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.
All growers—conventional and organic—should follow two proven steps to eradicate (eliminate) the gummy stem blight fungus from infested fields.
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.
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.
Dr. Tony Keinath’s remarks on fungicide stewardship:
Growers who are applying newer fungicides that are pre-mixes of two active ingredients do not need to add another fungicide to the tank mix. Pre-mixes can easily be identified by the two FRAC Codes on the label in the top right corner. Please consider the following points:
Many newer fungicides are sold as pre-mixes to reduce the risks of fungicide resistance in fungal and water mold pathogens. Mixing two active ingredients often is a way to prevent or slow resistance development, as long as both active ingredients work against the same disease.
Sometimes two active ingredients are mixed to expand the range of diseases controlled. For example, Quadris Top controls both anthracnose (the Quadris part) and gummy stem blight (the “Top” part, which is Inspire).
Mixing more than 2 pesticides, whether they are 3 fungicides or 2 fungicides plus an insecticide, increases the risk of pesticide injury (burn). Risks may be greater if a spreader-sticker is added, or when air temperatures are above 90 F.
Adding another fungicide to a pre-mix fungicide increases fungicide costs, often without increasing disease control.
Growers should contact their Extension agent before adding another fungicide, even a protectant, to a pre-mix fungicide to be sure it’s really necessary.
Justin Ballew reports, “Not much has changed in the midlands over the last week. We’ve received some scattered rain and it has been warm and humid. As a result, we are still seeing disease issues. We’re still picking tomatoes, squash, zucchini, cucumbers, sweet corn, and a few greens. We’re at an in-between stage in several fields where the spring crops have been finished and folks are preparing to plant fall crops. Some have already started fall cucurbit plantings. For anyone planting strawberries this fall, if you are still deciding which varieties to try, take a look at the NCSU’s 2020/2021 variety comparison data (pages 9 and 10).
Bruce McLean reports, “Vegetable crops are looking pretty good across the Pee Dee. Fields that have received rain or are irrigated look very good. Fields that have missed the rain are a bit drought-stressed. Sweet corn, peas, butterbeans, tomatoes, squash, watermelons, cantaloupes, and cucumbers are all being harvested in good volume. Okra is just beginning to be harvested well. Growers are still fighting cucurbit downy mildew (CDM) on cucumbers. Fields that have been sprayed with fungicides for CDM (Orondis Opti, Gavel, Ranman, and Omega) are relatively clean and producing well. Fields that have not received those products are in severe decline. Cowpea curculio is still being a challenge. Some growers have asked about adding PBO8 (Piperonyl Butoxide) synergist to their insecticide application. Research has shown some efficacy, so it does help. But, it is not the silver bullet that everyone is looking for. There really is no alternative to having a robust spray program, spraying every three to five days starting prior to bloom.
Many varieties of blueberries have already finished up, with only mid-late and late rabbiteye blueberries going now. Blackberries have finished up, as well. Summer primocane tipping and floricane removal has begun. Be sure to apply a broad-based fungicide to all open wounds/pruning cuts to prevent disease development (I.e. cane blight, etc). Muscadines are sizing up pretty well. Grape root borer traps have been placed in vineyards, but no moths captured yet.
Kerrie Roach reports, “Things in the Upstate were a little wet the last few days and it looks like showers will continue into the middle part of the week at least. Continue preventative measures for disease control and if/when a plant seems too far involved, remove and dispose of the entire plant to prevent further spread. Squash vine borer has been one of the continued problems in market garden production in the last 2 weeks. At this late point in the season and lifecycle, monitor plants closely and as soon as frass is seen, carefully cut the stem longitudinally and remove/kill the larval stage of the borer. You can mound soil on the cut part of the stem to help encourage new root growth. If done early enough, plants can continue to thrive despite the slice in the stem. New plantings of cucurbits set out in the last week or so should mature after the adults have finished laying eggs, but monitor closely for any wilting. Crop rotation, row covers, traps (yellow bowl of water), and pesticide applications can also be used as a part of a good IPM program. Check out the crop handbook for more specifics.
Andy Rollins reports, “We are having thrips issues on 3 crops peaches, pepper and cucumber. Thrips as seen in the picture cause surface damage to the outside make it aesthetically less appealing and marketable. On pepper they damage the leaves and can transmit viruses to the plant (on cucumber also). They are much worse in greenhouse and high-tunnel settings. I have been recently concerned about presence of chili thrips and I am waiting on definite identification. This is a good website about this new pest. I also found a rare disease called foot rot of squash in the upstate. It was identified by Dr. Tony Keinath. Rotation is very important with all of our vegetable crops. We are picking some excellent quality peaches in the upstate. Cold damage has limited our wholesale picking.”
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.
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:
The Coastal Research and Education Center Field Day is coming up this Thursday, June 17 beginning at 8:30. You must have registered to attend this year.
Zack Snipes reports, “We received some welcomed rain but 5+ inches in a day or so was a bit much. Conditions this week will dry things out. I cannot stress enough how important it is to get out fungicides once you can get in the fields. I saw a few squash fields going downhill last week. Upon closer examination, I found thousands of squash bugs. They tend to congregate on the crown of the plant and will hide under the plastic when you look for them.
Justin Ballew reports, “More rain fell in the midlands this past week. We’re up to a little over 4 inches of rain at my house for the month of June, more than March and April combined. Some places have received much more. Crops have been loving the rain, but it does come with a cost. As expected, we’ve seen diseases pop up here and there. We’ve seen some Southern blight and bacterial wilt in tomato fields, fusarium in watermelons, and more black rot in brassicas. Weeds are loving the moisture as well. Keep up with fungicide applications, as diseases can get out of hand in a hurry in these conditions.
Bruce McLean reports, “Vegetable crops are growing well. Sweet corn, squash, cucumbers are being harvested in good volumes. Downy mildew was observed in a smaller planting of cucumbers. Powdery mildew was seen on a smaller planting of squash. Tomato harvest is just beginning. Blossom end rot has been an issue on some early tomatoes. This can be contributed to environmental stresses to the plants (high UV, extremely low humidity, high air temps) from a couple of weeks ago. Blueberries are being harvested in good volumes. The quality of the blueberries is good (for the most part), but a bit undersized. Blackberries are looking good, but botrytis (gray mold) is likely right around the corner. Muscadines are looking good in most locations. Even with the frequent rains, calyptra release has been good. Thrips activity has been low so far. Some early-season ALS (Angular leaf spot) is starting to show. Rally is a good product to control ALS. Some fields are getting a bit wet, limiting equipment and harvest activity in the field.
Tony Melton reports, “Peppers and tomatoes are rotting at bottom of the plants from too much rain. Belly rot and Pythium leak is bad on cucumbers. Pickleworm is here and needs to be controlled. Southern stem blight is getting bad on tomatoes. Peas that are flowering need to be sprayed for cowpea curculio. Lots of potassium phosphide, metalaxyl, and mefenoxam are being used to keep down rots. Started side dressing, laying by, applying bifenthrin, and boron to sweet potatoes.”
Andy Rollins reports, “Bacterial canker of peach is much worse this year in the upstate. This disease is caused by a Pseudomonas sp. that is different than the bacteria that causes bacterial spot (Xanthomonas sp.). It is common in younger plantings, 3 to 6 years old. A common tell is dead blooms that remain on the tree. This is one of many sites where the bacteria will invade the plant. Fall pruning is a huge mistake as it creates even larger openings for bacteria to enter. This disease can be part of a disease complex (Peach tree short life) or can act independently. The presence of ring nematode would contribute in this case. Cold damage from southwest injury to the trunks is very common and also provides entry. Unfortunately, there is very little that can be done to remedy this problem after being found. Trying to limit the above mistakes is helpful and some research has shown positive results from painting pruning wounds. Other Rutgers research recommended early winter copper applications for bacterial canker in cherry.
The Coastal Research and Education Center Field Day is coming up on June 17 beginning at 8:30. You must register to attend this year. Registration may be found here.
Dr. Matt Cutulle reports, “The coastal rain events of the past week resulted in a lot of weeds popping up. For fields not planted yet this is a perfect opportunity to burn down the flush of weeds to reduce the weed seed bank in the field.”
Rob Last reports, “After some welcome rain in the area over the weekend, crops are looking good. However, given the humidity and rainfall, we are likely to find disease pressure increasing. Strawberries are all over. The wet humid conditions have spiked infections of botrytis and water soaked berries. Blueberries, blackberries and peaches are all coming to market with good quality and volumes with few insect or disease pressures being seen currently. Watermelons and cantaloupes are developing well with some early planted crops coming to harvest. In response to rainfall and increased humidity, keep an eye out for diseases such as bacterial spot in pepper and tomatoes. In addition, some cantaloupe crops are beginning to show Alternaria leaf spot. Make sure fungicide timings are good using a robust program.”
Zack Snipes reports, “We finally got a little bit of rain last week which knocked the dust down some. We could use more and hopefully we will get some this week. Incidence of Southern blight increased last week in tomato. Make note of these fields and avoid planting them in tomato, eggplant, pepper, squash, or melons next season. Overall, the crops look solid. We are in the thick of things when it comes to tomato harvest and rabbiteye blueberry harvest is starting this week as well. We have really humid and wet conditions coming this week so don’t forget to use protectant fungicides. I know everyone is busy, but during the busy season is when most of our insect and disease pressure spiral out of control.”
Justin Ballew reports, “We finally got some significant rain in the midlands this past week. I’m up to 1.8 inches at my house and there is a good chance in the forecast for more. It has been warm, overcast, and very humid the last several days, which is the perfect recipe for disease development. Make sure you are using preventative fungicide where necessary. Insects seem to be picking up a bit as well. Caterpillars are going strong, we’re still seeing plenty of mites on various crops, and I’m getting reports of stink bugs here and there. We’re harvesting brassicas, squash, zucchini, cucumbers, and herbs.”
Sarah Scott reports, “Much needed rain has occurred throughout the area this week with more predicted. With rain may come an increase in disease cases, growers should be aware of brown rot as it has been seen in several orchards this week. Removing fruit mummies and diseased fruit if possible can remove some inoculum from the field and limit future spread. Bacterial spot still isn’t being seen on a widespread basis, but again, after the moisture moving in for the extended forecast, growers should keep an eye out. Strawberries are wrapping up with some harvest still happening. Peppers and eggplants are progressing nicely and will also benefit greatly from some rain.”
Tony Melton reports, “1 pickle worm was found in a local squash. Processing and fresh market peppers will be ready to harvest on the week of June 13. Early tomatoes are just beginning to ripen. The rains have been mostly very beneficial but some spots have had excessive. Mudding through to pick pickles in spots. Very difficult to apply timely application of chemicals and some have to be reapplied after rains.”
Kerrie Roach, “Things are looking good in the Upstate with warm season crops starting to come off. It’s still early for too many disease and insect issues, but high humidity and a couple days of spotty rain will most likely increase pressure significantly.”
Rob Last reports, “As we mentioned last week, cucurbit downy mildew has been confirmed in cucumber crops locally. All cucurbit growers should be applying downy-mildew specific fungicides such as Ranman tank mixed with either chlorothalonil or mancozeb. Other pests and disease levels in cucurbits remain low. Tomatoes and peppers are developing well with good fruit development and low pest and disease pressure. A little bacterial spot is beginning to show in untreated crops. Applications of mancozeb or copper can help to hold the disease. Note applications are unlikely to “cure” bacterial disease. Fruit crops continue to progress with good quality and low disease. Strawberries are showing a little botrytis and anthracnose, but this is at a low level. Peaches are coming to market along with blueberries and blackberries.
Justin Ballew reports, “Dry weather has returned to the midlands. Strawberries have slowed down and while we’re still picking a few, we are seeing very few blooms. The heat we’re expecting this week is likely to cease bloom production. The heat could be a problem for tomato and cucurbit pollination as well, as we may see some blooms die. Brassicas have been growing well. There has been an increase in diamondback moth activity over the last week. If you start seeing large DBM populations on your farms, contact your local agent about doing a bioassay to test for insecticide resistance. This is a great way to figure out which materials may work best on your farm. We don’t charge anything for this service. Don’t forget downy mildew was found this past week in cucumber in multiple places in the coastal plain, so it’s time to start preventative fungicide sprays in cukes if you haven’t already.”
Sarah Scott reports, “We are in full swing with early variety peach harvest. Some peaches are developing a little smaller in size partly due to previous cold weather events. Something spotted in the field has been some nutrient deficiency. Growers may have been tempted to dial down nitrogen or calcium nitrate applications on peach blocks that suffered cold damage as they thought those blocks would not produce a viable harvest, however this could lead to leaf drop and less reserves for next year’s crop. If you skipped out on some of your fertilization keep in mind yellowing, spotting and leaf drop could occur. A soil application of calcium nitrate at 30lb/acre now and another in late July can help those trees recover some foliage and build up a nutrient supply for next year’s reserve. Be careful if you want a quick result and try foliar applications, as this could burn the leaves and cause more drop that you may not be able to afford.”
Bruce McLean reports, “Irrigated vegetables are looking good for the most part. Seeing some stress to tomatoes with a heavy fruit load – likely from very low humidity and very high UV. Nonirrigated crops are starting to show some drought stress. Strawberries are starting to wind down. Blueberries are finally starting to be harvested, much later than usual. Blooming on muscadines appears to really stretched out due to the Easter freeze. Early flower clusters on growth from primary buds appears to be somewhat on time. But, flower clusters developing from secondary buds are really behind… possibly 3-4 weeks (easily). Soil is really starting to get dry in many areas.”
Tony Melton reports, “The higher temperatures have started to speed-up crops. We will start to harvest pickles this week. Spraying downy mildew fungicides on cucurbits. Some farmers are starting to apply insecticide for pickleworm. They don’t want to put out insecticides this early but they see the gamble of an entire field of pickles being rejected because of 1 pickleworm as too much of a gamble. These farmers think we need a system set-up to detect pickleworm in the southern part of the state to give them a better indication of when pickleworm arrives. Harvesting greens, cabbage, and broccoli as quickly as possible. Started planting sweetpotatoes. Many farmers getting deer depredation permits for peas, beans, and sweetpotatoes. Also, many farmers are applying acephate and dimethoate to control thrips with the added benefits of repelling deer. Peppers and tomatoes are begin to load with fruit. Many farmers still harvesting strawberries.”
Kerrie Roach reports, “Things in the ‘Golden Corner’ are looking up for vegetable producers, despite the dismal projection for tree fruit crops because of the cold events in April. For market growers, cool season crops are at their absolute last with 90+ degree temps forecasted for this week. Warm season crops are starting to really get growing, and many growers are starting to see some potential harvests this week. With higher than normal temps projected, make sure irrigation is consistent, scheduled and set for early morning to avoid leaf wetness late in the day and overnight and reduce disease incidence.”
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.
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.
Should products with hydrogen peroxide, alone or combined with peroxyacetic acid, be used like fungicides on vegetables? It’s difficult to give just one answer to this question because there are so many different vegetables and diseases to consider. Here are a few important things to think about.
Hydrogen peroxide/peroxyacetic acid has no curative activity against any vegetable disease. Yes, it might look reassuring to see the dead centers drop out of leaf spots on tomato. The pathogen, however, is still in the leaf, where pathogens are naturally designed to live. All leaf pathogens—bacteria, fungi, and water molds—will survive inside the treated leaves.
Hydrogen peroxide/peroxyacetic acid has no (or a very short) residual activity or “staying power” on leaves after spraying. Conventional fungicides usually will last up to the minimum spray interval on the label, normally 7 days. Biofungicides also leave a residue on the leaves for at least a few days after spraying; there hasn’t been as much research on this as on conventional fungicides. The fact that labels like Oxidate recommend two applications per week suggests that the residue lasts no more than 3 days.
Oxidate did not control powdery mildew on cucurbit seedlings in the greenhouse. (Details available at https://doi.org/10.1016/j.cropro.2012.06.009). There was absolutely no effect in any of four experiments. Unlike other fungi, powdery mildew grows mostly on the outside of the leaf, so it was completely exposed to Oxidate.
The number of sprays per week needs to be considered when calculating the cost of hydrogen peroxide/peroxyacetic acid products. At $35/gallon and 1% solution, Oxidate costs $35/acre when sprayed twice a week at 50 gpa spray volume each time.
I recently tested Oxidate on kale affected with Alternaria leaf spot caused by the new species in South Carolina, Alternaria japonica. Oxidate (1%) was applied 2 days before harvest. Healthy leaves with no visible leaf spots were placed in sealed plastic bags with 100% relative humidity (RH) and stored at 41 F. for 1 week, then checked for disease symptoms. Based on two tests, 70% of Oxidate-treated leaves had Alternaria leaf spot, and 73% of water-treated control leaves did. That’s only a 5% improvement, and the difference is not statistically significant. Based on a kale price of $18/carton and 400 cartons/acre (for a once-over harvest), that’s an extra $359/acre.
Oxidate might have some benefits sprayed immediately before harvest. After harvest, leaves aren’t exposed to more pathogen spores from the air, so the short residual time (hours) isn’t as limiting. Storing produce at less than 100% RH also might have made a difference with less disease overall.
If you haven’t already filled out the Clemson Agribusiness Team’s COVID-19 Ag Impacts Survey, please take a minute to do so now. Click the graphic below or scan the QR code with your iPhone to access the survey. If you have any questions about the survey or the results, please reach out to Kevin Burkett, Kburke5@clemson.edu, 540-239-4602.
Rob Last reports, “Strawberry crops in the area are developing well with good fruit set. I am seeing a little gray mold around, so sanitation is going to be key as well as fungicide applications. There are also a few thrips in some crops, so scouting for these pests will be very important. Peaches and blueberries are blooming with little evidence of any chill injury from last weeks overnight lows in the upper 20’s. Asparagus crops are beginning to come to market with some chill affected spears early last week. With the dry weather conditions, field work and land prep is going well with plastic being laid for melon crops.”
Zack Snipes reports, “Great week of weather in the Lowcountry which has really improved the way things are looking. Greens, lettuces, root crops, and strawberries are looking good. The strawberries are absolutely loaded up with blossoms right now and are still putting on crowns. Fall planted cover crops are finishing up and being turned under. A bit of sad news this week as Author “Ikee” Freeman passed away. Ikee was a huge supporter of Clemson Extension and the farming traditions in the Lowcountry of South Carolina. His friendship, mentorship, leadership, and sense of humor will be missed by many.”
Justin Ballew reports, “We had another beautiful week of weather last week that really pushed plants along. Strawberries have a ton of blooms on them now. The weather has been pretty dry the last two weeks, but with a few days of rain in the forecast for this week, now would be a good time to throw one of the site specific fungicides into your fungicide rotation. Temperatures are still in the perfect range for Botrytis spore development (60-70 degrees F) and with moisture returning, we could see a lot of Botrytis in the next week or two.
Sarah Scott reports, “Last week’s warm temperatures have pushed peaches into bloom. There was a significant increase in open buds from the beginning to end of the week. Early varieties are near 90% bloom. Temperatures look like they will level out in the next 7 days which is good as temperatures that are way above normal can lead to developmental damage for fruit set. Some pruning is still underway, as well as bloom sprays for blossom blight using products like Captan or chlorothalonil.”
Andy Rollins reports, “So far peach season is off to great start. We got all of our chill hours in and are between pink and 5% blooms on all large scale varieties, with a few oddballs further along. Plum growers are even further along and many are preparing for bloom fungicide applications with primarily Bravo (Chlorothalonil). I will be repeating last years testing of new biological, Ecoswing, in bloom this week compared with Bravo. Strawberry production is progressing nicely. This week looks good as far as frost but things change quickly, so all growers will be intimately aware of every weather change at this point. I was very happy to help with a new 10 acre pecan orchard planted last week. It encouraged me because several local growers, University of Georgia specialist, a retired NRCS conservationist, and myself all worked together to help a widow in need. Her planning and execution was impeccable and diligent.”
From Clemson Fruit Pathologist Dr. Guido Schnabel and UGA Fruit Pathologist Dr. Phil Brannen.
Unfortunately, there appears to be a shortage of Captan products this year. For our fruit crop producers this may result in a change of strategy if and when reserves run out. Strawberry growers fortunately have the option to use Thiram for basic gray mold and anthracnose control. The two products are basically equal in efficacy but thiram has a slight edge over captan for gray mold control, but it is a bit weaker against anthracnose.
Peach growers rely on captan for cover sprays. These early to mid-season applications manage green fruit rot and possibly anthracnose. Fortunately, during bloom and preharvest we use different chemistries at our disposal listed in the spray guide. As we mention often, we prefer to hold other chemical classes, those for which Monilinia fructicola (brown rot) is most likely to develop resistance, for use in the pre-harvest sprays. During captan shortage, we recommend application of two applications of a QoI/SDHI (FRAC 7+11) product, such as Luna Sensation, Merivon, Pristine or Quadris Top (FRAC 3+11), for cover sprays as needed this year. Along with any remaining captan products, we hope that this will help us to limp along till next year, while still providing excellent control of cover spray pathogens. Sulfur alone in cover sprays, though sufficient for scab control, is not efficacious for green fruit rot or other diseases; if using a QoI/SDHI (7+11) product, these will control scab as well as green fruit rot, so sulfur would not be necessary for these applications. As always, please contact your local extension agent should you have questions.