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The Plant Health Instructor

Volume: 22 |
Year: 2022
Article Type: Case Studies

​​​​​​​​Battling Powdery Mildew on Organic Acorn Squash in Mesotunnels​

Kellie Damann and Sarah Pethybridge, Cornell AgriTech, Geneva, New York; and Mark Gleason, Iowa State University, Ames, Iowa

Date Accepted: 28 Nov 2022
 Date Published: 08 Dec 2022

Keywords: Disease Control, Pest Management, Powdery Mildew, and Vegetables

This case study is designed to be used in a single 50-min class, although it can be extended to fit a longer or multiple class periods. The target audience is an undergraduate class of 10 to 50 students who are majoring in plant pathology, entomology, horticulture, integrated pest management (IPM), sustainable agriculture, vegetable production, organic agriculture, or agricultural extension and outreach.​

It is recommended that students come prepared to the class by reading this case study ahead of time and designing a spray schedule in response to the first question under the suggested questions—this way the class period can be used to discuss questions that arise, allowing time for an in-depth discussion on the rest of the questions within the case study. The background text should be available to students who would like to read further information. A short, targeted quiz or discussion led by the instructor may encourage student engagement and enhance the depth of their experience with the background reading.

In a 50-min class period, the instructor should spend the first 5 to 15 min summarizing the content within the case study and answer the questions embedded within the study. For the following 20 to 30 min, students may split into groups of two to five people to discuss and agree on an IPM strategy the grower could implement next year. In the remaining 10 to 15 min of the class, the groups may propose their ideas to the larger group and have a discussion on the pros and cons of each strategy.

This case study was road tested by the Introduction to Horticulture (HRT 110-02) class at Finger Lakes Community College, Canandaigua, NY, in April 2022. Feedback from students and the instructor was incorporated, including links to access pesticide information to support the design of a spray schedule (Question 1).​


The goal of this case study is to engage students in the kind of problem-solving techniques and balancing act that growers perform in managing different diseases while maintaining optimal yields. Students will gain a better understanding of IPM approaches and consider multiple factors, including vegetable production under organic standards, insect-vectored transmission of bacterial pathogens, pathogen biology, the role of pesticides, and resistance to pesticides in pathogen populations.


After completing this case study, students will

  • Recognize the signs and symptoms of powdery mildew on cucurbits.
  • Understand the life cycle of the pathogen that causes powdery mildew of cucurbits.
  • Identify management strategies for both insect-vectored diseases and fungal diseases in organic systems.
  • Evaluate advantages and limitations of IPM-based approaches.​

Figure 1. Jesse (left) visiting Ronnie (right) at her farm. (Photo by Kellie Damann, Cornell University, NY)

Figure 2. An adult squash bug laying eggs (upper left) and squash bug nymphs along with eggs (upper right). High squash bug populations can lead to plant collapse due to Cucurbit yellow vine disease (botto​​m). (Photo by Kellie Damann, Cornell University, NY)​
Figure 3. Installation of nylon-mesh fabric over metal hoops. (Photo by Sandeep Sharma, Cornell University, NY)

Figure 4. A mesotunnel established to protect plants from insect pests and insect-transmitted diseases. (Photo by Kellie Dama​nn, Cornell University, NY)​

Figure 5. Squash bugs excluded from the crop because of the netting barrier. (Photo by Kellie Damann, Cornell University, NY)

Figure 6. Powdery mil​​dew on a cucurbit leaf. (Photo by Kellie Damann, Cornell University, NY)


Ronnie McArt (she/her; Fig. 1): Organic grower in western New York who has been specializing in cucurbit production for seven years.

Jesse Lyell (they/them; Fig. 1): Vegetable Extension specialist who focuses on organic crop production and protection.


(NOTE: See glossary for definitions of specialized terms.)

Ronnie McArt is an organic vegetable grower in western New York who has been operating her family's farm for the last decade. Over the past few years, she has been specializing in cucurbit crops, mainly winter squash. The farm has been in her family for three generations, and she takes great pride in its connection with the local community. Ronnie enjoys getting to know each of her customers and listening to their requests. She hosts a Community Supported Agriculture (CSA) enterprise on her farm and attends the markets in the region twice a week to sell her produce. Her customers always come back for her produce because, as they say, “It's simply the best."

During the past few years, she has been struggling with high populations of a pest insect called squash bug (Anasa tristis) (Fig. 2). Squash bugs carry the bacterium Serratia marcescens, which causes Cucurbit yellow vine disease (CYVD; Fig. 2). Ronnie lost 45% of her acorn squash crop last year and 60% the year before, which had a significant impact on her profit margins. Since she is an organic grower, only pesticides listed with the Organic Materials Review Institute (OMRI) can be used. She has been applying OMRI-approved insecticides frequently to control the squash bug populations but without much success. It's important to time spra​​​ys just right, as squash bugs are most vulnerable when they are young. Monitoring populations—making counts of squash bugs at regular time intervals during the season—helps in timing insecticide sprays for maximum impact, but this is a very time-consuming process. Ronnie is getting very discouraged and doesn't know what else to do.​

As a result of poor squash bug control, her farm operation is becoming less and less profitable. Also concerning are the negative comments from customers on social media due to the scarcity of her squash. If this crisis continues, she will need to abandon growing acorn squash, which will alienate many loyal customers.

At a loss for answers, Ronnie reached out to her local Extension office. She connected with Jesse Lyell, an extension specialist in vegetable production and organic systems, and asked about any potential solutions.

After much discussion about Ronnie's dilemma, Jesse suggested a new tactic. Rather than planting into bare ground and leaving the plants exposed all season, Jesse suggested using a full-season row cover, called a mesotunnel, to protect her squash from the squash bugs and help reduce insecticide applications. Jesse explained the system to Ronnie: “It's pretty simple to set up…. You just need some metal hoops, the nylon mesh fabric, and some sandbags—then presto, you have a tunnel!…. The row cover acts as a barrier between the plants and the insect; the bugs just can't figure out how to get in!"

Ronnie was very excited to hear about mesotunnels and decided to give them a try. This year she chose to transplant the squash crop instead of seeding into bare ground to give her crop a head start. She placed the plants in rows with a 2-ft spacing between each plant. After transplanting the squash, she draped the netting over the 4-ft-tall metal hoops and secured the netting to the ground with sandbags (Figs. 3 and 4).

Once the squash plants started to bloom, she inserted a bumblebee hive, which she purchased online, inside the tunnel to facilitate pollination. Jesse had mentioned that, “Cucurbit crops such as acorn squash need insects, particularly bees, to aid in pollination. Given that this system excludes natural pollinators, inserting a hive fulfills the need for pollinators. This way you do not have to take off the netting (except when inserting the hive) and expose the plants to insect pests."

Ronnie monitored the crop weekly and was impressed with how effective the netting was at excluding insects. She saved time and money from not having to apply insecticides. She noticed the squash bugs on the outside of the netting, but they could not get to the plants (Fig. 5). She also observed a significant decrease in the number of plants with CYVD and thought to herself, “Yay, significant progress!"

However, toward the end of the season she noticed white patches forming on a few leaves within the tunnel (Fig. 6). The white patches were mostly on the underside of older leaves. She had not been irrigating recently, despite it being a hot and dry summer, because she was worried about running out of water in her pond. She contacted Jesse, who came out to look at the plants and collect some samples.

Jesse told her, “It looks like powdery mildew is starting. This is a fungal disease caused by Podosphaera xanthii. If you don't act soon, Ronnie, a large portion of the crop could become infected. This season has been great for powdery mildew to take hold, as it prefers warm conditions with little rainfall." They then discussed the life cycle of powdery mildew, so she would have a clearer understanding of disease management (Fig. 7).

Figure 7. Life cycle of powdery mildew on cucurbits (image produced in BioRender).

Jesse explained to Ronnie why management of powdery mildew is important: “This fungus can weaken the structure of the plant and cause premature defoliation that reduces photosynthesis and plant health [Fig. 8]. It may cause the plant to grow fewer and/or smaller fruit or produce lower quality fruit due to sun scald." Ronnie was also alarmed to hear that powdery mildew has the potential to harm flavor and storability, because fruit ripen faster or even fail to ripen on a diseased plant. This threat could impact her returning customers, many of whom show up mainly to buy squash, and could damage her farm's reputation.​

Ronnie asked Jesse, “What can I do? After all this work setting up the mesotunnel, I don't want to lose the crop now!"

Figure 8. Damage from ​​powdery mildew on acorn squash. Shriveled foliage exposes the squash fruit to the sun, leading to sun scalding. (Photo by Kellie Damann, Cornell University, NY)
Jesse said, “My best suggestion would be to start a spray program with OMRI-listed fungicides, to slow down disease spread and prevent it from taking over. You can spray directly through this netting, so you don't have to be hassled by removing it before every spray. I also recommend regular scouting to see how fast the disease is spreading. A general rule of thumb I use for cucurbits is: if I see 1 of 50 leaves with powdery mildew, I recommend starting a spray program to try and stay on top of things. Once the horse has bolted there is no return!"

​Next, Ronnie did some research on powdery mildew. She quickly found OMRI-listed fungicides that she could use on her crop. However, these fungicides came with a type of restriction called a pre-​harvest interval (PHI), which requires a minimum number of days between the last spray and the harvest. There were only six weeks left before harvest, so she needed to pay attention to the PHI. She also likes to enter her field frequently to check it out, so she is conscious of the re-entry interval (REI). REI is the minimum time required between the fungicide application and when it is safe to re-enter the crop without proper protective equipment (PPE) like gloves, a TyvekÒ suit, safety goggles, etc. Ronnie also gathered information on the Fungicide Resistance Action Committee (FRAC) code for each product. This is important information, since rotating fungicides with different modes of action is important to prevent the fungus from becoming resistant to the fungicides—and thus no longer effective.

Armed with this information, Ronnie knows she has some important decisions to make. She thought to herself, “I need to keep the plants healthy enough so the squash is protected until I can sell them on my stand. What's the best strategy so my customers will be happy and keep returning?"


  1. Design a spray schedule, using two or more fungicides (refer to Table 1 in the Background section and the links provided below the table), that Ronnie could use to prevent powdery mildew from spreading in her acorn squash until she is ready to harvest.
  2. What are the benefits of rotating fungicides from different FRAC groups?
  3. What are some other reasons why powdery mildew is starting to take over in the tunnel? Are there other environmental conditions that may be favorable to this disease inside compared to outside the tunnel?
  4. Are there other preventative measures Ronnie could take next year so this outbreak does not occur?
  5. Putting yourself in Ronnie's place, how would you decide whether to use mesotunnels for your organic acorn squash production?​


IPM principles help all growers but are especially important for organic growers, who often combine multiple approaches to deter diseases and pests. Cultural control is an IPM approach that aims to suppress conditions that are conducive to pest or pathogen survival. This includes selecting sites and resistant or tolerant crop varieties to minimize the risk of outbreaks, as well as sanitation (cleaning up debris between crop cycles) and crop rotation (shifting the location of related crops from place to place on a farm, which minimizes the risk of pest and pathogen buildup from year to year). Timing can also play a substantial role in maintaining plant health. For example, a delay in planting by days or weeks may help avoid periods when specific pests or pathogens are particularly threatening. Mechanical control methods are often used in combination with cultural practices in organic systems. Mechanical measures aim to physically exclude the crop from the pest or disease. This may include removing alternative host plants for the pests and pathogens or establishing a physical barrier to keep pests away from the crops.

One IPM strategy of particular interest for organic cucurbit crops is row covers. Row covers provide a physical barrier between the crop and damaging insect pests. They come in many different types, including low tunnels, mesotunnels, and high tunnels, all of which are supported with hoops.

Mesotunnels are intermediate in size between low tunnels and high tunnels: about 3.5 ft tall compared to 1.5 ft for low tunnels and 16 to 20 ft for high tunnels (Fig. 9). Mesotunnels are covered with a nylon-mesh fabric that looks like window screen. Because mesotunnel fabric is breathable, plants don't overheat, so some cucurbit crops can remain protected from insect pests during all or most of the growing season. In contrast, low tunnels need to be removed as soon as plants start to produce female flowers, leaving the plants exposed for the rest of the growing season. High tunnels are generally considered too costly for profitable cucurbit production.

Bees play a major role in pollinating cucurbit crops as they transfer pollen from the male to female flowers, after which fruit will form. In mesotunnels, the cover remains over the plants for most or all of the growing season, which excludes pests but also bees. Bees can be purchased and placed in the tunnels to enable pollination and fruit set.

Figure 9. Mesotunnel (center) compared to a high tunnel (left) and low tunnel (right). 

As the growing season continues, new threats to profitable cucurbit production can appear. Diseases like bacterial wilt (BW) and CYVD can kill plants. Both diseases are caused by bacterial pathogens that are spread by insects. BW is caused by Erwinia tracheiphila, which is spread by cucumber beetles. CYVD is caused by S. marcescens, which is spread by squash bugs. An entire crop can be lost due to these diseases. Once a plant becomes infected with either of these pathogens, it's a death sentence for the plant. This is where the season-long barrier of mesotunnels becomes so important: if the pests can be kept away from the crop, the vectors of the bacteria that cause BW and CYVD can be excluded.

Other plant pathogens have a different battle plan. Many of the​m spread through the air as microscopic spores that are much too small to be excluded by the nylon-mesh screen on mesotunnels. Foliar diseases, including downy mildew (DM; caused by the oomycete Pseudoperonospora cubensis) and powdery mildew (PM; caused by the fungus P. xanthii) are significant threats to cucurbit production. Scouting to detect disease onset and monitoring environmental conditions conducive for disease development are critical to managing these diseases. Few OMRI-listed pesticides are available for controlling these diseases once they are established and are likely to provide only moderate control. However, if a grower observes favorable weather conditions for a disease outbreak, preventative measures can be taken before the disease is present. Using preventative fungicide applications is a good way to deter the disease from significantly impacting the crop, but constant monitoring is still needed.

Many factors can influence a grower's decision about which fungicide to use. These may include the PHI, REI, and mode of action. The mode of action is represented by the FRAC (Fungicide Resistance​ Action Committee) group. The FRAC group is important to comply with best management practices for fungicide resistance. Each FRAC group has a different mode of action. Table 1 shows some of the pesticides registered for use in organic cucurbit production.

Table 1. This​ chart provides information about different pesticides used in organic cucurbit production and highlights key categories to look for on the label when selecting a pesticide

Target Diseases Active Ingredient Mode of Action P​HI (hr) REI (hr) FRAC Group
Double Nickel WDGFoliar and soilborne diseases: downy and powdery mildews, crown and root rots25% Bacillus amyloliquefaciens strain D747Preventive biofungicide for contr​ol or suppression
MilStopAlternaria leaf spot, anthracnose, downy and powdery mildews85% Potassium bicarbonateInhibits enzymes involved in fungal cell wall formation01Not classified
RegaliaAlternaria blight, anthracnose, downy and powdery mildews5% Extract of Reynoutria sachalinensisInduced systemic resistance (preventative)04P5
TrilogySeveral insects and diseases, including Alternaria leaf spot, anthracnose, powdery mildew70% clarified hydrophobic extract of neem oilOil prevents fungal attack on plant tissues04Not classified
Serenade Opti Botrytis, powdery mildew, Xanthomonas, Sclerotinia26.7% QST 713 strain of Bacillus subtilisStops spore germination, disrupts cell membrane growth, and inhibits attachment to leaf0444

Many tools can help growers to identify potential high-risk areas for disease. Forecasting websites are available to facilitate tracking of the most important disease threats. Pesticide lists have also been created to make it easier for growers to search for suggestions, for example Cornell's Pesticides for Cucurbit Powdery Mildew (see link in Reference section), as well as websites created to easily access pesticide product information and labels (see NYSPAD link in Reference section). Additionally, many U.S. growers have local Extension offices they can reach out to with questions or concerns.


Thank you to Berna Ticonchuk for running this case study through the Introduction to Horticulture class at Finger Lakes Community College.


Gevens, A. J. 2016. Cucurbit powdery mildew. Vegetable crop update. University of Wisconsin-Madison Extension.

McGrath, M. T. 1997. Powdery mildew of cucurbits. Fact sheet. Cornell Cooperative Extension.

McGrath, M. T. 2021. Cucurbit powdery mildew​. Cornell Vegetables. Cornell University.

New York State Pesticide Attribute Database.

Pair, S. D., Bruton, B. D., Mitchell, F., Fletcher, J., Wayadande, A., and Melcher, U. 2004. Overwintering squash bugs harbor and transmit the causal agent of Cucurbit yellow vine disease. J. Econ. Entomol. 97:74-78.

Schumann, G. L., and D'Arcy, C. J. 2010. Essential Plant Pathology, 2nd ed. American Phytopathological Society, St. Paul, MN.

U.S. Department of Labor: Occupational Safety and Health Administration. Personal Protective Equipment. Personal protective equipment overview.

USDA National Agricultural Library. Community Supported Agriculture.