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.

Weekly Field Update – 8/23/21

If you haven’t already done so, please take a few minutes to fill out a survey share your thoughts with us on Extension meetings. We’ll be using the information collected to help plan meetings over the next year. It will take less than 10 minutes and is anonymous. Click here to get started. Thanks!

Midlands

Justin Ballew reports, “Last week was warm and humid again. Some of our sandier fields got dry enough early in the week that crops were wilting in between waterings. We got a bunch of rain over the weekend, though (a little over 2 and a half inches at my house). Overall, fall planted crops are coming along nicely. Some of the earliest planted fall squash and zucchini is being picked now. We’re still seeing the same disease problems that have plagued us all summer, though growers seem to be managing them fairly well. As far as caterpillars go, I’m seeing mostly diamondback moth and armyworms with a few loopers here and there. Get ready. This could be a high pressure fall for caterpillars.”

Even though we’ve had lots of recent rain, it doesn’t take long for the sandy soil in Lexington to dry out and let the plants wilt. Photo from Justin Ballew.

Phillip Carnley reports, “Not much has changed here in Orangeburg or Calhoun Counties. Its been hot and humid and we’ve had a few untimely showers delaying cucumber harvests. There is still plenty of downy mildew to go around. We are seeing loopers on pickling cucumbers that are ready for harvest. At that stage the loopers should be treated prior to harvest with Coragen, Harvanta, or Radiant. Fall brassica, peas, and tomatoes are just now being planted.  We are seeing increased amounts of scab in pecans this year, due to the rain and humid weather. For insect and disease management in pecans, have a look at the UGA Commercial Pecan Spray Guide.”

Caterpillar damage to fall cucumbers. Photo from Phillip Carnley
Loopers are causing some damage to fall cucumbers in the Orangeburg area. Photo from Phillip Carnley.

Upstate

Andy Rollins reports, “Excessive rains have caused cracking of fruit at several upstate muscadine farms. Powdery mildew is also present but I’m not sure how much of a role it is playing on the cracking part. The powdery is damaging the skin of the fruit. Topsin M is labeled and recommended with Captan but have to wait 7 days to pick so have to watch your PHIs. Also finding some insect pests in peach. Oriental fruit moth and sap beetles have been found last week but only a small amount of affected production. Herbicide control has been difficult this year because all the rain has caused excessive grass growth, especially in new orchards.”

Seeing some cracking of the skins in muscadines in the upstate. Photo from Andy Rollins.
This new grower has done a good job of keeping his understory clean. This will pay off for him in the spring of next year if he can keep up the diligent work. Photo from Andy Rollins.

Input Wanted for Fruit and Vegetable Meeting Planning

Clemson Extension has created a short, anonymous survey to help plan for future commercial fruit and vegetable meetings. The survey will take less than 10 minutes to complete and no personal or identifying information will be collected. Your responses will be used to make Extension meetings more useful and enjoyable for those who attend. Please take a few minutes to share your opinions with us.

Click here to complete the survey.

Weekly Field Update – 8/16/21

Coastal Region

Zack Snipes reports, “Its hot and humid in the Lowcountry. Fall tomato and watermelons are in the ground and enjoyed a week of mostly dry weather. Okra and sunchokes, aka Jerusalem artichokes, are loving this heat. With the exception of those crops, there aren’t too many crops in the ground right now.  I am seeing lots of summer cover crops. I love the idea of using a mixed species of cover crop. One reason is, it spreads out the risk that one of the species in the mix won’t germinate or will be eaten by deer. So by using multiple species, you can almost guarantee that something will be there covering the soil. Multi-species mixes also provide different benefits to the farm. Cowpeas may fix nitrogen while sorghum X Sudan hybrids may be a deer deterrent and shade out weeds.”

A beautiful mixed-species stand of cowpeas and sorghum X Sudan hybrid (sudex). Photo form Zack Snipes.
Buckweat is one of my personal favorites because of its ability to attract beneficial insects and mine potassium from the soil. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “Not much has changed here in the midlands over the last week. Its been warm and humid and we got a little rain a couple times throughout the week. Recently planted fall brassica and cucurbit crops have gotten off to a good start. We are seeing some caterpillar activity already. We are seeing increased amounts of scab in pecans this year, thanks to the rain and humid weather. I’m seeing some black pecan aphids causing damage as well. For insect and disease management in pecans, take a look at the UGA Commercial Pecan Spray Guide.”

Pecan scab is really showing up this year. If growing cultivars with poor scab resistance (Desireable, Pawnee, Kiowa, Oconee), stay on top of fungicide applications. Photo from Justin Ballew.
Black aphid damage on pecan foliage. Photo from Justin Ballew.

Phillip Carnley reports, “It’s that time of year to be on the look out for Southern stem blight (Athelia rolfsii) in hemp. To avoid possible infections of this pathogen, avoid planting in fields that have previously had tomatoes, peppers, eggplant, or peanuts. Crop rotation is your best avenue for mitigation as there are no fungicides labeled for control of Southern blight in hemp at this time. Also be on the lookout for plant stunting from girdling roots. For information on chemicals labeled for hemp checkout the EPA website.”

Southern blight mycelium (white fuzzy looking growth) and sclerotia (small, tan bb-shaped structures) developing around the base of a hemp plant. Photo from Phillip Carnley.
This hemp plant has wilted due to Southern blight cutting off its ability to transport water from the roots to the shoots. Photo from Phillip Carnley.

Upstate

Kerrie Roach reports, “Late summer season crops are still producing in the upstate, but many market producers are starting to lose the disease battle. When something is finished for the season, make sure to remove all parts of the crop. Do not leave diseased plant material in the field. Dispose of the diseased crop in an area far away from your fields or garden. Do not use diseased plant material for compost. Most home compost piles do not reach consistent and uniform temperatures at which pathogens will be killed. Dead plant material harbors insects and pathogens that can and will cause issues for fall season crops as well as next year’s crop.”

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.

Weekly Field Update – 8/9/21

Coastal Region

Rob Last reports, “Generally speaking, we are in the change over period from spring to fall crops, with some fumigants being applied to next year’s strawberry plantings. If fumigants are to be used, make sure soil moisture is good and beds are firm to gain maximum efficacy. One thing we have observed in blackberries and blueberries in the area is the emergence of bark scale. Bark scale is a new pest to South Carolina and has previously been noted in ornamentals. However reports for Asia, where the pest is native, indicate the bark scales can survive on Rubus species.

Bark scale egg sacks and adults. Photo from Rob Last.

The images show the egg sacks and adults of the bark scale. The insects appear white and are very waxy, similar in appearance to mealybugs. The wax coatings can make management complex, preventing insecticides from penetrating the layer to be effective against the insects. The addition of crop oils to the pesticide solution may enhance efficacy by helping to strip off the waxy coatings. In blueberries, adults can be found underneath the exfoliating bark, again making contact with the insecticides more difficult. When crawlers emerge, they will be pink and barely visible to the naked eye, and monitoring will be easier with the aid of a hand lens. We will find crawlers hatching in April or May, with a second flush emerging in late summer. An excellent way to detect crawler movement is to wrap the branches in double-sided sticky tape to help to catch the crawlers. Contact insecticides can be very effective against the crawlers. Insect growth regulators may also be effective for management. Unfortunately, systemic materials such as imidacloprid appear to have little effect. Please get in touch with an Extension Agent to help with identification and management options.”

Bark scales have a waxy coating that helps protect them from insecticides. Photo from Rob Last.

Zack Snipes reports, “It’s wet in the Lowcountry. We are getting heavy rain showers what seems like everyday. This is making it hard to get equipment in fields to spray or prepare for fall plantings. I have seen some watermelon and tomato fields and they look ok considering the rain and soggy conditions. I saw some bacterial spot on pepper and expect to see the same on fall tomato with the rain and humidity we’ve had. Get out your preventative fungicides, if you can. I also saw some leafminer damage on these crops which is unusual in my tenure. It seems plants were impacted a few weeks ago but the new growth looks to be unaffected. Melonworms were found in cucumbers, so get ahead of them.”

Leafminers are showing up on tomatoes on the coast. Photo from Zack Snipes.
Melonworms were found in cucumbers this week. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “This past week was a relatively nice, mild week compared to the week before. We had some rain to start the week and more to finish it out. Planting of fall crops continues and what’s been planted seems to be growing well. I looked at a few fields this week of seedling brassicas and I’m already seeing diamondback moth caterpillars and armyworms feeding. Remember to start scouting as soon as plants go into the ground. It doesn’t take many caterpillars to eat up seedlings and small transplants. Don’t forget we can run field bioassays to screen for insecticide resistance, so call us when you start seeing worms.

Diamondback moth caterpillars are already showing up on fall brassicas. Photo from Justin Ballew
It doesn’t take many caterpillars to eat up these small plants. Photo from Justin Ballew.

Pee Dee

Bruce McLean reports, “Vegetable crops are still harvesting well, for the most part. Other than cucurbit downy mildew affecting cucumbers and some powdery mildew affecting squash, there are no widespread vegetable diseases seen in the fields. Cowpea curculio is still widespread on peas and must be intensively managed (starting prior to bloom) to minimize damage. Spotted Wing Drosophila is still very active in late-season blueberries, with trap counts showing very high capture numbers. Grape root borers (GRB) emergence is starting to increase in muscadines. Trap captures for GRB is on the rise. Much of the state is outside the window of Chlopyrifos application for GRB… except on late harvesting cultivars (more than 35 from application to harvest) and maybe some wine grape cultivars in the Upstate. Pecan weevil emergence is just getting started (in pecans). Ground and lower canopy application of Carbaryl and/or a trunk application of Tanglefoot are effective methods of control. Monitor traps and weevil movement through September (especially after rains). Re-treatment of Carbaryl will be necessary and can be reapplied at (up to) 7 day intervals. Pecan scab is becoming more evident in pecan orchards. Also, fall armyworm numbers have exploded over the last two weeks. This is a pest that can affect a wide variety of crops.

Who is spitting in my muscadine vines? That is not actual spit. It is a sticky, frothy substance produced by the spittlebug for protection from predation. Photo from Bruce McLean.
This tiny insect is the spittlebug (what is inside the frothy spittle on muscadines and other plants). They feed upon the foliage of the muscadine vine and do not cause any noticeable damage to the plant. Photo from Bruce McLean.
Grapevine aphids populations on muscadines can be quite significant in late summer. They feed primarily on the foliage and late flower clusters. They seldom require chemical management except when present during spring bloom. Heavy rains and natural predation usually keep them in check. If extended periods of dry weather occur and their feeding does lead to noticeable problems (or if honeydew and sooty mold become evident on fruit and leaves) an insecticide application may be necessary. Photo from Bruce McLean.

Weekly Field Update – 8/2/21

Coastal Region

Zack Snipes reports, “Summer crops like okra are still coming in and looking good. We’ve had a lot of rain and some fields are soggy. More rain is coming. Some growers have started planting peppers and tomatoes. Remember to get out in the fields and destroy spring crop residue. Nematodes and other pests can really thrive on that old residue.”

Root knot nematode infestation on tomato roots. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “Last week was by far the hottest week we’ve seen so far this summer. It’s been a very mild summer, so last week was actually a reminder of what “normal” is here. Tomatoes, squash, zucchini, cucumbers, and sweet corn are still being harvested. More fall brassicas, cucurbits, and some tomatoes were planted this past week. I haven’t heard of any reports of serious caterpillar activity yet, but remember to start scouting as soon as you plant. It doesn’t take long for caterpillars to significantly damage brassica seedlings.”

We’re still picking some good looking sweet corn in the midlands. Photo from Justin Ballew.

Upstate

Kerrie Roach reports, “Things in the Upstate, like much of the rest of the state, have been particularly hot and dry this last week. Irrigation has been of utmost importance as well as mitigation of disease and insects. As we move later in the season, things are starting to slow down, but now is the time to start prepping and planning for any of those fall plantings. For our smaller market growers, season extension by utilizing fall crops can be a great addition. Many farmers markets are looking for growers to sell in the early fall, and competition is slim, often making sales easier. Do your homework BEFORE plating and start looking now at local markets ending dates, vendor loads, customer preferences, and plan accordingly. Check out the SE Veg Crop Handbook for fall planting dates from many of your favorite crops. 

SC Farmer Named to Fruit Grower News’ 40 Under 40

From the Fruit Grower News.

Corey Harmon of Titan Farms in Ridge Spring, SC has been named to the Fruit Grower News’ 40 under 40 Class of 2021. The Fruit + Vegetable 40 Under 40 Awards honor 40 outstanding individuals making their marks in the industry.

Corey Harmon of Titan Farms. Photo from Fruit Grower News.

Corey has been with Titan Farms since 2015. In that time, he has taken over as vegetable manager and has taken great pride in trying to advance the program through trial partnerships with the mindset of a more sustainable future for agriculture. As a second-generation farmer, he appreciates that every day is different and that every obstacle is a new opportunity to learn new management techniques and practices. Corey is also a huge supporter of Clemson Extension, with whom he has partnered on numerous research trials.

These 40 young professionals represent the best in the industry. The Fruit + Vegetable 40 Under 40 Class of 2021 will be honored at the Great Lakes Fruit, Vegetable & Farm Market EXPO, and recognized in the October 2021 issues of Fruit Growers News and Vegetable Growers News.

Congratulations, Corey!

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.

Weekly Field Update – 7/26/21

Statewide

The SC Specialty Crop Association is offering a new grant opportunity, the Enhancing Crop Packaging Cost-share Program. With this new cost-share program, growers can receive reimbursement up to $1,800 per grower for packaging needs. All that is required in addition to the application are copies of receipts used for purchasing packaging materials. You will also be required to fill out two surveys, one initially and one 12 months after submitting the application. All information is confidential. For more information, contact LauraKate McAllister. The application can be downloaded below.

Coastal

Zack Snipes reports, “We are in a summer weather pattern with warm, muggy days and occasional thunderstorms. Most crops have finished up or are in the process. Now is a great time to sit down and do some crop planning and field rotation planning. I collected many soil and root tissue samples lately and had them analyzed for nematodes. I was surprised at how many nematodes were present in the fields. Nematodes can interfere with growth, cause stunting, and lower overall yields. Sometimes the symptoms of nematodes can be very discrete so sampling right now is the best way to get a baseline of your populations and how to properly manage and rotate fields. If left unchecked, thousands of dollars are wasted before the first seed is planted into a field.”

Significant galling from root knot nematodes on a cucumber seedling. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “It was another fairly mild week with high humidity and some pretty decent rain. Not much has changed on the disease front. We’re still seeing plenty. Growers are still prepping fields for planting fall crops. Some fall cucurbits and brassicas have been planted already. More are on the way. As soon as brassicas go in the ground, start scouting for worms. Remember, we can perform bioassays to screen for insecticide resistance in diamondback moths populations. Reach out to your local fruit and vegetable agent when you start seeing worms to schedule one.”

Bacterial spot is common on tomatoes right now. Photo from Justin Ballew.
Recently planted kale is growing well. Start scouting for worms as soon as you plant brassica crops. Photo from Justin Ballew.

Pee Dee

Bruce McLean reports, “Vegetable crops are harvesting well, with good volumes of squash, zucchini, cantaloupe, watermelon, cucumber, butterbeans, peas, tomatoes and okra. Sweet corn is beginning to wrap up. Late season blueberries are still being harvested in some volume, but will be finishing soon. Muscadines are sizing well. Vineyards that were only slightly affected by the Easter freeze are looking good and should have a good crop. Vineyards that were more significantly affected by the freeze are very short on crop this year. Grape root borer traps in muscadine vineyards are starting to catch moths in all locations. Spotted wing drosophila (SWD) trap captures (in blueberries) have dramatically increased over the past few weeks, showing that even in late season when fruit is becoming less and less plentiful, the fly is still very active and must be managed.”

Bucket trap baited with the Grape root borer (GRB) pheromone lure in muscadines. Photo from Bruce McLean.
Where we did not see significant damage from the Easter freeze, there is a good looking crop of muscadines. Photo from Bruce McLean.

Upstate

Andy Rollins reports, “I identified a major scale problem on peaches. A grower from middle part of state called about red spots on peaches. Earlier in the year across the whole state we had red spots on leaves. We found prunus necrotic ring spot on all of those samples last year but we are still unsure of the origin. In this case, it is something much different. This is an insect that feeds on the fruit and the tree itself. The adult stage of this insect doesn’t move but the crawlers do. After consulting with Dr. Brett Blaauw, regional entomologist for Clemson, the grower decided to go ahead and treat now. On Friday, he sprayed Movento at the label rate. There is great concern because with this high of a population, the life of the entire trees at risk. The plan is to follow that application with chlorpyrifos and oil at low rates after the leaves drop. You have to be careful when doing this as the oil can damage the next years bud crop if temperatures are too hot. We will be trapping using black electrical tape wrapped around the limbs then double sided scotch tape around that. We will then look for the crawlers on the scotch tape. This ensures money isn’t wasted killing a pest that has already been controlled.”

Red spots have been common on peaches this year. Photo from Andy Rollins.
In this close up of the bark on a peach tree, you can see the tiny, black and grey colored scales. Photo from Andy Rollins.