Cucurbit Downy Mildew Found In SC

Cucurbit downy mildew was found in SC this week in Bamberg, Barnwell, and Calhoun Counties. In each case it was found on cucumbers and for now severity seems low. This is about two weeks earlier than in the past couple years.

Downy mildew symptoms on cucumber. Lesions are often limited by the veins in the leaves.
Dark-colored downy mildew spores developing on the underside of cucumber leaves.

If not already doing so, all cucumber and cantaloupe growers in SC should begin applying protective fungicides to help prevent or manage downy mildew. Ranman tank-mixed with a protectant, such as chlorothalonil or mancozeb, or applications of Zampro are good options for protecting plants prior to symptom development. For more info, see Dr. Tony Keinath’s CDM Management publication.

Fusarium Wilt in Watermelon

From Clemson Plant Pathologist Tony Keinath.

Fusarium wilt is showing up in watermelon fields and in research plots at Coastal REC in Charleston. The most telltale symptoms are wilting of a few leaves at the crown of the plant, wilting of one vine on a plant, or wilting without yellowing of a small plant.

One vine of this watermelon plant has wilted. This is a telltale sign of fusarium wilt.

A good field diagnostic trick is to cut a wilted vine close to the crown, split it open lengthwise, and look for reddish brown spots on the crosswise cut or streaks in the lengthwise cut.

Cross section of a watermelon stem showing the discolored, reddish brown spots.

The Fusarium fungus is most active when the soil temperature is below 81 degrees F. Although daytime temperatures were warm in April, the nights were still cool enough to allow infections. At this point, there is nothing that can be done to mange Fusarium wilt. All successful management practices must be done before transplanting. See: Keinath AP. Integrated Management for Fusarium Wilt of Watermelon. Land-Grant Press by Clemson Extension. 2019; LGP 1022.

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.

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.

Downy Mildew found in SC

From Clemson Plant Pathologist Dr. Tony Keinath:

The first SC report of cucurbit downy mildew this year came on June 6 from a crop consultant, who found it on cucumbers in Bamberg County. Growers should spray all cucumber and cantaloupe crops to prevent or manage downy mildew. The cheapest downy mildew fungicide is Ranman. It can be tank-mixed with chlorothalonil, mancozeb, or tebuconazole to add protection against fungal leaf spots, like gummy stem blight and anthracnose, that will start to spread with the rain. Another option is Orondis Opti, a pre-mix of Orondis and Bravo (chlorothalonil). Watermelon growers should be spraying with protectants, as downy mildew has been spotted on watermelon in south Georgia.

Downy mildew lesions on cucumber leaf. Note how they are delineated by the veins in the leaf. Photo from Dr. Tony Keinath.

For more info on downy mildew management in cucurbit crops, refer to this fact sheet.

Testing What Controls Downy Mildew in Cucumbers – and What Doesn’t

From Clemson Plant Pathologist, Dr. Tony Keinath.

Small growers can manage downy mildew on slicing cucumber and increase yields by growing a partially resistant cultivar, like Bristol, and spraying weekly with moderate-cost fungicides, like chlorothalonil rotated with Ranman. Trellising doesn’t help manage downy mildew. Read Dr. Keinath’s full article about testing cultural and chemical controls for downy mildew here.

The cucumbers ‘Speedway’ (front) and ‘Bristol’ (back) in non-trellised plots with foliage yellowing due to downy mildew on June 23, 2017. Photo by Anthony P. Keinath

American Foulbrood Found in Bee Hives in SC

The Department of Plant Industry (DPI) is informing the South Carolina Beekeepers Association (SCBA) Members that we have official confirmation from the Clemson DPI Bee Lab in Pendleton of American Foulbrood (AFB) in Pickens County. To be more specific, AFB was found near the town of Pickens and the positive AFB hives were removed from a small apiary there. We hope beekeepers will notify the Department if they are experiencing problems, especially in the affected area. This cooperation is very important since South Carolina does not require beekeeper registration.

In addition to the SCBA notification, the surrounding local associations in the affected area have been notified as well.

Below is a map of the affected area. Beekeepers should email or call if they are experiencing any issues.

AFB found in hives near the town of Pickens, SC.

This notification is provided to protect the beekeeping industry of South Carolina.

  • Clemson University
  • Department of Plant Industry Apiary Inspection
  • Brad Cavin
  • scavin@clemson.edu
  • 864-596-2933 ext.113