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 – 4/12/21

Statewide

Dr. Matt Cutulle reports, “I am starting to see some goosegrass popping due to soil temperatures being 65 F. Goosegrass will typically be problematic in more compacted areas of the field. In most broadleaf crops a Select or Poast post-emergent application will control emerged goosegrass. PRE herbicide options include Curbit and Dual Magnum (If crop is labeled). In rice it is important to remember that Quinclorac products will kill crabgrass but not goosegrass. The best rice product that will control goosegrass effectively in SC is Clincher.”

Coastal

Rob Last reports, “Crops are generally doing well in the area, with strawberries coming off with good volumes. On the whole, row covers or icing protected 97% of the susceptible flowers leading to 1-2% losses of flowers. The damaged flowers can increase grey mold pressure in the crops so, maintaining both sanitation and fungicide applications to strawberries will be crucial. As berries ripen, sanitation also becomes essential for reducing pest pressure from sap beetles. In some crops, where row covers were utilized, we see spider mite populations increasing and a few active thrips feeding on both flowers and berries. Other fruit crops in the area, such as blackberries and blueberries, look very good with low levels of damage from the freeze event last weekend. Peaches in the area are being thinned, with scouting being maintained for scale and plum curculio. Early planted watermelons did suffer from the frost in places, leading to 10-15% plant loss and hence the need to replant in a few areas. Other crops are moving slowly away from the injuries. Luckily a lot of crops were not beginning to vine and survived the worst of the damage. These plants are stressed, so care will be needed with any applications as well as scouting for pest and disease issues. Conversely, Cantaloupes in the area were direct seeded and have survived unscathed.”

Zack Snipes reports, “I was out and about last week as things are moving rather quickly in the fields. Spider mites are alive and active in every strawberry patch that I was in last week. You will see the translucent mite with 2 distinct spots as well as a reddish colored mite. The reddish colored mite is actually a two-spotted spider mite. We see this red form early in the season. Get out and scout as the weather is perfect for them. I am also seeing strawberry plants wilting down and dying. If you cut the crown you will see a brown/red rot in the center of the crown. Send these plants off for diagnosis. Most of what I have seen has been Phytophthora. We lost some cucurbit crops to the frost last weekend but some areas had no damage at all or slight damage. For some positive news, we are cutting some beautiful broccoli right now.”

Lower lying areas of fields or areas where the drip tape was nicked is where I am seeing some root rot issues. Photo from Zack Snipes.
Cutting the crown of wilting strawberry plants can help detect the pathogen responsible.  Sending off to the lab is the only way to get a 100% diagnosis of the problem. Photo from Zack Snipes.

Midlands

Justin Ballew reports, “Strawberry picking started on a wider scale this past week. This conveniently coincided with spring break for a lot of the public schools in our area, so U-pick operations have been busy. The weather was very mild last week, so everything is looking great. Spider mite and grey mold activity seems to be very low. There are a few thrips in certain places, so that’s something we need to keep an eye on. Brassicas are growing well. Caterpillar populations are still pretty low in most spring planted crops. There was a little injury to sweet corn from the recent frosts, but we expect the plants to grow out of it. Sweet corn’s growing point is underground until the 6 leaf stage. Since pretty much all the sweet corn was at the 3 leaf stage or younger, the growing points were protected and the damage is just superficial.”

Strawberry season has finally arrived in the Midlands. Photo from Justin Ballew.

Sarah Scott reports, “We are still assessing peach crop damage from freeze events happening between 4/1 and 4/3. It looks like anywhere from 1/4 to 1/3 of fruitlets were damaged by the cold event but with no additional damage we should still be on track for a good crop this year.  Strawberries were delayed a bit from the cold but are recovering nicely. Planting continues for crops like kale and other spring greens.”

Peach fruitlets at shuck off. Photo from Sarah Scott.

Pee Dee

Bruce McLean reports, “Strawberries are really starting to come off. The quality is very good, and the plants are in good health. Disease is relatively low and spider mite activity is moderate. Damage was minimal from the freeze before Easter. We did see some damage on blueberries, though. Blueberries without frost protection (in especially cold locations) did see some significant injury to both early southern highbush and early rabbiteye cultivars. Injury of 80% was observed in Star and O’Neal (southern highbush) and Premier (rabbiteye). Blueberries with frost protection fared much better. This shows the importance of having well-designed frost protection if you are going to grow early-blooming cultivars. Muscadines, being at budbreak, did not show any significant injury. Farmers are taking advantage of this absolutely beautiful weather to plant vegetables. Sweet corn, peas, butterbeans, cucumbers, squash, melons, tomatoes, etc. are going in the ground as fast as they can. Blueberries are being planted now too.

Black and brown seed and tissue within the berry shows that the fruit was injured from the freeze and will not develop. Photo from Bruce McLean.
Damage to early fruitlets on rabbiteye blueberry. Photo from Bruce McLean.

Tony Melton reports, “Most strawberries are doing well and really starting to produce a lot of fruit. Cabbage is starting to head and is growing well. Brassica growers are applying products for diamondback moth and Sclerotinia. Some pickles are emerging and many acres will be planted this week. Some butterbeans are up and more are being planted. We will start to planting peas this week. Cool temperatures slowed sweetpotato slip growth a little, but most beds are covered with slips. Collards, turnips, and kale are growing fast. Hundreds of acres of tomatoes and peppers are already planted. A few acres of watermelons and cantaloupes are planted, but our main market is after the 4th of July.”

Weekly Field Update 11/30/20

Coastal

Zack Snipes reports, “After a long Thanksgiving nap, I was able to waddle out in the fields and look at some strawberries.  We have had some really good strawberry growing weather especially considering most folks got their plants out somewhat late this year.  We need some cold weather to slow them down a bit in places.  I am seeing a tiny bit of plant collapse and death in some spots within the fields.  It is very important to send these plants into our lab to get a positive identification of the pathogen.  Phytophthora crown rot and anthracnose crown rot can cause similar symptoms but are managed completely different. For information on how to submit a sample during COVID times, click here. I am also keeping my eye on a good bit of leaf spotting in some fields to make sure its not the new disease, Neopestalotiopsis. I don’t think we have it yet, but being proactive is better than being reactive.  More information on that disease can be found here.”   

A healthy and a diseased plant side-by-side.  Perhaps a positive identification of the pathogen can help with management to protect the healthy plant. Photo from Zack Snipes.
Determining the pathogen responsible for plant collapse can be tricky in the field.  Send in a plant pathology sample to our lab.  Is this anthracnose, phytophthora, or another pathogen? Photo from Zack Snipes.

Midlands

Justin Ballew reports, “The mornings were nice and cool last week and we saw light frosts in a few more areas. We’ve been getting a fair amount of rain also. This has the brassica crops looking great. Caterpillar populations are still fairly high. Don’t give up on scouting as it gets cooler this week. Diamondback moth caterpillars and adults can survive for several hours at temperatures well below freezing, so a few nights in the upper 20’s is unlikely to affect them, other than slowing down their life cycle a bit. Don’t give up on scouting for mites in strawberries either. Even though we’ve had some wet weather lately, they’re still out there.”

Lacinato kale is growing well and looking good. Cropping has already started in this field. Photo from Justin Ballew
Keep up with scouting for caterpillars as the weather gets cooler. Winters in SC don’t get cold enough to wipe out diamondback moth populations. Photo from Justin Ballew.

Pee Dee

Tony Melton reports, “We still have some sweet potatoes in the ground. Greens are growing well except for bacterial diseases. Some diamondback are hard to kill. We are trying everything.”

Upstate

Kerrie Roach reports, “With a low of 30 degrees Fahrenheit predicted tonight, and 26 degrees F predicted tomorrow night(Tuesday), growers in the Upstate should be making preparations for a hard freeze event. Wind speeds from 10-25 miles per hour have begun, and are expected to continue through Tuesday. So make sure any protective measures are held down tightly!”