Managing Cucurbit Downy Mildew on Organic Farms

Video Transcript

This video will focus on managing cucurbit downy mildew on organic farms. Downy mildew (caused by the oomycete Pseudoperonospora cubensis) has always been a disease of cucurbit crops. There are other downy mildews that affect other crops, but these generally cannot also infect cucurbits.

Dr. Michael Mazourek (Cornell University, Plant Breeding & Genetics Section): This is the characteristic symptom of downy mildew on cucumber. A lot of cucurbits get downy mildew, but the symptoms are very obvious on cucumber. It's this kind of quilted appearance on the top, and on the bottom is where you see the reason it has its name. There are these black spores underneath the yellow regions. If there's any uncertainty, you could put a leaf in a wet paper towel overnight, and see if it starts to sporulate, but if it's wet like in this high tunnel, there's no need to do that—you'll see the sporulation right away.

Downy mildew is particularly severe when the crop experiences alternating wet-dry cycles, as these are needed by the pathogen to reproduce and infect new hosts via airborne spores. If you have questions about whether your crop has downy mildew, you can consult a plant pathologist at your local land grant university.

Dr. Michael Mazourek: This leaf has a mixed infection—multiple pathogens are affecting it. We can see there's some downy mildew affecting it, and confirm that by seeing the quilted regions, the squares of the downy mildew spores on the back, that are black. It also has Alternaria. It would be hard to tell for sure that this is Alternaria, without actually looking at the spores under a microscope—like your local diagnostic lab might be able to help you with—but this is also, I know, a cultivar that's susceptible to Alternaria, and the cultivar we were just looking at before is not susceptible (it's resistant) to Alternaria, so that makes this quite a telling diagnosis.

As you are getting some diagnostics done, it's important to have a leaf just in the initial stages, because you'll have secondary infections moving in that are taking advantage of the diseased plant. So if you were to provide a sample like this to your diagnostic lab, there's just so much going on and taking advantage of the infected plant tissue that you can't really tell what was the cause—there are many other pathogens moving in and further colonizing this leaf that aren't necessarily the ones that caused it to die.

Zaid Kurdieh (Norwich Meadows Farm, Norwich, NY): At Norwich Meadows Farm, the cucumber crop is definitely the most difficult crop for us to take care of because of pests and diseases. It seems like downy mildew is getting worse and worse every year. We're getting it, it seems like, earlier and earlier; it did not used to be as bad years ago—say ten years ago.

Until 2004, downy mildew-resistant varieties of cucumber were commercially available and widely used. In 2004, the disease changed radically, and cultivars that were resistant to the old strain are now susceptible.

This video will provide information about finding cultivars that are resistant to this new strain, and the timing, chemical, and cultural strategies that you can use to reduce your losses due to cucurbit downy mildew.

There are currently very few choices for downy mildew-resistant cucumber cultivars. Some Asian-type cucumbers, like Suyo Long, tend to have usable resistance. In slicing cucumbers, there are cultivars with moderate to high levels of resistance coming out of the Cornell University program: Marketmore 97, Marketmore 420, and more recent releases DMR-264 and DMR-401.

Melon is the second most susceptible cucurbit to downy mildew. The Mazourek program at Cornell has identified resistant cultivars that are older, like Seminole, and developed a new cultivar, Trifecta, that's also commonly available. Hopefully, there will be many more choices in the future. While summer squash is the least susceptible to downy mildew, it can still suffer significantly.

It is important to remember that if you're growing both susceptible and resistant cultivars, or if you're growing a highly susceptible variety upwind of your resistant variety, the resistant cultivar can be overwhelmed. This high tunnel, for example, is downwind of an infected field of a susceptible variety of squash, so the cucumbers inside are being exposed to high levels of the airborne spores. Plan to grow susceptible cultivars downwind of resistant ones.

You need only use resistant cultivars when you know you are at risk for the pathogen. CDM IPM Pipe is a helpful website that provides forecasting information, and histories of when the disease has arrived in certain areas. Importantly, it can advise to the vulnerability of your crop (given your location), so you can learn how to time your interventions during times when the pathogen is likely to be in your area.

Knowing when you typically need to use a resistant cultivar can be a useful strategy for your farm. You should know that when you are working with resistant cultivars, there are often trade-offs for agronomic or quality characteristics. As you can see in this graph, there are many other great varieties to grow earlier in the season, when cucurbit downy mildew is not a problem. Use resistant cultivars when and where appropriate.

One OMRI-listed pesticide that has been shown to be an effective and therefore potentially useful tool for controlling cucurbit downy mildew is Zonix™, a rhamnolipid biosurfactant that kills zoospores.

For more information about Zonix™ and a variety of other chemical control methods, consult up-to-date resources as people explore the best ways to integrate biological and chemical controls.

NOTE: Before applying ANY product, be sure to 1) read the label to be sure that the product is labeled for the crop you intend to apply it to and the disease you intend to control in your state and local area, and 2) make sure that the brand name product is listed your Organic System Plan and approved by your certifier. For more information see Can I Use this Product for Disease Management on my Organic Farm?

Zaid Kurdieh:  Zonix™ definitely helps. This year, I think, because of the intense pressure, certain cultivars succumbed, they just couldn't handle it; the pressure was way too high.

Increasingly, growers are using high tunnels to have more control over their growing environment.

There are different designs of high tunnel, and differences in architecture affect the conditions inside. Gothic high tunnels, where trellises are hung from the rafters and humidity can be controlled through gable end vents, tend to be much drier. Under drier conditions, downy mildew cannot reproduce, so if your high tunnel design allows you to control humidity and leaf wetness, you'll be able to control downy mildew more effectively. Rounded high tunnels tend to be more humid than vented gothic high tunnels, but are still a more controlled environment than the field.

Even in a high tunnel, downy mildew can establish itself, leading to crop loss. In the drier high tunnels, we typically see the disease on the perimeter, where the plants are subject to outside conditions.

Because high tunnels change the environment, growing in a high tunnel requires varieties that are well suited to high tunnel production. If the high tunnel is being used to exclude striped cucumber beetles and other pests, it will be necessary to grow parthenocarpic cultivars that do not require pollination to set fruit, such as Beit-alpha cucumbers. Seed catalogues will include the term parthenocarpic in their descriptions—look to determine whether netting the high tunnel will be useful or detrimental based on the cultivar you're growing.

In a high tunnel, with increased transpiration, there can be an increased severity of wilt pathogens that might not be a problem outdoors in the Northeast, such as bacterial wilt.

Dr. Michael Mazourek: Here is a cucumber suffering from bacterial wilt. It is a bacterium transmitted by striped cucumber beetles—as they feed they'll infect the plant—and you have this appearance very much like the plant is shutting like an umbrella. Also accompanied with it, you can see in this leaf symptom here, is that there will be this browning between the major veins in the leaves that often accompanies it. There are many other wilts that can affect a cucumber plant, but bacterial wilt is by far the most common, especially in high tunnels, in my experience.

There is one bacterial wilt-resistant cucumber cultivar: please contact Dr. Michael Mazourek at Cornell University if you are interested in growing this variety.

There are a few considerations to keep in mind when growing under high tunnels. First, it is a good idea to rotate crops in your high tunnel to break up disease cycles.

Zaid Kurdieh: Those rotations are critical. Rotations are very, very critical.

Growing a wide range of crop families can make a significant difference in disease pressure, and be a valuable tool in soil fertility management. Nutrient management requires greater attention under high-tunnel cultivation, as the lack of soil exposure to rain can facilitate the buildup of soluble salts.

One way to address this is to have a rolling high tunnel, like this one at Cornell's Dilmun Hill student farm. The rails allow the tunnel to be moved from one field to another; this way, one plot is exposed to precipitation every 6 months, allowing salts to leach more effectively from the soil.

Alena Hutchinson (Cornell University, Mechanical and Aerospace Engineering '18): Here at Dilman Hill, we have a movable high tunnel. It's just like a traditional high tunnel, but it's on a set of rails, and the tunnel itself is mounted on pipe-gate rollers, which are actually used in chain-link fences. And that allows the tunnel to be moved by just one or two people from one site to another.

The plot that's uncovered by the high tunnel is exposed to the elements, and also, generally, we have a cover crop planted there. And what that does is while your cover crop is replenishing the good nutrients that you do want your plants to have for the next season, the rain and snow and other elements that that side is exposed to, (it) has kind of the buildup, that you don't want, washed away.

We designed the high tunnel here at Dilman Hill, and the blueprints are freely available on our website.

You can also look at the economic analysis tool that we've developed to help you consider the costs and benefits of these interventions. It will be hosted here on the resource page of the Eastern Sustainable Organic Cucurbit Project.

In the end, you'll have the best results when you combine all of these practices into a holistic strategy for managing cucurbit downy mildew. Understanding when your farm is at risk, which cultivars provide you protection and when to use them, which chemical controls might be appropriate for your production, and how to grow cucumbers under high tunnels, can all increase your ability to successfully manage the disease on your organic farm.

Many thanks to the USDA's National Institute of Food and Agriculture for supporting this work.

Additional Resources

High Tunnel Blueprints:

• Dilmun Movable High Tunnel—Track Assembly Guidelines https://cuaes.cals.cornell.edu/sites/cuaes.cals.cornell.edu/files/shared/documents/Dilmun%20Movable%20High%20Tunnel%20-%20Assembly%20Guide.pdf
• Dilmun Movable High Tunnel—Endwall Design https://cuaes.cals.cornell.edu/sites/cuaes.cals.cornell.edu/files/shared/documents/Dilmun%20Movable%20High%20Tunnel%20-%20Endwall_.pdf
• Dilmun Movable High Tunnel—Sidewall Designhttps://cuaes.cals.cornell.edu/sites/cuaes.cals.cornell.edu/files/shared/documents/Dilmun%20Movable%20High%20Tunnel%20-%20Sidewall_.pdf