Bacterial wilt is a major challenge for North Central Region cucurbit growers. This disease, caused by the bacterium Erwinia tracheiphila, can wreak havoc on all types of cucurbit crops except watermelon. Leaves and stems wilt and dry up, and infected plants typically die. See photo below.
Cucumber beetles are part of the bacterial wilt story. Two species, striped and spotted cucumber beetles, carry the bacterium from plant to plant, and infection often happens through beetle feeding wounds. Fighting bacterial wilt means fighting cucumber beetles; the main defense against the disease is stopping the beetles. Many strategies have been tried to beat the beetles. Most growers rely on insecticides, but chemical warfare can require many applications per year, which is expensive and may also damage non-target insects, including the bees that pollinate cucurbit crops. Other tactics, such as trap crops and chemical lures, are still in the experimental stage. Organic growers have an especially difficult struggle to grow highly wilt-susceptible crops such as cucumber and muskmelon, because organically approved insecticides are not very effective against cucumber beetles. In fact, some organic growers won’t plant these highly susceptible crops due to worries about bacterial wilt. ROW COVERS AND THE FIGHT AGAINST BACTERIAL WILT Row covers are made of lightweight fabric and are suspended above the plants on wire hoops. See photo below. All edges of the fabric are secured in the soil to hold the fabric in place and to exclude pests. Spunbond polymer row covers such as Agribon(R) and Reemay(R) have gained a foothold with North Central Region cucurbit producers because the covers warm the soil, speed up crop maturity, and protect against extreme early-season weather (frost, hail, wind, heavy rain). Row covers also have potential as a defense against bacterial wilt because they keep out cucumber beetles. As the soil warms up in the spring, cucumber beetles emerge from the ground with one mission: zero in on cucurbits. As they munch on the leaves, bacteria from their mouthparts and feces (frass) end up on the feeding wounds and enter the plants. Soon the plants start to wilt. Row covers keep out the beetles during the vulnerable early-season period. Most growers remove row covers once flowering starts. Our research at Iowa State University (ISU) several years ago showed that deploying row covers from transplanting until the start of flowering could delay bacterial wilt. But sometimes the wilt would catch up by the end of the season, so the end result was still dead plants. What if the row covers could stay in place a bit longer? Researchers in Canada and Africa found major insect-pest control benefits if they kept the row covers in place for 10 more days past the start of bloom. We thought this delayed-removal idea was worth a try in Iowa. RESEARCH APPROACH With funding from North Central IPM Center and EPA-PESP grants, we ran 6 field trials at ISU research farms in Gilbert and Muscatine (central and eastern Iowa respectively) in 2007, 2008, and 2009. Muskmelon (cv. Athena) seedlings were transplanted to field plots and immediately after transplanting, seedlings were covered with Agribon® AG-30 row covers supported by wire hoops, with the edges buried in soil. Treatments were:
- Row covers were removed at start of flowering.
- Row cover ends were opened at start of flowering to enable pollinator access, and covers were removed 10 days later.
- After a bumble bee hive (Koppert Biological Systems Inc.) was inserted under one end of the row cover at start of flowering, the row cover was re-sealed and then removed 10 days later.
- No row covers (control).
Symptoms of bacterial wilt in muskmelon. Photo by Jean Batzer.
Row cover on wire hoops over muskmelon with ends open to allow entry of pollinators. Photo by Erika Saalau.
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