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 – 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.

Weekly Field Update – 5/3/21

Coastal

Rob Last reports, “Botrytis (gray mold) is evident in places following rainfall. Sanitation and removal of the infected berries are critical to reducing inoculum loading. Fungicide applications will also be required to prevent spore germination and further spread of the disease. Where spider mite treatments have been applied, they have effectively reduced the populations but will still require scouting and monitoring.  Yields achieved are very good. Cucurbit crops continue to develop well with a low incidence of chill injury from the cooler temperatures observed on April 21/22. Crops are running well, with some flowering being observed. Cucumber beetles are being monitored, and a few aphids are present in places. Cucumber beetles observed are currently below the threshold of 5 beetles per plant. Squash bugs are also being found on some sites. Careful monitoring will be required because numbers can increase rapidly. Crops are free from disease; however, protectant fungicides are still a critical input. Blueberries are swelling well and beginning to blush.  Blackberries are flowering with excellent fruit set and bud formation. Spider mites have been evident in these crops too; however miticide applications are proving very effective.

Squash bugs are showing up in the coastal region. Photo from Rob Last.

Zack Snipes reports, “We had another nice week of weather in the Lowcountry. We are starting to dry out again so some rain would be nice. All of our crops look really great right now with no huge issues. The biggest issue I see right now is spider mites on all crops which makes sense with the warm dry weather. I’ve gotten a few calls about some strawberry plants putting out runners. We need to get in the fields and pinch those off while we are cleaning up dead berries, blossoms, and tissue. I have been in several vegetable fields lately and seen some inconsistencies in plant growth and vigor. Upon further inspection of some tomato and squash crops, I found galling on the roots which is an indicator of root-knot nematodes. If this is the case on your farm, pull up a few plants and look at the roots. We need to keep good notes on where these areas or fields are and practice good crop rotation for next season or use resistant cultivars. There are some cover crops that help with suppression as well. If you want to pull a soil sample to check for nematodes, give your local agent a call. We are doing a statewide survey looking for the guava root-knot nematode and can assist with sampling. In case you missed the “Update from the Tomato Fields” talk last Wednesday, here is the link.

Smaller squash plants in a large field made me question root-knot nematode damage.  Upon inspection of the roots, I found galling which is an indicator of nematode pressure. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “The weather last week was mild and it is very dry again. The inch of rain we got last weekend didn’t go very far. Strawberries are still yielding well, though in some fields, we’re starting to see berry size decrease, like we frequently see towards the end of picking. Folks had to remove water-damaged berries from their fields following the rain this past weekend. I guess the silver lining in this dry spell is we haven’t had many water-damaged berries yet this year. We’re still seeing some mites here and there, but disease pressure remains low. I found a berry last week with an anthracnose fruit rot lesion for the first time this season. So even though disease pressure is low, we still need to keep up with fungicide programs. There is some rain in the forecast for this week, so now would be a good time to throw in one of the site specific fungicides.”

Fungicides with the best efficacy on gray mold (left) and anthracnose (right) Photo from MyIPM app.
Cabbage with some cold damage from the cold a little over a week ago. Photo from Justin Ballew

Sarah Scott reports, “Along the Ridge, crews are still finishing up thinning in peach orchards to ensure good sizing on fruit. We are well into our summer cover spray programs for the season. During pit hardening it is critical to do preventative treatment for bacterial spot. Growers should be watching for plum circulio activity as well as scale crawlers in the next week as well. Sime powdery mildew has been found on foliage. Refer the the 2021 SE Peach, Nectarine, and Plum Management Guide for specific treatment recommendations. Strawberries are performing well. Some spider mite activity as well as some botrytis in the fields, which could worsen with several days of wet weather in the forecast. Transplanting of summer crops continues, including tomatoes and peppers. Curcurbits are becoming established and harvest will begin soon. 

Peach tree after thinning. Notice all of the green fruit on the ground surrounding it. Photo from Sarah Scott.

Pee Dee

Tony Melton reports, “Tomatoes, peppers, cucumber, and sweet potato slips are really growing since it has warmed up. Getting ready to sidedress and plow cucumbers. Fresh market cabbage will be ready to harvest soon. Processing collards are almost ready to harvest- some downy mildew has been seen. Asparagus suffered from excessive rains last year. Saw some sunscald and chemical burn on strawberries, so watch what products you are applying foliar, especially fertilizers. With the small amount of rain we had last week many acres of peas have been planted; however, it is awfully dry again.”

Field Update – 10/7/19

Statewide

Dr. Tony Keinath reports, “Growers who have “slacked off” on fungicide applications during the dry spell should resume biweekly or weekly fungicide sprays in areas that are or have received rain. For most fungal diseases, the amount of rain determines how severe the disease becomes. The more rain, the more fungicide sprays are needed. Note that many fungicide labels now state that the product may only be applied once every 7 days; 5-day spray schedules are going away.

Coastal Region

Zack Snipes reports, “Another hot and dry week in the Lowcountry. Non irrigated crops are really starting to suffer.  Many farms are waiting on rain to plant fall crops.  We are beginning to prepare for strawberry season but dry conditions are making it hard to lay plastic. Festivals, corn mazes, pumpkins, and haunted trails are in full gear in the Lowcountry.”

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Sunflowers can add extra income to the farm during the summer and fall seasons. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “Last week was hot, but temperatures finally dropped over the weekend. It finally feels like fall.  Dry weather remains, though.  A few areas got some light showers, but it didn’t amount to much.  Growers have laid their plastic for strawberries and planting is approaching quickly.  Caterpillars are still very active in brassica crops and we are seeing some whiteflies as well.  Keep scouting and stay on top of the insects.”

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Plastic laid for strawberries. Planting will start soon. Photo from Justin Ballew.

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Whiteflies are showing up on brassica crops in the midlands. Photo from Justin Ballew.

Sarah Scott reports, “We have had spotty rain throughout the Ridge but conditions are still very dry. Cooler night temperatures are bringing some relief. Field preparation for new peach tree plantings are underway including soil fumigation.  With the recent discovery of the root knot nematode, Meloidogyne floridensis, in fields in Edgefield County, growers can request a PCR test if nematode samples test positive for root knot nematodes.

Lalo Toledo reports, “Phytophthora blight on bell peppers has been found in Clarendon and Orangeburg County. Phytophthora blight on peppers is extremely damaging and can result in total loss of the crop prior to the first harvest. Proper fungicide applications and resistant cultivars can be used to suppress this disease. Sweet potatoes are being harvested, as well as eggplants. White-fly populations have been found in broccoli and mustard.

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Phytophthora blight on pepper. Photo from Lalo Toledo.