Are Hydrogen Peroxide Products Effective Fungicides?

From Clemson Plant Pathologist Dr. Tony Keinath.

Should products with hydrogen peroxide, alone or combined with peroxyacetic acid, be used like fungicides on vegetables? It’s difficult to give just one answer to this question because there are so many different vegetables and diseases to consider. Here are a few important things to think about.

  1. Hydrogen peroxide/peroxyacetic acid has no curative activity against any vegetable disease. Yes, it might look reassuring to see the dead centers drop out of leaf spots on tomato. The pathogen, however, is still in the leaf, where pathogens are naturally designed to live. All leaf pathogens—bacteria, fungi, and water molds—will survive inside the treated leaves.
  2. Hydrogen peroxide/peroxyacetic acid has no (or a very short) residual activity or “staying power” on leaves after spraying. Conventional fungicides usually will last up to the minimum spray interval on the label, normally 7 days. Biofungicides also leave a residue on the leaves for at least a few days after spraying; there hasn’t been as much research on this as on conventional fungicides. The fact that labels like Oxidate recommend two applications per week suggests that the residue lasts no more than 3 days.
  3. Oxidate did not control powdery mildew on cucurbit seedlings in the greenhouse. (Details available at https://doi.org/10.1016/j.cropro.2012.06.009). There was absolutely no effect in any of four experiments. Unlike other fungi, powdery mildew grows mostly on the outside of the leaf, so it was completely exposed to Oxidate.
  4. The number of sprays per week needs to be considered when calculating the cost of hydrogen peroxide/peroxyacetic acid products. At $35/gallon and 1% solution, Oxidate costs $35/acre when sprayed twice a week at 50 gpa spray volume each time.

I recently tested Oxidate on kale affected with Alternaria leaf spot caused by the new species in South Carolina, Alternaria japonica. Oxidate (1%) was applied 2 days before harvest. Healthy leaves with no visible leaf spots were placed in sealed plastic bags with 100% relative humidity (RH) and stored at 41 F. for 1 week, then checked for disease symptoms. Based on two tests, 70% of Oxidate-treated leaves had Alternaria leaf spot, and 73% of water-treated control leaves did. That’s only a 5% improvement, and the difference is not statistically significant. Based on a kale price of $18/carton and 400 cartons/acre (for a once-over harvest), that’s an extra $359/acre.

Alternaria leaf spots that appeared a week after this kale was harvested and stored at proper storage temperatures. Photo from Dr. Tony Keinath

Oxidate might have some benefits sprayed immediately before harvest. After harvest, leaves aren’t exposed to more pathogen spores from the air, so the short residual time (hours) isn’t as limiting. Storing produce at less than 100% RH also might have made a difference with less disease overall.

For more information on Oxidate and vegetable diseases, see this list of trials with biofungicides approved for organic use with brief comments on efficacy: https://rvpadmin.cce.cornell.edu/uploads/doc_582.pdf.

Weekly Field Update – 3/15/21

If you haven’t already filled out the Clemson Agribusiness Team’s COVID-19 Ag Impacts Survey, please take a minute to do so now. Click the graphic below or scan the QR code with your iPhone to access the survey. If you have any questions about the survey or the results, please reach out to Kevin Burkett, Kburke5@clemson.edu, 540-239-4602.

Coastal

Rob Last reports, “Strawberry crops in the area are developing well with good fruit set. I am seeing a little gray mold around, so sanitation is going to be key as well as fungicide applications. There are also a few thrips in some crops, so scouting for these pests will be very important. Peaches and blueberries are blooming with little evidence of any chill injury from last weeks overnight lows in the upper 20’s. Asparagus crops are beginning to come to market with some chill affected spears early last week. With the dry weather conditions, field work and land prep is going well with plastic being laid for melon crops.”

Zack Snipes reports, “Great week of weather in the Lowcountry which has really improved the way things are looking. Greens, lettuces, root crops, and strawberries are looking good. The strawberries are absolutely loaded up with blossoms right now and are still putting on crowns. Fall planted cover crops are finishing up and being turned under. A bit of sad news this week as Author “Ikee” Freeman passed away. Ikee was a huge supporter of Clemson Extension and the farming traditions in the Lowcountry of South Carolina. His friendship, mentorship, leadership, and sense of humor will be missed by many.”

A fall planted daikon radish cover crop is flowering which provides early season nectar, pollen, and habitat for beneficial insects and bees. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “We had another beautiful week of weather last week that really pushed plants along. Strawberries have a ton of blooms on them now. The weather has been pretty dry the last two weeks, but with a few days of rain in the forecast for this week, now would be a good time to throw one of the site specific fungicides into your fungicide rotation. Temperatures are still in the perfect range for Botrytis spore development (60-70 degrees F) and with moisture returning, we could see a lot of Botrytis in the next week or two.

Lots of blooms present now that we need to protect from Botrytis. Photo from Justin Ballew.
These are the site specific fungicides listed on the MyIPM app with the best efficacy for Botrytis control on strawberries. It would be a good idea to apply one of these ahead of the rain this week.

Sarah Scott reports, “Last week’s warm temperatures have pushed peaches into bloom. There was a significant increase in open buds from the beginning to end of the week. Early varieties are near 90% bloom. Temperatures look like they will level out in the next 7 days which is good as temperatures that are way above normal can lead to developmental damage for fruit set. Some pruning is still underway, as well as bloom sprays for blossom blight using products like Captan or chlorothalonil.”

Upstate

Andy Rollins reports, “So far peach season is off to great start. We got all of our chill hours in and are between pink and 5% blooms on all large scale varieties, with a few oddballs further along. Plum growers are even further along and many are preparing for bloom fungicide applications with primarily Bravo (Chlorothalonil). I will be repeating last years testing of new biological, Ecoswing, in bloom this week compared with Bravo. Strawberry production is progressing nicely. This week looks good as far as frost but things change quickly, so all growers will be intimately aware of every weather change at this point. I was very happy to help with a new 10 acre pecan orchard planted last week. It encouraged me because several local growers, University of Georgia specialist, a retired NRCS conservationist, and myself all worked together to help a widow in need. Her planning and execution was impeccable and diligent.”

10 acres of new pecan trees planted last week. Photo from Andy Rollins.

Captan Could Be Scarce in 2021

From Clemson Fruit Pathologist Dr. Guido Schnabel and UGA Fruit Pathologist Dr. Phil Brannen.

Unfortunately, there appears to be a shortage of Captan products this year. For our fruit crop producers this may result in a change of strategy if and when reserves run out. Strawberry growers fortunately have the option to use Thiram for basic gray mold and anthracnose control. The two products are basically equal in efficacy but thiram has a slight edge over captan for gray mold control, but it is a bit weaker against anthracnose.

Peach growers rely on captan for cover sprays. These early to mid-season applications manage green fruit rot and possibly anthracnose. Fortunately, during bloom and preharvest we use different chemistries at our disposal listed in the spray guide. As we mention often, we prefer to hold other chemical classes, those for which Monilinia fructicola (brown rot) is most likely to develop resistance, for use in the pre-harvest sprays. During captan shortage, we recommend application of two applications of a QoI/SDHI (FRAC 7+11) product, such as Luna Sensation, Merivon, Pristine or Quadris Top (FRAC 3+11), for cover sprays as needed this year. Along with any remaining captan products, we hope that this will help us to limp along till next year, while still providing excellent control of cover spray pathogens. Sulfur alone in cover sprays, though sufficient for scab control, is not efficacious for green fruit rot or other diseases; if using a QoI/SDHI (7+11) product, these will control scab as well as green fruit rot, so sulfur would not be necessary for these applications. As always, please contact your local extension agent should you have questions.

Downy Mildew Found on Watermelon in SC

Downy mildew was found yesterday, June 17, 2020, in one watermelon field in Bamberg County, South Carolina. All growers should immediately spray watermelon with Ranman, Revus, or Gavel to protect their crops from downy mildew. In addition to direct yield loss, loss of vine cover can expose fruit to sunburn (when the sun comes out again). Growers who find downy mildew in a field should apply Orondis Ultra or Orondis Opti in a weekly rotation with Ranman or Gavel. For more information on downy mildew, see the updated Cucurbit Downy Mildew Management for 2020.

2015-07-13 DM on WM

Downy mildew symptoms on watermelon foliage. Photo from Dr. Tony Keinath.

Alternaria Leaf Blight Common This Year

From Clemson Extension Plant Pathologist Tony Keinath.

During the 2019 Cantaloupe Disease Survey, Alternaria leaf blight has been found in several fields. It was more common in fields that had not been sprayed recently than in fields sprayed on a regular schedule. It also was found in a field rotated only one year out of cantaloupe.

ALB cantaloupe.JPG

Alternaria leaf blight lesions on cantaloupe leaf.

The FRAC Group 11 fungicides (Cabrio, Quadris, Flint) are the recommended fungicides. Although the gummy stem blight fungus is resistant to this group of fungicides, they still are very effective against Alternaria leaf blight on cantaloupe and anthracnose on watermelon.