Sustainable Disease Control In Cucurbits
April 29, 2015 For growers of cucurbit crops, keeping diseases such as downy mildew and Phytophthora out of your fields can be particularly challenging, whether you are an organic or conventional producer. You strive to be sustainable and use your resources wisely because you want to continue to farm for years to come. As a result, you need to take several things into consideration when determining a disease-control strategy.
To help simplify your strategy and stay on top of plant health, Anthony Keinath, a professor and vegetable pathologist at Clemson University Coastal Research & Education Center in Charleston, SC, encourages growers to follow five general principles.
Cucumber leaves often
show a characteristic
“checkerboard” pattern
with downy mildew.
Photos credit: Anthony Keinath
Exclusion. This principle focuses on keeping pathogens off your farm in the first place. To exclude pathogens, begin with clean, disease-free seed and transplants, he says.
Eradication. Make sure materials and equipment used in the field are clean and not harboring pathogens, Keinath explains. Eradication also means getting rid of potentially contaminated crop debris that can move from field to field when the wind blows.
“[Eradication] can be difficult as it is not easy to get rid of anything completely, but you can make an attempt to lower the pathogen level,” he says.
The next three principles, Keinath says, involve strategies to implement if exclusion and eradication aren’t going to work and you have to live with the pathogen on your farm.
Host plant resistance. Use varieties that have disease resistance if they are available.
For example, Keinath mentions SV4220CS, a new slicing cucumber variety from Seminis that has some resistance to downy mildew. In addition a zucchini, Spineless Perfection from Syngenta, tolerates downy mildew and is resistant to powdery mildew. Plus, a straight-neck summer squash, Lioness, from Harris Moran has resistance to four viruses.
Crop protectants. Use registered fungicides and remember that most of these products must be applied preventively.
Avoidance. Implement cultural practices, such as planting when the disease won’t be in your area, provide proper plant nutrition to encourage healthy growth, and use drip irrigation to keep leaves dry.
How To Avoid Downy Mildew
Avoiding disease in the first place is sometimes the simplest way to reduce the chance of your crop becoming infected. In some instances this can be done by changing your planting date.
“To reduce the incidence of downy mildew, and because the pathogen moves from south to north, you want to plant early so you get as much plant growth as possible before the pathogen arrives in your area,” he explains.
Keinath also points out that in the South some growers have been successful growing cucumbers on trellises. The use of a trellis allows the leaves to dry faster after irrigation, making them less susceptible to infection. In addition, he encourages growers to spray the trellised cucumbers with a copper fungicide and use resistant or tolerant varieties, if they are available.
Citing downy-mildew-tolerant varieties of summer squash as an example, Keinath says when the disease is prevalent and the tolerant squash eventually become infected, most of the time yield is not reduced.
“More than 50% of leaves need to succumb to disease before you see a reduction in the number of squash,” he explains. “That is the definition of tolerance. The plant still gets the disease but it doesn’t significantly reduce the yield.”
He also encourages implementing multiple control measures, such combining the use of resistant or tolerant varieties and crop protectants, when one by itself won’t get the job done. Keinath says copper is the main organic fungicide option to protect cucurbits from downy mildew. Conventional growers can use products such as Ranman (FMC Corp.) or Gavel (Gowan Co.), he adds.
Prevent Phytophthora
Unlike downy mildew, which can be carried by the wind, Phytophthora survives directly in the soil and has the potential to be devastating to cucurbits as there are no varieties available that offer resistance with the exception of a few hard-shelled pumpkin varieties.
As the disease primarily spreads by moving water, Keinath says to keep potentially contaminated water off plants, use raised beds and drip irrigation, and don’t plant in low areas that collect water in heavy rains. It is the low areas where Phytophthora will start and then spread to the rest of the field.
He also recommends using mulch to help reduce the incidence of disease because it acts as a barrier between the pathogen in the soil and the plant.
Keinath tells organic growers to implement cultural controls, such as keeping equipment and materials used in the field clean and regulating water to help keep Phytophthora at bay. Conventional growers can alternate between Revus (Syngenta) and Presidio (Valent U.S.A.) to keep the disease in check.
“For small acreages, you should remove diseased fruit from the field because most of the reproduction of Phytophthora in the field is on the diseased fruit,” Keinath adds. “As a result, you will reduce the amount that is in the soil that will carry over to the next year.”
Protectant Products Are Not Rescue Materials
According to Anthony Keinath, a professor and vegetable pathologist at Clemson University Coastal Research & Education Center in Charleston, SC, one thing many growers — both organic and conventional — don’t understand is that copper and sulfur fungicides are protectants only. That means to be effective, they need to be applied before disease is found in the field. For organic producers, however, these materials are designated to be used as
a “last resort.”
“If you read the National Organic Standards, [the approved fungicides] are to be applied after you have applied biopesticides and they have not worked,” he says. “At that point, though, it is too late to save the crop.”
The solution for an organic grower, he says, may be to discuss this situation with his organic certifier. “Have the certifier agree that you can use copper and sulfur as protectants and not wait until the end to use these products. I think the certifier would agree to this if the grower has evidence that the year before he used the products as a last resort and lost his crop.”
The Importance Of Compost And Cover Crops
By staying on top of plant health, your cucurbit crop will be better prepared to fend off diseases. Anthony Keinath, a professor and vegetable pathologist at Clemson University Coastal Research & Education Center in Charleston, SC, says the use of soil amendments will help improve soil health and ultimately plant health for conventional and organic growers.
“On-farm freshly prepared compost is the best,” he says. “[The compost] has beneficial, active microorganisms. Depending on how long it has been since it was composted, bagged, and applied, I have seen dramatic, noticeable growth response with vegetables when fresh compost is used.”
He also suggests the use of cover crops. Those with more experience with cover crops typically use mixes, which means they are using at least two types of cover crop, such as a grain and legume or a brassica and a grain. Keinath recommends two cover crops because using more than one can increase the range of benefits you can get for your soil.
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