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.”

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.

Weekly Field Update – 8/17/20

Statewide

Dr. Tony Keinath with a word on crop rotation. “To keep the soil on your farm productive over the long term, do not replant the same vegetable, or a related crop, in the same field “too often.” How often is “too” often depends on the crop and the pathogens present in the soil. Almost always, “too often” is less than 12 months between disking the old crop and planting the new crop. A general rule of thumb is 24 months between related crops, and in some cases, 36 months (3 years) is needed.

The main risk in replanting “too often” is building up root pathogens that survive in soil for years. Even in the heat, diseased roots and stems take several months to decay enough so they are not a source of pathogens. Thicker tissue, like the crowns of cantaloupe, can take 2 years to decay.

Another risk is foliar diseases that start on volunteers from the previous crop. The pathogens may be in or on some of the seed that sprouts, for example black rot on leafy brassica greens or gummy stem blight on cantaloupe and watermelon. A small number of infected volunteers means the disease has a head start right at the beginning of the crop.

Controls for soilborne pathogens (fungi, water molds, and nematodes) are limited.

  • Many vegetable crops have no resistance to these pathogens.
  • Fungicides do not penetrate soil well, or they are quickly inactivated.
  • Fumigants have many restrictions that require time-consuming record keeping and air monitoring.

Root-knot nematodes are a special problem, because they form galls on many vegetables and some field crops (cotton, for example) grown in rotation with vegetables. Summer cover crops of sunn hemp can lower nematode numbers. See https://edis.ifas.ufl.edu/in892 for a list of other warm season cover crops that help manage nematodes.

Without crop rotation, more fungicide sprays will be needed, which raises the risk of fungicide resistance. Fungicides and fumigants are not a substitute for good crop rotation.”

Midlands

Justin Ballew reports, “Many areas got a little more rain last week and we had a break from the heat over the weekend. We have a lot of the fall crop planted now including squash, zucchini, tomatoes, and brassicas. So far everything is mostly growing well. We are seeing bacterial wilt develop in some of the fall-planted tomato fields. Bacterial wilt loves hot soil temperatures, which is typical this time of year. Be sure to follow a proper crop rotation plan (at least 3 years) to help manage bacterial wilt buildup in fields. Since the heavy rain we got a few weeks ago, we’ve also seen plenty of bacterial spot in what’s left of the spring tomatoes.”

To test for bacterial wilt in tomatoes, place the cut stem in a jar of clean water. If the plant is infected, within about 30 seconds you will see milky, white bacterial ooze begin to stream from the stem. Photo from Justin Ballew.
Bacterial wilt commonly causes discoloration of the tissue within the stem. Photo from Justin Ballew.

Pee Dee

Bruce McLean reports, “Most summer vegetable crops are starting to wind down. Peas, okra, watermelon, cantaloupe still harvesting at some volume. Late summer/fall squash, cucumbers and tomato harvest are a couple weeks away. Fresh market muscadines are being harvested now. Juice and wine muscadines are getting close – maybe 7-14 days away, depending upon location. In most juice and wine muscadine vineyards, Carlos is around 40% colored (ripe), Noble is around 60%, and Doreen is around 25%. Grape root borer (GRB) flight is still occurring, with moderately high moth counts in traps.”

Eumorpha pandorus, a.k.a Pandorus Sphinx Moth caterpillar, found in the muscadine vineyard at Pee Dee REC. Photo from Bruce McLean.
‘Noble’ muscadines getting close to harvest. Photo from Bruce McLean.

Tony Melton reports, “‘Another Crop Bites the Dust’ from spreader/stickers. I have seen too many farmers sing this sad song. Be careful and know what you are doing when adding a sticker/spreader when spraying vegetables. Short season, tender vegetable crops will burn very easy in our heat and do not have time to come back like long season row-crops. Our state’s second major watermelon/cantaloupe season is in full season in Chesterfield County. Harvest is in full swing and will continue until frost. Curculio sprays are beginning to be applied to the fall pea crop. Looks like pickle harvest will continue until frost.”