Not Planting to Manage Phytophthora Blight by Reducing Disease Risk

From Clemson Plant Pathologist Anthony Keinath and Clemson Agricultural Economist Felipe Silva.

After an absence of several years, Phytophthora blight reappeared in South Carolina in July 2021 on three farms. As expected, the outbreaks were on two of the three most susceptible vegetable crops: 2 cases on pepper and 1 case on pumpkin. (Summer squash is the other very susceptible crop.)

A white powdery layer of Phytophthora capsici spores covers this pumpkin fruit. A watery rot will soon follow. Photo from Dr. Mary Hausbeck, Michigan State University

Five management options are listed in Land-Grant Press 1014: Managing Phytophthora capsici Diseases on Vegetables. One of the recommendations under Soil Management is to not plant low areas in an infested field, because that is where the Phytophthora blight pathogen, Phytophthora capsici, will first become active. Rain or overhead irrigation will spread the pathogen to the rest of the field. Remember that once a field is infested with Phytophthora capsici, The. Field. Remains. Infested. Period.

Based on the calculations shown in Table 1, a grower that considers the likelihood of disease outbreaks and plants only the well-drained 4.5 acres in a 5-acre infested field would earn an expected average of $1,600 per 5 acres per year. A grower who does not consider the disease occurrence pattern and plants all 5 acres can expect an average loss of over $2,600 per year (see the column labeled “Avg. Net Return”). These estimates consider the likelihood of disease occurrence based on the different planting sizes and areas.

The difference between planting 5.0 vs. 4.5 acres in an infested pumpkin field totals a positive net return of $4,300 per 5 acres. Why does not planting—and forfeiting yield—make more money? Assume that an outbreak of Phytophthora blight reduces the entire farm yield by 50%. Not planting the low area of the field, where disease is likely to start, will cut the number of outbreaks of Phytophthora blight in half (see the column labeled “Disease likelihood”), decreasing the chances of an outbreak from 60% to 30%. This estimate comes from vegetable pathology colleagues in other states who have worked on Phytophthora blight for many years.

Even in an infested field, by reducing the disease risk, the expected net return increases by more than $4,000. Note that over half of the gain in profit comes from reducing input costs by not planting the 10% of the field that probably will not yield anything. Although this example is calculated for pumpkin, the risk of the pathogen spreading from diseased peppers in a low spot in the field is just as likely or greater, because the pathogen produces spores readily on the fruit. Reducing disease risk is the key to increasing profits.

Accidentally Infesting Fields by Moving Soil on Equipment or How to Exclude Soil Pathogens from Fields

From Clemson Plant Pathologist Tony Keinath.

Most growers probably have heard that it’s possible to infest a “clean” (pathogen-free) field by moving soil on equipment. The question is how much infested soil is too much. The answer depends on the pathogen and where the soil is deposited.

Some pathogens are present in soil at very high numbers. One of the worst is the clubroot organism that infects all brassica vegetables. According to a new report from North Dakota, presented at the 2021 Plant Health meeting last week, there are enough spores in 1/8 teaspoon of moist soil to infect one plant. So, if all the soil stuck on a small dozer, like the one shown here, landed in one spot at the edge of the field, a patch of clubroot surely would develop there.

Soil caked up on cultivating implements, dozer tracks, and tractor tires can transfer soil borne pathogens and nematodes from infested fields to clean fields.

In addition to the clubroot organism, root-knot nematodes and the water mold that causes Phytophthora blight are pathogens at high risk of being spread in infested soil. They’re not as concentrated in soil as the clubroot organism, but moving as much soil as is on the dozer above is enough to create a patch of diseased plants. It takes only about 1/3 cup of soil with young nematodes in it to cause a gall on a susceptible crop.

On the other end of the spectrum, since 2005 I have had a field at Coastal REC infested with Fusarium oxysporum f. sp. niveum, the fungus that causes Fusarium wilt on watermelon. As far as I know, the pathogen has not spread, or has not spread enough, to other fields to cause Fusarium wilt, even though the equipment is not cleaned after it is used in the infested field. Even though Fusarium is present in the soil, it’s not at high enough levels to be spread easily.

It’s true that environmental conditions must favor the pathogen for disease to develop from the pathogen inoculum moved in soil on equipment. However, all four pathogens mentioned in this blog are long-term soil residents. They will simply wait in the new field until environmental conditions are right for them to infect a susceptible crop.

The bottom line is equipment, especially dozers that carry a lot of soil with them as they are moved from field to field, must be cleaned by power washing every time they are used in a different field. Power washing should be a standard practice, not just done when the equipment is used in a “known” infested field, because sometimes you don’t know if a field is infested until it’s too late to take precautions.

Preparing for Gummy Stem Blight in Fall Cucurbit Crops

From Clemson Plant Pathologist Tony Keinath.

Gummy stem blight is more common and more severe on fall cucurbit crops than crops grown in the spring. The cooler weather and longer dew periods in the fall provide an ideal environment for the fungal pathogen to grow and spread. Gummy stem blight is most common on watermelon and may also be seen on cantaloupe, cucumber, pumpkin, and winter squash foliage. Butternut squash fruit are susceptible to black rot, the fruit rot phase.

Gummy stem blight lesions on watermelon foliage.

All growers—conventional and organic—should follow two proven steps to eradicate (eliminate) the gummy stem blight fungus from infested fields.

  1. Rotate away from all cucurbit crops for 2 years to allow time for the gummy stem blight fungus to die out in infested crop debris. The timeline starts when the first (diseased) crop is disked. It takes a full 24 months for 90% of the debris to decay under South Carolina weather conditions.
  2. Promptly disk cucurbit crop debris after harvest to stop the spread of airborne ascospores from fruiting bodies that form on vines, stems, crowns, petioles, tendrils, and leaves. Burying crop debris helps it decay faster.

Four fungicides provide good control of gummy stem blight on watermelon, the most susceptible cucurbit grown in the fall: Miravis Prime (FRAC Codes 7 + 12), Switch (FRAC Codes 9 + 12), Inspire Super (FRAC Codes 3 + 9), and Luna Experience (FRAC Codes 7 + 3). Note that because these fungicides share active ingredients in FRAC groups 3, 7, 9, and 12, the only products that can be rotated with each other are Miravis Prime and Inspire Super. Another option is to rotate a generic formulation of tebuconazole (FRAC Code 3) with Miravis Prime or Switch. None of these fungicides controls downy mildew or anthracnose. See Watermelon Fungicide Guide for 2021 for a sample spray program for fall watermelon that covers all major foliar diseases.

What is NOT Proper Crop Rotation

From Clemson Plant Pathologist Tony Keinath.

There seems to be some confusion about what is a proper crop rotation to manage diseases caused both by pathogens that survive a few years in soil and pathogens that survive in soil longer than anyone can measure. Here are some rotations I have heard about that are not proper crop rotations.

1. A susceptible crop in year 1 and a different susceptible crop in year 2. Example: Pepper followed by tomato or tomato followed by pepper in a field infested with Phytophthora capsici, the water mold that causes Phytophthora blight. Simply switching between two susceptible crops is not proper crop rotation.

2. A susceptible crop in the spring and another crop in the same plant family in the fall. Example: Watermelon in the spring and cucumber in the fall on the same plastic. There are too many potential disease problems with this crop sequence to even mention all of them. Gummy stem blight, root knot, and Phytophthora blight are among the top three.

3. Multiple plantings of a short-season crop in the same field in the same year, then rotating the following year. Example: pickling cucumbers. The same comments made in #2 apply here. Root knot nematodes on an early summer crop will infect a crop planted later in summer.

4. A susceptible crop in the fall and a different susceptible crop in the spring in the same field. Although I don’t have a specific example this time, the short break over winter is not long enough to reduce pathogen levels.

In general, a proper crop rotation is a sequence of crops that are in DIFFERENT plant families. Although not every disease affects all plants in the same family—e.g. early blight on tomato and potato but not on eggplant or pepper—enough diseases do affect closely related vegetables that it is best to avoid planting them too often right before or after each other.

Downy Mildew on Watermelon Found in SC

From Clemson Plant Pathologist Tony Keinath.

Downy mildew was found on watermelon this week in Allendale and Barnwell counties. Although downy mildew does not infect fruit, it reduces sugar content once 1 in 4 leaves (25%) are infected.

Downy mildew symptoms on the underside of a watermelon leaf.

All watermelons should be sprayed with a fungicide effective against downy mildew. See pages 214-215 in the 2021 Southeastern U.S. Vegetable Crop Handbook. Any fungicide listed there can be used except Aliette, Previcur Flex, or Curzate. Do not use these fungicides to manage downy mildew on watermelon, as the isolate on watermelon in 2020 was resistant to them. Gavel, Ranman, and Elumin are the least expensive choices. Growers should apply a downy-mildew specific fungicide this week, a protectant (chlorothalonil or mancozeb) next week, and repeat this sequence until one week before the last harvest. See the following publications for more info on watermelon disease management:

Powdery Mildew on Watermelon

Cucurbit Downy Mildew Management

Watermelon Fungicide Guide

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.