Background Information
Organic agriculture
The United States Department of Agriculture (USDA) describes organic agriculture as the application of a set of practices that support recycling of on-farm resources, promote ecological balance, and conserve biodiversity.
Organic pest management. Organic pest management adopts a systems approach – implementing pest management techniques based each component of a grower's production system (i.e. soil, water, plants, insects, microbes, animals) and how they interact to provide long-term pest control and minimize agriculture's negative off-farm impacts. For example, by using cover crops, a grower can not only protect soil health and reduce soil erosion, but can also trap nutrients, reduce weed pressure, and provide habitat for beneficial insects that serve as natural predators of pest insects. While the benefits of cover crops alone may not be as effective as synthetic chemical herbicides, fertilizers, or insecticides for the first few years, they can be combined with other strategies to gradually improve overall effectiveness of pest control and reduce the use of synthetic chemical inputs. The use of most man-made herbicides, fertilizers, insecticides, and other materials is generally prohibited in organic agriculture, but there are a few exceptions. For example, row covers and plastic mulch used to control weeds are made of synthetic polymers, but their use is allowed because they are not incorporated into the soil and because managing weeds organically without them would be extremely difficult. More information on organic agriculture and pest management can be found in the USDA Guide for Organic Crop Growers (www.attra.ncat.org) as well as the ATTRA (Appropriate Technology Transfer for Rural Areas) Organic IPM Field Guide (https://attra.ncat.org/attra-pub-summaries/?pub=148).
Transitioning to organic. To market produce as organic, the land on which it was grown as well as the practices and inputs used to grow the produce must be certified by the USDA National Organic Program (NOP). The NOP is the USDA branch that develops rules and regulations for production, handling, labeling, and certification of organic products. The NOP Handbook provides organic growers and certifiers with an overview of organic regulations (https://www.ams.usda.gov/rules-regulations/organic/handbook). Growers may choose to certify their land as organic for a variety of reasons. Some may wish to take advantage of price premiums associated with organic crops. Others may aim to preserve or improve soil quality while also earning a higher price for their produce. In some cases, a grower's customer base may demand organic produce so he/she will need to meet their demand in order to stay in business. Transitioning land from conventional to certified organic production can take up to three years if non-organic inputs were used prior to the transition. This means that organic practices must be used in that 3-year period, but produce cannot yet be marketed as certified organic. Without earning the price premiums associated with certified organic produce in that 3-year window, transitioning can be expensive. However, programs exist to make the transition process more feasible for growers: https://www.sare.org/Learning-Center/Bulletins/Transitioning-to-Organic-Production/Text-Version/Making-the-Transition.
Bacterial Wilt
Some of the most widely produced cucurbit species in the U.S.A. are susceptible to bacterial wilt – including muskmelon, cucumber, and squash. However, the disease is limited to the Midwestern, mid-Atlantic, and Northeastern regions of the U.S. Cucurbit plants become infected when striped cucumber beetles (Acalymma vittatum) and spotted cucumber beetles (Diabrotica undecimpunctata howardi Barber) excrete frass (excrement) containing the causal agent—a bacterium called Erwinia tracheiphila—onto plant wounds. Infected plants show symptoms of wilt and die within a few weeks. To prevent bacterial wilt infection, cucumber beetles must be prevented from feeding on plants and spreading the bacterial pathogen.
Beetles that vector the bacterial wilt pathogen. The causal agent of bacterial wilt, Erwinia tracheiphila, is spread to cucurbit plants by striped cucumber beetles and spotted cucumber beetles. Striped cucumber beetles are believed to be the most important vectors (agents of spread) of Erwinia tracheiphila because they are able to harbor the bacterium in their digestive tracts for extended periods. Furthermore, they overwinter throughout the eastern half of the U.S. and appear in cucurbit fields early in the spring, whereas spotted cucumber beetles migrate north from Mexico and the southern U.S. and appear in cucurbit fields several weeks later than striped cucumber beetles. Striped cucumber beetle adults overwinter just below the soil surface and emerge in the spring to seek out freshly planted cucurbits. A small percentage of the beetles that emerge in the spring are carriers of the bacterium, which they acquired during the previous growing season while feeding on infected cucurbit plants. After emerging from the soil, cucumber beetles seek out cucurbit plants to feed and lay their eggs. They are highly attracted to volatiles (biochemicals in the vapor phase) emitted by plants in the cucurbit family, especially muskmelon and cucumber. Once striped cucumber beetles discover a suitable cucurbit field for a food source, they emit aggregation pheromones to attract other beetles. They lay their eggs in the soil at the bases of cucurbit plants, and the larvae feed on the roots to support their growth into adult cucumber beetles. Over the course of one growing season in Iowa, as many as three generations of cucumber beetles can be produced. Cucumber beetles can kill young plants from feeding damage alone. They can also kill cucurbit plants by spreading E. tracheiphila that cause bacterial wilt.
Bacterial pathogen. E. tracheiphila can survive in the digestive tract of cucumber beetles and inside the xylem of living cucurbit plants. The beetles acquire the bacterium when they feed on infected cucurbit plants and the bacterium then attaches to the lining of the digestive tract. By attaching and reproducing, the bacterial population within the beetle can survive overwintering and replenish itself throughout the course of a beetle's life. E. tracheiphila-colonized beetles deposit infested frass when they feed on leaves. The Erwinia bacterium excreted by a beetle gains access to a plant's xylem when it touches any of the abundant feeding wounds created by a large population of beetles. Because cucumber beetles fly, the bacterium can be deposited on many plants in a field as the cucumber beetles travel from plant to plant feeding and excreting.
Plants. Xylem is the part of plant's vascular system that transports water and dissolved nutrients up from the roots to the leaves. Plant tissues remain rigid and upright when water flows normally through the xylem, but normal water flow is interrupted when the xylem is infected with E. tracheiphila. The bacterium reduces water flow by multiplying and clogging the xylem, causing the leaves to wilt. After initial wilting symptoms appear, the leaves and vines turn brown and die within a couple of weeks—usually before any marketable fruit can be harvested. In that 1- to 2-week period between initial infection and plant death, an infected plant is even more attractive to cucumber beetles than a healthy plant. Foraging cucumber beetles preferentially feed on the diseased cucurbits in a field, which increases their chances of ingesting E. tracheiphila and spreading it to healthy plants after the diseased plants die.
For a detailed review of the cucurbit bacterial wilt pathosystem, see Bacterial Wilt of Cucurbits: Resurrecting a Classic Pathosystem (https://apsjournals.apsnet.org/doi/10.1094/PDIS-10-14-1068-FE).
Disease triangle
The disease triangle is a basic concept of plant pathology. The triangle represents a plant disease, and each of its sides represents a component required for plant disease to occur: a susceptible host plant, an environment conducive to disease, and a disease-causing agent, also known as a pathogen. Like a triangle, which needs three sides, a plant disease needs all three of these components to occur. A useful application of the disease triangle is that it emphasizes that there are always three potential options for managing a plant disease: focusing on the host, the pathogen, and/or the environment. For example, for anthracnose, a fungal disease, to develop in a cucumber plant, the anthracnose pathogen (Colletotrichum orbiculare) must come in contact with a susceptible cucumber host, and the environment must be wet and warm enough for the pathogen to infect. Managing cucurbit anthracnose can be accomplished by targeting all three components of the disease triangle: the pathogen can be targeted by spraying a fungicide, the environment can be modified by using drip irrigation rather than overhead irrigation, and the host can be targeted by planting only cucumber cultivars that are known to have resistance to the disease.
While the disease triangle accurately models cucurbit anthracnose and many other plant diseases, cucurbit bacterial wilt and other diseases whose pathogens require vectors are special cases. Targeting the pathogen by spraying bactericides would be ineffective because the pathogen would be untouched (it resides only inside the beetle vectors and the plant xylem). Thus, strategies to deter the cucumber beetle vectors should be pursued to manage bacterial wilt. Furthermore, muskmelon growers can do little to modify the host for this pathosystem because bacterial wilt-resistant cultivars of this crop have not been developed. Further information on the disease triangle and inclusion of vectors in the model can be found in The Disease Triangle: A plant pathological paradigm revisited (http://www.apsnet.org/edcenter/instcomm/teachingarticles/pages/diseasetriangle.aspx).
Management strategies
Low tunnels. Low tunnels can be used at the start of the season to physically protect young plants from cucumber beetles. The 3.5-ft-tall tunnels consist of spunbond polypropylene fabric that is suspended over the plants on wire-hoop supports immediately after transplanting seedlings. The edges of the fabric are held down to the soil with rock bags, sand bags, or bare soil to prevent the beetles from gaining access to the plants. In addition to functioning as a pest barrier, the fabric traps heat around the plants. This early-season heat allows growers to plant earlier in the spring when temperatures would otherwise be too cold, and results in an earlier harvest that earns a higher price. However, spunbond polypropylene tears easily and is difficult to repair. The need to replace the fabric annually adds to the expense and creates waste. Furthermore, the fabric must be removed permanently after 3 to 4 weeks because: (1) female flowers appear at that time and pollinators must visit flowers for cucurbits to bear fruit, and (2) the fabric traps so much heat that it would kill plants if left on beyond pollination. Once the fabric is removed, cucumber beetles must be managed by other means.
Mesotunnels. Mesotunnels are modified low tunnels that use nylon-mesh fabric to protect plants from cucumber beetles. The mesh is small enough to keep cucumber beetles from entering. Because the mesh allows airflow to the plants, temperatures remain similar to ambient air temperatures and the fabric can remain on plants the entire season without killing plants by overheating them. Because mesotunnels protect plants until they are full-grown and bear fruit, they are taller than low tunnels. The ProtekNet fabric is suspended over plants on 1-inch galvanized metal conduit pipes bent into a hoop shape and staked into the soil at a 3.5-foot height. Pollination in the mesotunnels can be accomplished naturally by removing the fabric for a two-week time period when female flowers appear. Insecticides will likely need to be applied during those 2 weeks to manage cucumber beetles. After 2 weeks, the fabric is reapplied and left to protect plants until fruit are ready to harvest. Alternatively, instead of removing the fabric and exposing plants to cucumber beetles during pollination, bumblebee hives (Koppert Biological, Co.) may be placed under one edge of the tunnel at the first appearance of female flowers and left for several weeks. A drawback of both mesotunnel systems is the cost of the ProtekNet fabric – it's about three times the expense of the spunbond polypropylene fabric used in the low tunnel system. However, two seasons of testing resulted in up to four times the yield compared to the low-tunnel system, which may make mesotunnels cost-effective. Furthermore, the tendrils of muskmelon plants can wrap around the mesh netting and create small holes in the fabric during removal. This limits the lifespan of the material to a maximum of three years. A benefit of this material is that holes can be mended with fishing line, but that could be very time consuming.
Perimeter trap cropping. In this strategy, a cucurbit crop that is highly attractive to cucumber beetles is planted as a two-row border around a main cucurbit crop that is less attractive to cucumber beetles. The perimeter crop is used as an attraction area where insecticides are sprayed to kill the cucumber beetles before they begin entering the main crop. When used with buttercup squash as a trap crop, this strategy effectively reduced the number of insecticides applied to a muskmelon main crop and reduced the incidence of bacterial wilt; however, use of this strategy did not result in any yield gains. For more information on perimeter trap cropping, see “Perimeter Trap Cropping Works!" (http://ipm.uconn.edu/documents/raw2/Perimeter%20Trap%20Cropping%20Works/Perimeter%20Trap%20Cropping%20Works.php).
Rotation. Rotation is an indispensable strategy that can be used to manage a variety of pests and diseases in many cropping systems. Rotating land out of cucurbit production for 2 years is sufficient to reduce cucumber beetle populations, but additional years may be required for management of other pests and diseases. When determining crop rotations, information about pest behavior and biology must be understood. For example, it is important to know that cucumber beetles typically migrate only about 900 feet—a little less than the length of three football fields. On a small farm of a few acres, rotation may or may not be helpful depending on what is grown on neighboring land. For more information on using crop rotation to manage insect pests, see “Management of Insect Pests with Crop Rotation and Field Layout": https://www.sare.org/Learning-Center/Books/Crop-Rotation-on-Organic-Farms/Text-Version/Physical-and-Biological-Processes-In-Crop-Production/Management-of-Insect-Pests-with-Crop-Rotation-and-Field-Layout.
