Pseudoperonospora cubensis to fungicides in OH, 2019

Management of Plectosporium blight of pumpkins, 2019

Evaluation of Regalia and Stargus for the control of black rot of cabbage, 2019

Evaluation of biorational products for the control of bacterial leaf spot of cilantro, 2019

Vegetable Disease Diagnostics, 2019