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Bees, Beetles, and Bacteria: the Cucurbit Bacterial Wilt Dilemma

Erika Saalau Rojas, Laura Jesse, and Mark L. Gleason

Department of Plant Pathology and Microbiology
Iowa State University

Rojas, E. S., Jesse, L., and Gleason, M. 2014. Bees, Beetles, and Bacteria: the Cucurbit Bacterial Wilt Dilemma. The Plant Health Instructor. 10.1094/PHI-T-2014-0312-01


Answers to Questions - Part A

1. What is the role of cucumber beetles in this disease system? Does this role change your view of the disease triangle? Why or why not?

The disease triangle is a basic plant pathology concept that illustrates the interaction of three factors required for biotic disease to occur:  a susceptible host, an infectious pathogen, and a favorable environment for disease development. Although this concept may be applied to most infectious diseases, it fails to represent the role of insects as pathogen vectors. Erwinia tracheiphila, the causal agent of cucurbit bacterial wilt, is overwintered and transmitted by cucumber beetles. Without bacteria-infested beetles a bacterial wilt epidemic is unlikely. Students may argue that striped cucumber beetles are implicitly represented in the disease triangle or may suggest adding a fourth factor, the vector, as a critical component. Students may suggest adding a link between the pathogen and the host to maintain a linear format for the disease triangle. A three-dimensional representation of the disease triangle may also be acceptable by drawing an additional vertex to the triangle and converting it into a disease pyramid. More variants may be discussed among students and as long as the main factors for disease development are accounted for all answers should be correct.

2. What are the signs and symptoms of cucurbit bacterial wilt?

Signs: bacterial ‘ooze’

Symptoms: wilting of leaves and vines followed by necrosis

3. Describe the cucurbit bacterial wilt disease cycle.

The bacterial wilt disease cycle is closely linked to the striped cucumber beetle life cycle. Primary inoculum comes from adult overwintering beetles harboring bacteria in their digestive systems. Beetles become active in spring and seek cucurbit plants to feed on. Transmission of the pathogen occurs when bacteria-infested beetles feed on plant tissues and deposit bacteria-infested frass (feces) onto fresh feeding wounds. Once inside the plant’s vascular system, the bacterium multiplies and plugs the xylem, which leads to wilting of leaves and vines 1 to 3 weeks after infection. Subsequently, plant tissues collapse and the plant dies.

Overwintering beetles feed and mate in cucurbit fields and females lay their eggs at the base of cucurbit plants. Cucumber beetle larvae feed on roots, pupate, and emerge in the field. Newly emerged beetles do not harbor E. tracheiphila, but they may acquire and spread the pathogen by feeding on infected plants.  Depending on the region, multiple generations of beetles may be observed in a single season. Later in the season beetles seek overwintering sites near fencerows and plant debris and, if they harbor the pathogen in their guts, the disease cycle will begin next spring.

4. Can bacterial wilt be controlled by using fungicides or bactericides? Why?

No, bacterial wilt cannot be controlled by fungicides or bactericides because E. tracheiphila is a vascular pathogen. Once a plant is infected it cannot be cured and should be removed. Control of bacterial wilt relies on preventing infection by controlling cucumber beetles.

5. What do you think about Henry’s statement: "I did nothing legally wrong!"? Can the beekeeping association prove that he is responsible for poor health of the bees in their hives?

It is unlikely that that the beekeeping association will be able to blame Henry for their problems because he is following regional pesticide recommendations to control cucumber beetles. In addition, many factors have been implicated in poor honey bee health problems and there is no evidence pointing to a single factor. However, Henry’s statement may lead to a discussion about what is known about colony collapse disorder and whether it is advisable to pursue a lawsuit. Some points that may be considered for discussion in class could be the following:

  • ‘The label is the law’. If Henry is following all the instructions outlined in the labels of the chemicals he is using, technically he is not violating any laws. This concept can be reinforced by searching online for labels of commonly used insecticides and have students formulate questions beekeepers could ask Henry about his insecticide applications practices.
  • A lawsuit is expensive and time consuming. Students may evaluate whether it is worth Henry’s resources to pursue a lawsuit against beekeepers. Although there might not be enough evidence to support the beekeepers claims, it may take months or years to reach an agreement. In addition, a lawsuit involving pesticides can attract detrimental publicity and may deter local clients from buying Henry’s produce.
  • Beekeepers are located a mile away from Henry’s farm. State departments of agriculture across the U.S.A. have an array of rules that strive to regulate and protect honey bees. In Iowa, for example, students can discuss compliance with the Iowa Department of Agriculture and Land Stewardship (IDALS) “Pesticide/Bee Rule”. In other states discussion points could revolve around apiary registration laws, commercial applicator distance from apiary, and timing of pesticide applications.
  • Farmer awareness and responsibility. Government agencies (EPA, USDA, U.S. Wildlife Service) and universities invest many resources to encourage farmers and general public to decrease pesticide use which could potentially harm pollinators. Calendar-based pesticide applications are often discouraged and, instead, agencies advocate for integrated pest management (IPM) strategies that reduce chemical applications in the environment. Although Henry might not be breaking any laws, do his practices pose a threat to pollinators? This dilemma may be brought up in the classroom to discuss whether Henry should be more concerned about his pesticide use and whether implementing IPM strategies should be an ethical responsibility for farmers.

Answers to Questions - Part B

1. Based on your knowledge of cucurbit bacterial wilt, in which part(s) of the growing season do you think that risk of wilt infection is highest?   Why?

Bacterial wilt risk is highest during early spring, since overwintering bacteria- infested beetles serve as the source of primary inoculum. In addition, overwintering beetles become active in spring, around the same time when cucurbit fields have been recently planted or transplanted. Young seedlings are highly attractive to cucumber beetles and are more susceptible to bacterial wilt and insect damage (Figure 10).

Figure 10

2. Discuss potential solutions to the potential problem of delaying harvest (and possibly receiving lower prices per fruit as a result) while using row covers.

Pollination can be enabled while using row covers by several methods:

  • Removing row covers during female flowering (anthesis) to allow pollination and place row covers over plants until harvest.
  •  Placing bee hives under row covers at anthesis (Figure 11)
  • Other methods such as opening the ends at anthesis can effectively protect plants from cucumber beetle damage and bacterial wilt (Figure 12). However, it is unclear why this method enables insect pollinator access under row covers and deters cucumber beetles.

Figure 11

Figure 12

3. What are some practical challenges that need to be overcome when implementing a perimeter trap crop (PTC)?

  • PTC’s should be planted earlier than the main crop, which could interfere with planting practices for the main crop.
  • Management of two different crops may be expensive and knowledge-intensive. Pests and diseases will differ between the main crop and the PTC, which implicates that farmers need to employ different management practices to prevent PTC failure. This may result in additional costs and labor. 
  • Produce marketing. Produce harvested from PTC may require different marketing channels which may be a limitation for farmers with an established clientele or limited marketing opportunities.  

4. In order for a perimeter trap cropping system to be successful against bacterial wilt; which characteristics are desirable in the perimeter plants? Hypothesize what could go wrong if Henry chooses an inappropriate cucurbit variety for his perimeter.

Higher attractiveness to cucumber beetles than the main crop, lower disease susceptibility, and marketability of produce.

1. Attractiveness: the PTC crop must be more attractive to cucumber beetles than the main crop. Otherwise the PTC, traditionally planted before the main crop, could serve as a lure to attract beetles around the main crop. If the main crop is more enticing to beetles, they could easily move into the main crop from the perimeter.

2. Low disease susceptibility: since a PTC serves as a barrier, the crop must be hardier than the main crop and less susceptible to bacterial wilt. A highly susceptible crop or variety could serve as source of inoculum for the main crop.  

Marketability: some commercial and wild cucurbit species are highly attractive to cucumber beetles and have a high tolerance to bacterial wilt infection. However, low yield or poor acceptance of the product by clients makes these species non-profitable choices for PTC.

5. Use all the information available and your own experiences to construct an IPM-based management plan against cucurbit bacterial wilt that could solve Henry’s situation.

Row covers: Discuss the pros and cons of the following options:

  • Deploy row covers at transplant and choose one of the following:

Insert bees under covers at anthesis. In our classroom experience some students suggested reaching an agreement with the beekeepers to use their hives in Henry’s farm.

Remove row covers during anthesis, protect exposed plants with insecticides, and reapply row covers until the end of the season.

Open ends of row covers at anthesis to enable pollinator access

Remove row covers at anthesis to enable pollination and spray insecticides triggered by scouting the field 2 to 3 times per week. Although this might reduce insecticide use from transplant until anthesis (average of 4 to 5 applications), insecticides may have to be used weekly after row cover removal if beetle populations are too high.

Perimeter trap crops: Discuss the feasibility of using one or more of the following options to reduce the risk of PTC failure:

Biweekly scouting in the PTC and main crop and spray at 1 beetle/plant

Spray the PTC weekly (less insecticide applied per area)

Use the same cucurbit species used in PTC squash systems

Try different cucurbit varieties in different parts of the farm

Plant a muskmelon PTC earlier than the main cropDouble the amount of recommended PTC area to avoid risk of bacterial wilt infection

Scouting: Discuss the feasibility of implementing scouting and discuss the following:

Scouting alone can save one or two insecticide applications. However, if beetle populations are high throughout the entire growing season, Mr. DeSoto might have to spray on a weekly or biweekly basis. Will that be acceptable?

Insecticide reduction if scouting implemented in combination with other alternatives.

6. When choosing among different IPM strategies to combat pests and diseases, what kind of considerations do you think farmers will keep in mind when making a decision?

  • Cost of the strategy and profitability
  • Effectiveness of the strategy
  • Practical considerations such as availability of land, labor, materials, and technology