By Mark Gleason
Plant Pathologist
Iowa State University Extension
If you are a young cucumber or muskmelon plant, you are living on the edge. The garden is teeming with tiny critters that want to kill or eat you, and not necessarily in that order.
Cucumber beetles must terrify muskmelon plants. These yellow-and-black insects not only munch on tender leaves and shoots, they also carry a devastating payload: wilt bacteria. Once beetles open feeding wounds in the helpless seedlings, the bacteria slither in through the wounds. The next thing you know, the plants are dying, courtesy of bacterial wilt.
Bacterial wilt is not a pretty sight. If you’ve ever nurtured a cucurbit (melon, squash or cucumber) crop for weeks or months, only to see it collapse to the ground in a tangle of withering vines, you’ve probably experienced bacterial wilt. Many home gardeners and organic growers simply avoid planting cucumber and muskmelon after watching too many of their crops crash and burn.
Farmers and gardeners do their best to stop the beetles and bacteria, but they are limited to some fairly crude weapons. Chemical insecticides can stop the two species of cucumber beetles (striped and spotted), but need to be applied many times per season. Unfortunately, this chemical warfare can endanger health, injure bees and other pollinators, and increase the risk of insecticide resistance by the beetles.
We have no real weapons against the wilt bacterium, Erwinia tracheiphila. Resistant varieties are few, and bactericides don’t cut the mustard. So how can we solve the bacterial wilt puzzle?
We are still looking for the answer to that question. But part of the answer may be to rethink how we fight off cucumber beetles. Row covers turn out to be a useful weapon in that fight.
Row covers are synthetic fabrics that are draped over crops to protect them. These fabrics, with brand names like Reemay and Agribon, have a texture that is somewhere between cheesecloth and a plastic sandwich bag. They can breathe, but their mesh is too small to let insects through.
The main attraction of row covers for vegetable farmers is that they increase something called earliness. Since time is money, growers who bring their crops to market earliest in the season tend to get the best prices. Protecting melon plants inside an 18-inch-high tunnel of row cover, supported on wire hoops, warms up the soil and speeds growth enough to advance harvest by a couple of weeks.
Where does bacterial wilt come in? Row covers protect tender plants from rapacious cucumber beetles while the row covers are in place. No beetles means no bacterial wilt.
The catch is that the row covers need to be removed in order to allow other insects (mainly bees) to pollinate the crop. So the covers are usually peeled off three or four weeks into the season, just as the first flowers start to open. Unfortunately, no more row cover means no more protection from beetles and wilt.
But suppose we could somehow prolong the covered period for a bit longer; say 10 more days. Would it help the plant avoid bacterial wilt?
According to our research at ISU over the last two years, the answer is yes. Simply delaying removal of row covers for 10 days after the start of flowering manages to stop bacterial wilt for the rest of the season.
How could such a small change have such a big impact? It seems that the extra 10 days are critical for protection against cucumber beetles. Once the covers are finally removed, fewer beetles are flying, and the ones that do appear may be carrying fewer bacteria. The melon plants are 10 days larger when they are uncovered, and so may be less susceptible to wilt. So for beetles, bacteria and melons, as elsewhere in life, timing is everything.
If you are still reading, you may have noticed a large fly in this ointment. What about pollination? If my plants are row-covered while blooming, how can insects get access to the plants to pollinate them? It would be ironic to end up with wilt-free plants that are also melon-free.
We are testing two possible ways around this dilemma. One approach is to open the ends of the row covers when bloom starts. The idea is that pollinating insects (good guys) will zoom in through the open ends of the row covers and do their business, but the cucumber beetles (bad guys) will be too confused to find their way in.
Plan B is to insert a beehive under the row cover for the extra 10 days. Interestingly, you can buy boxes of bumble bees through the mail. We enclosed the bee boxes under the covers, popped open the access hole in the box, and the bees seemed to know what to do.
So far, both the open-end and bee-box tactics seem to be working. We have been getting good yields, with almost no bacterial wilt. More experiments need to be done, but the general approach looks promising. Row covers, these humble mini-greenhouses, can speed up the crop while stopping bacterial wilt.
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