Not Planting to Manage Phytophthora Blight by Reducing Disease Risk

From Clemson Plant Pathologist Anthony Keinath and Clemson Agricultural Economist Felipe Silva.

After an absence of several years, Phytophthora blight reappeared in South Carolina in July 2021 on three farms. As expected, the outbreaks were on two of the three most susceptible vegetable crops: 2 cases on pepper and 1 case on pumpkin. (Summer squash is the other very susceptible crop.)

A white powdery layer of Phytophthora capsici spores covers this pumpkin fruit. A watery rot will soon follow. Photo from Dr. Mary Hausbeck, Michigan State University

Five management options are listed in Land-Grant Press 1014: Managing Phytophthora capsici Diseases on Vegetables. One of the recommendations under Soil Management is to not plant low areas in an infested field, because that is where the Phytophthora blight pathogen, Phytophthora capsici, will first become active. Rain or overhead irrigation will spread the pathogen to the rest of the field. Remember that once a field is infested with Phytophthora capsici, The. Field. Remains. Infested. Period.

Based on the calculations shown in Table 1, a grower that considers the likelihood of disease outbreaks and plants only the well-drained 4.5 acres in a 5-acre infested field would earn an expected average of $1,600 per 5 acres per year. A grower who does not consider the disease occurrence pattern and plants all 5 acres can expect an average loss of over $2,600 per year (see the column labeled “Avg. Net Return”). These estimates consider the likelihood of disease occurrence based on the different planting sizes and areas.

The difference between planting 5.0 vs. 4.5 acres in an infested pumpkin field totals a positive net return of $4,300 per 5 acres. Why does not planting—and forfeiting yield—make more money? Assume that an outbreak of Phytophthora blight reduces the entire farm yield by 50%. Not planting the low area of the field, where disease is likely to start, will cut the number of outbreaks of Phytophthora blight in half (see the column labeled “Disease likelihood”), decreasing the chances of an outbreak from 60% to 30%. This estimate comes from vegetable pathology colleagues in other states who have worked on Phytophthora blight for many years.

Even in an infested field, by reducing the disease risk, the expected net return increases by more than $4,000. Note that over half of the gain in profit comes from reducing input costs by not planting the 10% of the field that probably will not yield anything. Although this example is calculated for pumpkin, the risk of the pathogen spreading from diseased peppers in a low spot in the field is just as likely or greater, because the pathogen produces spores readily on the fruit. Reducing disease risk is the key to increasing profits.

Weekly Field Update – 8/23/21

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Midlands

Justin Ballew reports, “Last week was warm and humid again. Some of our sandier fields got dry enough early in the week that crops were wilting in between waterings. We got a bunch of rain over the weekend, though (a little over 2 and a half inches at my house). Overall, fall planted crops are coming along nicely. Some of the earliest planted fall squash and zucchini is being picked now. We’re still seeing the same disease problems that have plagued us all summer, though growers seem to be managing them fairly well. As far as caterpillars go, I’m seeing mostly diamondback moth and armyworms with a few loopers here and there. Get ready. This could be a high pressure fall for caterpillars.”

Even though we’ve had lots of recent rain, it doesn’t take long for the sandy soil in Lexington to dry out and let the plants wilt. Photo from Justin Ballew.

Phillip Carnley reports, “Not much has changed here in Orangeburg or Calhoun Counties. Its been hot and humid and we’ve had a few untimely showers delaying cucumber harvests. There is still plenty of downy mildew to go around. We are seeing loopers on pickling cucumbers that are ready for harvest. At that stage the loopers should be treated prior to harvest with Coragen, Harvanta, or Radiant. Fall brassica, peas, and tomatoes are just now being planted.  We are seeing increased amounts of scab in pecans this year, due to the rain and humid weather. For insect and disease management in pecans, have a look at the UGA Commercial Pecan Spray Guide.”

Caterpillar damage to fall cucumbers. Photo from Phillip Carnley
Loopers are causing some damage to fall cucumbers in the Orangeburg area. Photo from Phillip Carnley.

Upstate

Andy Rollins reports, “Excessive rains have caused cracking of fruit at several upstate muscadine farms. Powdery mildew is also present but I’m not sure how much of a role it is playing on the cracking part. The powdery is damaging the skin of the fruit. Topsin M is labeled and recommended with Captan but have to wait 7 days to pick so have to watch your PHIs. Also finding some insect pests in peach. Oriental fruit moth and sap beetles have been found last week but only a small amount of affected production. Herbicide control has been difficult this year because all the rain has caused excessive grass growth, especially in new orchards.”

Seeing some cracking of the skins in muscadines in the upstate. Photo from Andy Rollins.
This new grower has done a good job of keeping his understory clean. This will pay off for him in the spring of next year if he can keep up the diligent work. Photo from Andy Rollins.

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 – 6/1/21

Coastal

Rob Last reports, “On the whole disease pressure in most crops remains low. The exception is cucurbits where we are finding powdery mildew in cucumbers as well as downy mildew. Cucurbit bacterial wilt has been found in isolated fields. This disease is characterized by wilting of one vine or the whole plant. Once cut the stem will ooze a sticky sap from the wound. It is transmitted by the feeding activities of cucumber beetles. Strawberries are beginning to slow down while tomatoes peppers and peaches are doing very well.”

Midlands

Justin Ballew reports, “We had an unseasonably hot week last week followed by a much cooler weekend. Some places received a little rain early in the week and some places received a little on Saturday. However, the Midlands are very dry overall. Crops responded well to the heat last week. Brassicas and cucurbit crops progressed extremely quickly. Squash and zucchini are setting fruit and some are being picked. Brassica and herbs are still being harvested. Tomatoes are setting fruit, but we still have a little time before picking on a large scale.”

Squash has grown rapidly in the last week in the Midlands. Photo from Justin Ballew
A bumble bee pollinating a squash blossom. Photo from Justin Ballew.

Sarah Scott reports, “Peaches are still going strong in the Ridge. Some varieties are showing more cold damage than anticipated, but there is still a good crop out there. It seems that the very early varieties and then some of the very late varieties are the ones with the most damage. Some bacterial spot is showing up but not near as bad as last season, mainly due to dry conditions. We’ve had some really hot days but this past week we finally got some relief from the temperatures and some cloudy days. Hopefully rain is in the future as growers are irrigating heavily now. Some second croppings of Camarosa strawberries are still being harvested this week. Squash, zucchini, and cucumbers are producing well. Insect pressure is starting to occur more heavily, but disease, for the most part, is still at bay.”

Pee Dee

Bruce McLean reports, “Blueberry harvest, after getting a late start, is starting to provide good quantities. Strawberry harvest is winding down for most. Last week’s heat caused much of the strawberries to become soft and unmarketable. Some varieties are still holding up though, with limited harvest. Some early blackberries are being harvested in good volume, with very good quality. Peach harvest is light and fruit is a bit small. Tomato harvest should begin late this week/early next week. Tomatoes have been experiencing some environmental stress (heat, low humidity, high UV), causing some leaf curl. This should subside with the increased moisture and lower temperatures. Squash are starting to be harvested. Cucumbers harvest isn’t far away. Peas and beans have been affected by thrips, but are growing out of that damage. Sweet corn harvest is getting near. Vegetable planting is starting to resume with the recent moisture and reduced temps.”

Although small and difficult to see, thrips (larva pictured above) can be a limiting factor for yield in muscadines. Thrips feed upon the blooms of the crop, damaging the flowers and early developing fruits. Checking for their presence during bloom is critical for their control. Placing a sheet of white paper underneath the clusters of blooms and forcefully bumping the wire or cordon with your fist should dislodge the insect, allowing it to drop onto the paper for detection. The size of the insect is small – about 0.5 – 1.0 mm in length.

Tony Melton reports, “Peppers and tomatoes are setting fruit. Harvesting pickles consistently. Applying downy mildew and pickleworm materials regularly. Finishing up first cut of collards for processing. Started 2nd cut of processing turnips. Harvesting processing cabbage. Snap beans and butter beans are flowering. Setting sweet potatoes as fast as slips become available.”

Cucurbit Downy Mildew Found In SC

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.

Downy mildew symptoms on cucumber. Lesions are often limited by the veins in the leaves.
Dark-colored downy mildew spores developing on the underside of cucumber leaves.

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.

Field Update – 6/29/20

Coastal

Rob Last reports, “In our area crops are generally looking good with watermelons and cantaloupes coming to harvest.  From a pest perspective, we are finding some early pickleworm and melon worm damage occurring. In addition, cucumber beetles and squash bugs remain active. Cucurbit downy mildew is being found in the area and as such protectant fungicide applications remain viable options. If in doubt scout.”

Zack Snipes reports, “Last week was full of heavy rain and heat. It finally feels like June.  Crops are either going one of two ways right now:  they either look great or they are succumbing to disease. Tomatoes are picking great and I’ve seen some really nice watermelons finish up this week. Peppers are loving this heat but I have seen an uptick in bacterial leaf spot (BLS) in the crop. Keep up with spray programs (copper and Manzate) for BLS in pepper. Tomatoes are also starting to look rough with all the heat and rain. If your tomato crop dies, please identify the culprit so we can better manage it next year. Soilborne diseases such as bacterial wilt and southern blight will not go away next year. I will be more than happy to work with folks on crop planning, cover crops, and rotation on their farms so we can avoid crop failures.

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A quick cut of the stem of a wilting plant and a dip into water can help to positively identify Bacterial Wilt in tomato. The presence of the pathogen will yield a clear to white ooze coming from the plant after a few minutes in the water. Photo from Zack Snipes.

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Bacterial Leaf Spot is spreading in pepper due to the heavy rains and increased heat. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “We finally got some widespread rain mid-week and it has rained in places most days since. Some areas received enough rain to cause temporary flooding in lower-lying fields. Remember, according to produce safety guidelines, any produce that was flooded may not be harvested. We should expect diseases to worsen in the coming weeks. Powdery mildew in cucurbits and bacterial spot in tomatoes has certainly increased in the past week. Crops are still developing very rapidly and we are picking lots of squash, zucchini, cucumbers, tomatoes, eggplant, sweetcorn, greens, beans, etc.”

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This field of cucumbers was temporarily flooded by the heavy rain last week. Photo from Justin Ballew

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A crew picking squash in Lexington. Squash and zucchini are growing like wildfire in the midlands. Photo from Justin Ballew.

Pee Dee

Tony Melton reports, “Raining almost every day some storms causing downed trees.   Root rot bad applying a lot of potassium phosphide. A few strawberry growers still picking around rains. Downy mildew bad but Ranman and Orondis are doing a good job of control. Peas are maturing and will not be too long until harvest. We need to hurry to get the second crop planted on the same land. Still planting sweet potatoes. Okra and tomatoes just started to bare. Southern stem blight is bad and we’re spraying Fontelis. Some first crop butterbeans are being harvested.”

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 – 8/19/19

Coastal Region

Dr. Tony Keinath reports, “Phomopsis blight is showing up on eggplant at the Coastal REC. On susceptible cultivars, like ‘Black Beauty,’ the disease starts as leaf spots; later in the season stem cankers and fruit rot appear. Although several fungicides are registered on eggplant, none are specifically labeled for Phomopsis blight. Aprovia Top and Priaxor might give some control; more testing is needed before a definite recommendation can be made.”

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Phomopsis blight on eggplant foliage. Photo from Dr. Tony Keinath.

Midlands

Justin Ballew reports, “Last week we had some scattered rains, but some areas remain bone dry.  Irrigation is running a lot in those areas.  Muscadine harvest has started and so far, this is the largest yield we’ve seen in recent years.  Squash, zucchini, eggplant, cucumbers, and tomatoes are also being harvested. Quality looks good, but volume is down a bit in some areas.  More fall brassicas are being planted and strawberry growers are just starting the process of prepping their fields.”

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Great crop of muscadines in the midlands this year. Photo from Justin Ballew.

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Good looking eggplant harvested in the midlands. Photo from Justin Ballew.

Pee Dee Region

Bruce McLean reports, “Much of the upper Pee Dee Region received beneficial rains late last week. This helped to drop temps back to about normal, following the excessive heat from a few days earlier. On most crops, the rain really helped to push production again. Late season cucumbers, squash, zucchini, and okra are looking good. Canary melons are looking excellent. Volumes on these crops are good. Last week’s heat did take a toll on watermelons, though. Volumes are significantly down from the prior week and are likely winding down for the season. Muscadines are a bit varied across the region. In some locations, ‘Carlos’ and ‘Noble’ (wine and juice varieties) are getting very close to harvest, while other vineyards are ripening a bit slower. Harvest on the most mature wine and juice muscadines could begin as soon as next week.

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Fall cucumbers were showing some downy mildew earlier on. Fungicide applications seem to have held down further development. Photo from Bruce McLean.

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Canary melons are looking excellent. Photo from Bruce McLean.

Tony Melton reports, “Heavy rain in certain area but none in others.  Some fresh market sweet potatoes are ready for harvest.  Processing sweet potato harvest will begin end of August.  Pickling cucumbers for machine harvest are still being planted most growers are favoring the parthenocarpic varieties.  Brassica planting has begun – seeds and transplants.  Fall bearing blackberries are yielding well.”

Upstate

Mark Arena reports seeing pecan scab. “Pecan Scab fungus is a weather dependant disease. There are varieties/cultivars that exhibit tolerance to this disease as shown below in the photos. Observed in Picture #1, we see both the nuts and foliage are showing signs of infection (the brown lessions). Picture #2 shows no sign of disease on either the nuts and/or foliage at the present time. Pecan scab can go undetected for days before the lessions appear, therefore, it is imperaitive to use a preventitive fungicide program to insure nut production.

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Nuts and leaves infected by pecan scab. Photo from Mark Arena.

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Nuts and leaves appear free of pecan scab. Photo from Mark Arena.

 

Alternaria Leaf Blight Common This Year

From Clemson Extension Plant Pathologist Tony Keinath.

During the 2019 Cantaloupe Disease Survey, Alternaria leaf blight has been found in several fields. It was more common in fields that had not been sprayed recently than in fields sprayed on a regular schedule. It also was found in a field rotated only one year out of cantaloupe.

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Alternaria leaf blight lesions on cantaloupe leaf.

The FRAC Group 11 fungicides (Cabrio, Quadris, Flint) are the recommended fungicides. Although the gummy stem blight fungus is resistant to this group of fungicides, they still are very effective against Alternaria leaf blight on cantaloupe and anthracnose on watermelon.