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.

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.

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.

Weekly Field Update – 4/26/21

Join us this Wednesday (3/24/21) at 12:30 pm for an update from the tomato fields with Zack Snipes. It will be a relatively short meeting, lasting 30-45 minutes, so tune in while you eat your lunch. Click here to register.

Coastal

Rob Last reports, “Crops in the area continue to develop well, and I expect this to continue after some welcome rainfall over the weekend.  Following the rain over the weekend and returning to warm temperatures, keep scouting for diseases in crops and ensure fungicide applications are made promptly. Spider mites show activity in a range of crops from strawberries to peppers, tomatoes, and blackberries. Always remember to use a specific miticide for spider mite control to avoid flaring populations. Cucumber beetles are beginning to be found in sticky traps. Currently, no damage is being seen to crops. Treatment options include neonicotinoid insecticides applied as a foliar spray or through the drip system.  When treatments are made, it is possible feeding damage will be seen as the pests need to ingest the pesticide.”

Zack Snipes reports, “We finally got some rain last week that I think will benefit all of our crops. Spider mites were the talk of the community last week. I saw high populations on strawberries as well as blackberries. On farms with mixed produce, you will want to scout all crops, even if they are at a stage where they normally wouldn’t have mites. I found mites on tomatoes last week because they were adjacent to a strawberry crop. We need to get on top of this pest before its too late. There are plenty of IPM techniques and strategies for this pest. Also, if you have sprayed a product in strawberry and are considering treating other crops nearby, you may want to rotate products/chemistries from what you sprayed on strawberry. Chances are those are the same genetically similar spider mites so if you had any resistance in strawberry, or another crop, then you may see it on tomato or watermelon.”

Taking good photos of spider mites is tough but on this leaf you can see several two-spotted spider mites. Don’t wait until you see symptomatic damage to manage this pest.    

Midlands

Justin Ballew reports, “We had a cold event in the midlands that seems like it got around 5 degrees colder in most areas than what the forecast called for. It got to 35 at my house Wednesday night/Thursday morning. South of Columbia and Lake Murray, I only heard a couple reports of light frost, but north of Columbia, I heard reports of the temperatures reaching 31. We’re certainly going to see some damage in those areas to spring crops that weren’t covered. On another note, strawberries are yielding really well right now. Growers are reporting that yields are at times outpacing sales. We finally got some rain this weekend (just under an inch at my house), so we’re going to see some water-damaged berries and the moisture will give grey mold an opportunity to increase. The rain was good news for spider mite management, as their populations had been thriving in the dry conditions. Diamondback moth populations are increasing in brassicas, so keep up with scouting.”

Strawberries have been yielding well and berry size has been quite large on some cultivars. Photo from Justin Ballew
We haven’t seen much grey mold lately, but the rain we had over the weekend may allow it to make a comeback. Photo from Justin Ballew

Pee Dee

Bruce McLean reports, “Last week’s brief kiss of frost doesn’t appear to have caused any significant problems on the crops. But, it is starting to get a bit dry. Sweet corn and pea emergence has been looking good, with most locations having an excellent stand. Cucurbit crops are starting to emerge, as well. Transplant tomatoes are looking very good. Everyone is taking advantage of this beautiful weather to plant vegetable crops and blueberries… just don’t forget to irrigate. Strawberry harvest is peaking right now and the fruit looks really good. Disease pressure is pretty low, but spider mite activity is high in many locations. In blueberries, much of the damaged fruit (from the Easter freeze) is beginning to shed off of the plant. Hopefully, this will continue… so lingering damaged fruit will not slow down harvest (starting in a few weeks). Muscadines are starting to show some early flower bud development.”

Early flower bud development in Carlos muscadine. Photo from Bruce McLean.
Crimson clover cover crop in a muscadine vineyard is a beautiful sight to see. It serves as a great habitat for beneficial insects. Photo from Bruce McLean.
Blueberries planted on plastic mulch covered beds. The plastic reduces the development of weeds, thereby reducing competition for the newly planted blueberry plants. Photo from Bruce McLean.

Tony Melton reports, “Peppers, cucumber, and tomatoes are just sitting and not growing. Extra fertilizer will do little for growth in these cool conditions. Cabbage needs to be sprayed for thrips, sclerotinia, and caterpillars. Cabbage, collards, and greens are loving the cool weather and hauling butt, so side-dress to keep growing. Strawberries are loving these cool conditions and are really producing. We are having an overabundance of local strawberries making them hard to sell – many are being discarded. Just a small amount of wind damage. Tomatoes, peppers, and cucumber plants were not big enough to act like a sail and be blown (twisted-off) in the wind. Got just enough rain to start planting dryland peas. I know I will regret saying this – “but I hope it warms up soon.”

Upstate

Kerrie Roach reports, “Every day brings us a little closer to May in the Upstate, and with that brings a little less chance of a frost or freeze event. Last week Thursday brought on lows of 30-32 degrees F in many places causing damage to warm season crops as well as landscapes across the upstate. Strawberry growers have slowed since the cool weather, but production will start to pick back up with warmer days and nights this week.”

Weekly Field Update – 1/11/21

Coastal

Rob Last reports, “Vegetable crops are growing out of the impacts of frost well. There is active Alternaria in places on brassica crops. Insect activity in vegetable crops in the area remains low. Strawberry crops are moving well with a few spider mites and aphids being observed. Remember if mite treatment is needed use a specific miticide to target the pest to avoid flaring populations. If you need a second pair of eyes to help scout then please give me a shout.”

Zack Snipes reports, “I’ve been getting a good many calls about strawberries in recent weeks.  The warm weather has really pushed our berries, perhaps too far along for this time of year.  I know of a couple of farms that are already harvesting which I’m not sure is a great thing this early in the season.  Most fields look good with great growth but we only have a few crowns for each plant.  Hopefully some cool weather will come in and slow them down.  Make sure to sanitize the plants by removing all dead tissue and put out a preventative spray once you are done sanitizing.  Good preventative sanitization right now can do wonders for disease management later in the season.  Now is a good time to manage weeds before they get too large.  And while I am at it…now is the perfect time to get ready for the season by checking sprayers, getting fertigation systems set up and calibrated, and purchasing pesticides you know you will need for the season.” 

A sanitized plant and the dead and diseased tissue that came off of it. This needs to be taken out of the field and disposed of. Photo from Zack Snipes.
Corn spurry is a weed that needs to be managed now before its too late. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “The weather has remained cool, so everything is growing pretty slowly. We had another very rainy day last week and we got a little over 2 inches at my house. That’s over 6 inches for me so far in 2021 and I’ve had some folks tell me they’ve gotten over 8. We are seeing some cold damage to strawberry foliage, but nothing to worry about long term. Just make sure to sanitize any dead leaves and flowers as the temperatures warm in the spring. I’ve already seen some Botrytis developing on dead flowers, so we definitely need to remove these sources of disease inoculum. I’m counting 2-3 crowns per plant right now. If you’re behind that, it may be helpful to put row covers on for a couple weeks. Just scout for spider mites carefully first.”

Cold damage around the margins of strawberry leaves from the hard freeze right after Christmas. It didn’t get cold enough to damage the crowns. Photo from Justin Ballew
This bloom was killed by cold weather. It’s difficult to see here, but there are already a few botrytis spores developing on the flower. If not sanitized, this could become a significant source of inoculum. Photo from Justin Ballew.

Pee Dee

Tony Melton reports, “Cold hurt green winter strawberries more than ripe ones due to increased sugar in ripe ones. Still got some squash producing in high tunnels if covered inside tunnel with row covers. We’re bedding green fields to allow weeds to germinate so they can be killed using stale-bed culture.”