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.”

Field Update – 5/20/19

Statewide: Dr. Guido Schnabel reports, “Green fruit rot is starting to show up in commercial peach orchards. This disease is caused by the fungus Monilinia fructicola. In spring we had an extended period of bloom with lots of rain. That led to blossom blight caused by the same fungal pathogen. Growers must take this disease very seriously as it can cause significant preharvest and postharvest fruit rot. Many take advantage of our lab service to determine potential weaknesses in fungicide spray programs. For more information contact your local county agent or Dr. Schnabel directly at schnabe@clemson.edu.”

Green fruit(Monolinia fructicola) rot on a peach. Photo from Dr. Guido Schnabel.

Coast: Zack Snipes reports, “We have had nice warm weather that is helping the development of irrigated crops.  We are starting to get dry and could use some rain for dry land crops and to settle dust. We are in the middle of squash and cucumber harvest.  We are starting to see powdery mildew show up on cucurbit crops. The tomato crop looks promising this year despite the usual bacterial wilt common in fields.  To test for bacterial wilt, select a wilting plant, cut it through the stem, and put into a jar of water.  If the pathogen responsible for bacterial wilt is present (Ralstonia solanacearum), you will get what’s known as bacterial streaming which is a milky white stream coming from the cut end of the plant.

Bacterial wilt ( Ralstonia solanacearum) on tomato. Photo from Zack Snipes
Powdery mildew on squash leaf. Photo from Zack Snipes.

Midlands: Justin Ballew reports, “The weather this past week was pretty mild, but dry. Thrips have become a problem in strawberries in some areas, but we are so close to the end of picking that most growers would not benefit from a treatment. Production has really decreased and some growers have already begun redirecting picking labor to other crops. Spring planted squash and peppers are starting to come into production and everything is looking pretty good. Keep an eye out for spider mites on tomatoes as it gets hot and stays dry this week.

Thrips damage on developing strawberry.
Peppers are developing well in the midlands. Photo from Justin Ballew

Sarah Scott reports, “We have begun picking early varieties of peaches across the Ridge. Things are looking good for a nice crop this year. Green fruit rot (Monilinia fructicola), also known as brown rot, has shown up in a few spots around the Ridge. We are continuing to monitor stink bug populations in orchards. Traps have shown high numbers of insects but damage has been scattered in this area.

Peaches are looking great on the Ridge. Photo from Sarah Scott.
Stink bug trap on the edge of a peach orchard. Photo from Sarah Scott.