​​​​Background Information​​ ​​Instructor Resources ​
Learning outcomes​

1. Strengthen understanding of the disease triangle concept
2. Understand how disease management differs for organic and conventional growers
   

Objectives

1. Understand how cucumber beetles spread cucurbit bacterial wilt
2. Develop a realistic disease management plan for an organic vegetable crop​
   

CAST OF CHARACTERS

Quinn Appleseth: Commercial fruit and vegetable grower

Graciela Hernández: Plant disease diagnostician​

THE CASE​​

Several factors prompted Quinn to initiate the process of certifying his land for organic production last year. In recent years, more and more residents of Quinn's growing rural community have expressed interest in buying organic produce. Furthermore, Sullivan Sisters, a regional chain of natural food stores dedicated to providing organic foods, opened a store this year in a large city just 1.5 hours from Quinn's community. He was confident he could market organic produce locally and to Sullivan Sisters. Quinn also saw the switch to organic as an opportunity to adopt growing practices that would better preserve his land and provide an opportunity to include his son, Andrew, in the farm's management. As an agronomy student at Iowa State University, Andrew was well informed about current best practices and resources available to growers. He had been urging his father to try new growing practices such as diversifying the farm's habitat to increase populations of beneficial insects and incorporating cover crops in his crop rotation sequence to enhance soil fertility. Quinn thought going organic would be a great way to meet local demand while also adopting growing practices that would spark Andrew's interest in the business. He knew there would be plenty of education and hard work up front to implement organic growing practices, but he expected the higher prices he could gain from selling organic produce would make his efforts worthwhile.

​​ Figure 1. Striped cucumber beetles defoliating a young muskmelon seedling.​

This year was Quinn's first attempt at growing produce using organic methods on the land he was trying to certify. Because he had experienced great success in growing muskmelon (also known as cantaloupe) conventionally in previous years, Quinn looked forward to growing it organically this year. He ordered organic-approved pesticides to prepare for his usual melon pests - the fungal disease anthracnose, along with two species of cucumber beetles (striped and spotted) and the bacterial wilt pathogen (Erwinia tracheiphila).  Unfortunately, his crop really struggled. Shortly after transplanting seedlings into the field in late May, Quinn noticed very high numbers of cucumber beetles causing feeding injury to his seedlings. Despite spraying organic-approved insecticides weekly to combat them, the beetles persisted. After a month, several young seedlings were killed from beetle feeding damage (Figure 1), and about 60% of the remaining plants began to wilt and collapse. Quinn was very unhappy with the results. He had never seen such severe damage in his conventional muskmelon fields. Quinn immediately sent fresh plant samples and photos of the damage to the Iowa State University Plant and Insect Diagnostic Clinic to diagnose the cause of wilting. From Quinn's high quality samples, Graciela Hernández—a diagnostician at the clinic—was able to detect the presence of Erwinia tracheiphila, the causal agent of cucurbit bacterial wilt. She called Quinn to explain her findings.

Quinn: “Hi, Graciela. Thank you so much for figuring out the problem with my muskmelons. What can I do to save my plants?"

Graciela: “Hi, Quinn. I'm so sorry. Unfortunately, once a muskmelon plant is infected with Erwinia tracheiphila it can't be saved. It's a death sentence."

Quinn: “So how do I prevent my plants from becoming infected in the first place?"

Graciela: “Well, it's really difficult to do with organic management. You have to keep cucumber beetles from feeding on your crop because they spread the bacterium that causes bacterial wilt."

Quinn: “So in addition to causing feeding damage to my seedlings, the cucumber beetles also spread bacterial disease?"

Graciela: “That's right, Quinn. The bacterial pathogen survives inside cucumber beetles and cucurbit plants. To manage the disease you have to manage one of those two things. Unfortunately, there are no bacterial-wilt-resistant muskmelon cultivars, so the beetles have to be managed. As you've learned the hard way, organic-approved insecticides often don't provide enough protection against cucumber beetles, so you'll need to include some other strategies in your pest management toolbox. As you probably know, some seasons are worse than others for cucumber beetles, and the risk of damage by cucumber beetles and bacterial wilt is very unpredictable. Some years you may have lots of beetles but no bacterial wilt, other years you might have no beetles and no wilt, but then you get some years with large numbers of beetles and a high risk of bacterial wilt. Let me send you some basic information about bacterial wilt and a description of recommended organic management strategies. You might be interested in giving some of these a try."

Quinn: “Thanks, Graciela. I've got to figure this out. I can't afford another failure like this!"

From reading the information provided by Graciela, Quinn learned that bacterial wilt affects plants in the cucurbit family including muskmelon, summer and winter squash, pumpkin, and cucumber, but watermelon is highly resistant.  Plants wilt after the bacterium enters the xylem, and the only way the bacterium can enter the xylem is through feeding wounds created by striped and spotted cucumber beetles. The beetles ingest the bacterial pathogen when they feed on infected plants, and then they survive the winter in the soil as adults. When temperatures warm up in the spring, the hungry beetles emerge from the soil and search for cucurbit plants to feed on, often causing significant damage with their chewing mouthparts. Some of the beetles may carry E. tracheiphila in their digestive tracts; as they feed on plants, they produce droppings (frass) that contain the bacterial pathogen, which can enter the plant's xylem through feeding wounds created by the beetles, then spread throughout the plant. In the one or two weeks between initial infection of the plant and its death, any cucumber beetles feeding on the infected plant may become new carriers of E. tracheiphila. In this way, the bacterium can spread rapidly from plant to plant.

In the two weeks he spent waiting for results from the Clinic, most of Quinn's wilted plants died and several more plants were showing the same wilting symptoms (Figure 2). He was really upset to lose his crop. Transitioning to organic was not cheap. He had taken out a loan to pay for the transition process. Because Quinn had managed his land conventionally prior to this year, he had to undergo a 3-year transition process wherein his land would be managed organically but his produce could not be marketed as certified organic. This meant that he would not be able to earn the price premiums associated with organic produce in his first three years despite investing more time and money on certification, labor for weeding, and organic inputs such as fertilizer, seed, and pesticides. With already tight profit margins, another crop failure could force him to default on his bank loan. ​

​ Figure 2. Healthy muskmelon leaves (left) compared with wilted leaves infected with Erwinia tracheiphila (right).​​​

DISCUSSION QUESTIONS

1. For what possible reason(s) might a grower need to take out a loan to convert from conventional to organic management?
   
2. Row covers and black plastic mulch are examples of synthetic, man-made inputs that are allowed in organic agriculture. Why do you think these inputs are allowed, but other synthetic inputs are not?
3. If you were to construct a bacterial wilt disease triangle, what would be the pathogen, host, and environment? Would you modify the disease triangle in any way to make it better reflect this particular pathosystem?
4. What are three aspects of cucumber beetles that make them so damaging as a vector (agent of spread) for E. tracheiphila?
   

DISEASE MANAGEMENT

Quinn knew he could get a really good price for organic muskmelons from Sullivan Sisters if only he could figure out how to grow organic melons without such disastrous results. He eagerly scanned the information that Graciela provided him about management strategies.​

Rotation

Rotation is a practice in which a crop that is susceptible to damage from an insect or pathogen is not replanted in the same field for a certain amount of time after a cropping season ends. This practice cuts the risk of pest outbreaks by limiting the pest's access to resources that are essential to its survival (e.g. food and shelter). Rotating fields out of cucurbit crops for a minimum of two years is an indispensable practice for cucurbit growers trying to manage cucumber beetles and bacterial wilt. This rotation encourages most of the beetles that overwinter at the end of a cucurbit growing season to migrate elsewhere at the start of the second season to find cucurbits. Even fewer beetles survive after two consecutive years of planting non-cucurbit crops, which means fewer beetles will be around at the start of the next cucurbit growing season. One drawback of rotation is that it is not 100% effective. Rotation does not eliminate cucumber beetles, but will make management easier by reducing their population size in a field. During rotation years, Quinn can grow non-cucurbit crops for profit.

Perimeter trap cropping

​​​​​ Figure 3. Perimeter trap cropping. Buttercup squash (foreground) surrounding muskmelon.​

Perimeter trap cropping (PTC) (Figure 3​) takes advantage of the fact that cucumber beetles prefer some types of cucurbit crops more than others and will feed mainly on the preferred crops. In this strategy, a main cash crop—muskmelon, in Quinn's case—could be bordered by two rows of Buttercup squash, which is a more attractive crop to cucumber beetles. The PTC idea is that cucumber beetles entering the field will encounter the more attractive perimeter crop on the field border and then stay there, where they can be sprayed with organic-approved insecticides to (hopefully) kill them. Drawbacks of PTC are that two crops – squash as well as muskmelon - must be managed in the same field instead of just one, and that the organic pesticides may not be effective enoug​h to prevent the cucumber beetles from migrating into the muskmelon main crop. However, PTC is relatively low-cost, and could reduce insecticide use by limiting applications to only the perimeter trap crop rows rather than the main crop. ​

Row covers: Low tunnels and mesotunnels

Row covers are physical barriers that prevent pests from feeding on plants by placing a layer of fabric between the plants and the pests. In Quinn's case, the fabric would be suspended above rows of muskmelon plants to prevent cucumber beetles from reaching the plants and spreading disease. Graciela shared information about two distinct types of row cover systems for Quinn to consider: low tunnels and mesotunnels (Figure 4).

​​​ Figure 4. Low tunnel (left), mesotunnel (right).​

Low tunnels. Low tunnels are used to protect transplants early in the growing season. They consist of a breathable, semitranslucent, spunbond polypropylene fabric that is suspended about 18 inches high on wire hoops above rows of seedlings. The edges of the fabric are held to the soil with rock-filled bags or soil to prevent the beetles from gaining access to the plants. In addition to functioning as a pest barrier, the fabric creates a warm, sheltered early-season environment well-suited to cucurbits, which would enable Quinn to plant and harvest muskmelons earlier and earn ​a higher market price. However, there are downsides. The spunbond fabric tears easily so it must be replaced every year, adding to expense and creating waste. Furthermore, the spunbond fabric must be removed three to four weeks after transplanting so that bees can visit flowers to pollinate them, and to avoid overheating plants once summer temperatures climb. Essentially, the benefit of low tunnels as a pest and disease barrier is limited to a small portion of the entire season, so Quinn would have to use another strategy to manage the cucumber beetles and bacterial wilt after removing the low tunnels.​

​​ Figure 5. Bumblebee hives placed inside mesotunnels and sheltered from rain using a laundry basket.​​

Mesotunnels. Mesotunnels are another type of row cover system used to protect plants from cucumber beetles. The barrier fabric is different: a nylon-mesh insect netting with holes that are small enough to keep out cucumber beetles. The mesh allows air movement inside the tunnels and can remain on plants the entire season without overheating them. So that plants will have adequate space to grow to maturity, mesotunnels are more than twice the height of low tunnels. The mesotunnel fabric is supported on sturdier hoops made of 1-inch-diameter galvanized metal conduit. To ensure that pollination can occur while plants are covered, purchased boxes of bumblebees (Figure 5) are placed inside mesotunnels once female flowers begin to appear.  Drawbacks of mesotunnels include the expense and durability of the mesh fabric. Muskmelon tendrils (at the tip of growing vines) can poke small holes in the fabric. Although the fabric can be easily mended, unlike low tunnel fabric, its use is limited to about three seasons before it must be replaced. The cost of the fabric is about three times the expense of the spunbond polypropylene used for low tunnels. Mesotunnels would be Quinn's most expensive option, but they might also provide the most protection against cucumber beetles and bacterial wilt. Two seasons of testing by researchers at Iowa State University showed the mesotunnel system could produce up to four times the marketable yield of the low-tunnel system - with zero insecticide applications. This sort of performance could potentially make mesotunnels cost-effective.  

Quinn is glad to learn there are some alternatives to the organic-insecticide-based system he has been using that resulted in severe crop losses. But because the risk of losses from beetles and the bacterium is so unpredictable from year to year, he is struggling to decide how much to invest in protection. He knows there is a strong market for organic muskmelons and hopes one of the new methods can help him to turn his losses into profits. But which method(s) should he try? Each one – insecticides only, rotation, PTC, low tunnels, and mesotunnels – has pluses and minuses.  Your challenge is to propose a plan for Quinn's organic muskmelon production, and explain your reasoning in favor of that plan.

DISCUSSION QUESTI​ONS

1. Which management strategy would be the most consistent and reliable? Why is it important for Quinn to know that the beetles and bacterium are so unpredictable?
2. In the interest of reducing his on-farm cucumber beetle population, should Quinn plant acorn squash instead of muskmelon next year? Explain why or why not.
3. What additional information would you request to help Quinn to create an effective disease management plan?