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 – 7/6/21

Statewide

Dr. Tony Keinath reports, “Basil downy mildew was found in mid-June in Charleston. Symptoms start as faint yellowing of leaves, which eventually show brown spots surrounded by yellow areas. To see the spores, look on the bottom of a symptomatic leaf. Sometimes it helps to hold the leaf up to a light source (but don’t look directly into the sun). Seeing spores is useful to rule out nutrient deficiency or sunburn on leaves. Growers who use conventional fungicides should rotate two of these three labeled fungicides: Revus, Presidio, or Ranman. See page 203 of the 2021 Southeastern U.S. Vegetable Crop Handbook. Potassium phosphite products can be used as a preventative and by home gardeners. I do not know of any cultivars that truly are resistant or any organic biopesticides that are effective. Once downy mildew spores arrive in South Carolina, the disease will be present until frost kills the basil host.”

Basil downy mildew symptoms on the upper side of the leaves. Photo from Dr. Tony Keinath.
Basil downy mildew spores developing on the underside of a leaf. Photo from Dr. Tony Keinath.

Coastal

Zack Snipes reports, “Crops are still looking good coming off. Typically July 4th week is our busiest week in the field. One thing I saw this past week in some melon fields was crown decline. Crown decline is characterized by a yellowing of the crown leaves which makes the plant look weak overall. The disease can be mistaken for a nutrient deficiency. This disease is important to diagnose because yields can be reduced and fruit quality can be impaired at the middle to end of the season which can impact your bottom line. Read up on this disease and management options here. I am seeing some gummy stem blight and anthracnose in watermelon right now as well. Get your fungicides out before the tropical storm this week.

Crown decline appears late in the season and can be confused with a nutrient deficiency. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “Things are progressing well in the midlands. Temperatures have been pretty mild and we saw about half an inch of rain at my house Thursday afternoon. It looks like Tropical Storm Elsa will be coming through Thursday, so plant diseases will continue to be our major issue for at least a little while longer. I’ve been seeing plenty of downy mildew, powdery mildew, anthracnose, and bacterial spot over the last week. Japanese beetle numbers are pretty high right now also.”

Japanese beetles feeding on developing muscadines. Photo from Justin Ballew.

Pee Dee

Tony Melton reports, “Sweetpotato vines are covering the beds. We’re starting to harvest processing tomatoes. We’re planting fall butterbeans and peas and picking processing peppers for the second time.”

Downy Mildew on Watermelon Found in SC

From Clemson Plant Pathologist Tony Keinath.

Downy mildew was found on watermelon this week in Allendale and Barnwell counties. Although downy mildew does not infect fruit, it reduces sugar content once 1 in 4 leaves (25%) are infected.

Downy mildew symptoms on the underside of a watermelon leaf.

All watermelons should be sprayed with a fungicide effective against downy mildew. See pages 214-215 in the 2021 Southeastern U.S. Vegetable Crop Handbook. Any fungicide listed there can be used except Aliette, Previcur Flex, or Curzate. Do not use these fungicides to manage downy mildew on watermelon, as the isolate on watermelon in 2020 was resistant to them. Gavel, Ranman, and Elumin are the least expensive choices. Growers should apply a downy-mildew specific fungicide this week, a protectant (chlorothalonil or mancozeb) next week, and repeat this sequence until one week before the last harvest. See the following publications for more info on watermelon disease management:

Powdery Mildew on Watermelon

Cucurbit Downy Mildew Management

Watermelon Fungicide Guide

Fusarium Wilt in Watermelon

From Clemson Plant Pathologist Tony Keinath.

Fusarium wilt is showing up in watermelon fields and in research plots at Coastal REC in Charleston. The most telltale symptoms are wilting of a few leaves at the crown of the plant, wilting of one vine on a plant, or wilting without yellowing of a small plant.

One vine of this watermelon plant has wilted. This is a telltale sign of fusarium wilt.

A good field diagnostic trick is to cut a wilted vine close to the crown, split it open lengthwise, and look for reddish brown spots on the crosswise cut or streaks in the lengthwise cut.

Cross section of a watermelon stem showing the discolored, reddish brown spots.

The Fusarium fungus is most active when the soil temperature is below 81 degrees F. Although daytime temperatures were warm in April, the nights were still cool enough to allow infections. At this point, there is nothing that can be done to mange Fusarium wilt. All successful management practices must be done before transplanting. See: Keinath AP. Integrated Management for Fusarium Wilt of Watermelon. Land-Grant Press by Clemson Extension. 2019; LGP 1022.

Downy Mildew Found on Watermelon in SC

Downy mildew was found yesterday, June 17, 2020, in one watermelon field in Bamberg County, South Carolina. All growers should immediately spray watermelon with Ranman, Revus, or Gavel to protect their crops from downy mildew. In addition to direct yield loss, loss of vine cover can expose fruit to sunburn (when the sun comes out again). Growers who find downy mildew in a field should apply Orondis Ultra or Orondis Opti in a weekly rotation with Ranman or Gavel. For more information on downy mildew, see the updated Cucurbit Downy Mildew Management for 2020.

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Downy mildew symptoms on watermelon foliage. Photo from Dr. Tony Keinath.

Field Update – 5/26/20

Statewide

Dr. Tony Keinath reports, “Powdery mildew was found late last week on watermelon at the Coastal REC, Charleston. All watermelon growers should look at the photo below to be sure they can identify powdery mildew in the early stages. The spots are pale yellow, and, unlike squash, may not have white powdery growth under the spot on the bottom of the leaf. See Powdery Mildew on Watermelon Land-Grant Press 1019 for spray recommendations (https://lgpress.clemson.edu/publication/powdery-mildew-on-watermelon/).”

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Light yellow spots on watermelon leaf from powdery mildew. Photo from Dr. Tony Keinath.

Coastal

Zack Snipes reports, “I saw a good bit of powdery mildew last week on all cucurbit crops. I have had several questions this spring about injury to watermelon. I think the strong winds, sand, and spraying damaged the crowns of melons. They are growing out of it now but have had some folks concerned. Bacterial wilt is showing up in tomato as temperatures climb and fruit is loading up in the crop. Keep up with scouting for insects and diseases. Overall things look great.”

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Crown injury on watermelon due to high winds and sandblasting. Photo from Zack Snipes.

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Conditions have been perfect for the development of powdery mildew on cucurbit crops. Photo from Zack Snipes.

Rob Last reports, “Watermelons generally look very good with great color and potential with early fruit set occurring.  Fusarium wilt is active at low levels in some places, as to be expected in areas of impaired drainage.  Also, I observed a little hail damage to watermelons and cantaloupe in Bamberg County.

Midlands

Justin Ballew reports, “It’s been raining a lot in the Midlands, but we really needed some rain. I’ve had almost 6.5 inches at my house since last Monday (5/18). This has slowed down strawberry picking and we have a ton of water damaged berries. Botrytis is loving all the moisture. Most fields look like they will keep producing for a few more weeks,  just stay on top of fungicide programs. The moisture and warm temperatures have most other crops growing rapidly and looking good. Keep an eye out for disease.

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These strawberries sitting in water will be damaged and need to be removed from the field before botrytis begins to develop. Photo from Justin Ballew

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Tomatoes are loving the warm weather. Photo from Justin Ballew.

Pee Dee

Tony Melton reports, “Anyone have an ark?  Many fields are flooded. Nutrients are washing away and strawberries are water-soaked.  Some strawberry farmers have stopped picking.  Water-logged soil causing crops to stop growing and we have to rely on airplanes to spray crops.  Weeds are enjoying the rain.”

Bruce McLean reports, “Last week was rainy in most locations, and this week looks like more of the same. Disease pressure is elevated due to excessive moisture. Seeing gummy stem blight showing up in cucumbers and bacterial spot in tomatoes. Thrips pressure had been high going into last week (especially on beans, peas, and cucumbers), and could still be high in the few locations that missed the heavy rains. But everywhere else, the heavy rains likely reduced thrips activity. Strawberries are finishing up. Heavy rains damaged most of the remaining red fruit. Blueberry harvest is starting to increase. Prior to rains, fruit was looking really good. The excess moisture did cause some fruit splitting, but no long term damage to the crop. Haven’t seen much thrips activity in muscadines yet, but now concerned with increased problems with calyptra release, a condition known as stuck cap. It’s a little early to tell, but could reduce yields.

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Thrips damage to southern peas. Photo from Bruce McLean.

Upstate

Kerrie Roach reports, “We are building the ark to send down to Tony in the Pee Dee!  Lots of flooded fields last week (and today) with many places seeing 4-6 inches of rain over 3 days. Strawberry growers halted picking and worked around the rains. Vegetable producers are replanting washed out crops and draining fields.  Peaches and apples continue to be on track for a good season. Localized hail damage is showing up on apples in Mountain Rest from one of the storms a few weeks ago.”

Field Update – 5/18/20

Statewide

Dr. Tony Keinath reports, “With the cooler-than-normal spring weather, two cool-season pathogens also may be active longer than normal. Downy mildew on brassicas, especially kale and collard, mainly affects the lower leaves. Sometimes it will move up onto the larger leaves in the middle of the plant. With a hand lens, you can see white mildew growth in lesions on the bottom of the leaves. In my spring 2020 trial, Presidio, potassium phosphite, and Zampro rotated with potassium phosphite worked well. Organic growers can use Badge X2 copper, which also performed well.

Fusarium oxysporum f. sp. niveum, the soilborne fungus that attacks watermelon, infects roots when soil temperatures are below 82F. At Coastal REC, I am still seeing new plants showing wilt symptoms. Remember that all control measures, and I want to stress all of them, must be applied before or at transplanting. There is nothing that can be done at this stage of crop growth. It is too late to apply fungicides, which will be a waste of money.”

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Tiny dark flecks of downy mildew on the bottom of ‘Blue Dwarf’ kale. Photo from Dr. Tony Keinath.

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Sporulation of downy mildew on the underside of ‘Tiger’ collard. Photo from Dr. Tony Keinath.

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Wilting and yellowing of lower leaves due to Fusarium wilt on watermelon 6 weeks after transplanting. Photo from Dr. Tony Keinath.

Coastal

Zack Snipes reports, “Things are coming along in the Lowcountry.  It has been very windy and I’ve had several farmers tell me that crops are using more water now because of the wind than if it were hot and humid.  Squash, zucchini, cukes, potatoes, and greens are coming off in good volume right now. Tomatoes are just a few weeks away and are in the sizing up stages right now. I have not seen any major pests or diseases as of late.  The conditions are ripe for the development of Powdery Mildew so be on the lookout when scouting watermelon, squash, brassica, and tomato.”

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Tomatoes are looking good and are just a few short weeks away. Photo from Zack Snipes.

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Powdery mildew on the shaded side of a collard leaf. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “It’s dry in the midlands. It’s been great because we haven’t seen much disease lately (especially on strawberries), but we need some rain. The forecast looks like we may get some this week. Strawberries are still yielding fairly well, though we’re starting to see fewer blooms. Fruit size is getting smaller, but taste has still been great. The first few plantings of sweet corn are tasseling now. Brassicas are still growing well, though there is some black rot out there.  Tomatoes, peppers, and cucurbits have been growing fast the last few days also.”

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The earliest plantings of sweet corn are tasseling. Photo from Justin Ballew.

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Black rot symptoms on the margins of cabbage leaves. Photo from Justin Ballew.

Sarah Scott reports, “Peach season is here! Picking several early varieties now and running packing lines. The crop looks good for this season. Bacteriosis started to show up on leaves and fruit in fields. Still picking strawberries. Summer crops like bell pepper and squash are progressing nicely.”

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Phomopsis or twig blight in a peach orchard. Lesions on twigs cause dieback, gumosis and curling at tips. Remove damaged wood and burn. Photo from Sarah Scott.

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Twig dieback from Phomopsis twig blight. Photo from Sarah Scott.

Lalo Toledo reports, “Cool temperatures have slowed down the growth of many vegetables, but most vegetable plants are looking great. Please be aware of possible diseases coming in this week. Wet and hot conditions will be conducive for pests and diseases. Please spray accordingly and scout every two days, if possible.”

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Grasshopper damage on eggplant. Photo from Lalo Toledo.

Pee Dee

Tony Melton reports, “Hail has destroyed at least 10 A squash, 200 A peaches, 30 A strawberries, 8 A blackberries, many acres of field corn, tobacco, rye, and wheat.  Damaged another 35 A strawberries, 300 A peaches.  Thank goodness that the wind has let up for a while and temperatures have risen on tomatoes, peppers, peas, beans, and sweet potatoes.  Thrips have been awful and imidacloprid is only partially controlling them. If labeled use dimethoate, acephate, etc.   More herbicide damage than usual this year because of cool temperatures and wind, even on labeled crops and drift to non-labeled crops has been awful.”

Upstate

Kerrie Roach reports, “With a week of good growing conditions, things are looking great in the Upstate for fruit & vegetable producers. In higher elevations, there were some losses of young tender plants during isolated frost events early last week. With rains expected most of this coming week, things should start to really push for our market vegetable growers. Peaches and apples are on track for a good season.