By Mark Gleason,
Plant Pathologist
Iowa State University Extension

Have you ever tried to grow cucumbers or muskmelons in Iowa? They can break your heart. Just as the fruit are shaping up, the vines droop. A little while later, they dry up and die. What happened to turn your cucurbit dream into a nightmare?

The usual culprit is a disease called bacterial wilt. It’s caused by a bacterium called Erwinia tracheiphila. This microbial bad boy multiplies in the water-carrying passages of muskmelon and cucumber (and sometimes squashes too), choking off the water flow.  Left without a water supply, the vines up and die.

That’s only part of the story. The other bad boys are striped and spotted cucumber beetles. These evildoers are yellow with black trimmings and a bit smaller and skinnier than lady beetles.  The beetles carry the bacterium from plant to plant, spreading the disease.

The key to beating bacterial wilt is stopping the cucumber beetles. Unfortunately, this is not so easy. Most commercial growers rely on frequent sprays of chemical insecticides once the beetles show up. But many home gardeners are not excited about waging chemical warfare on their own property.

Many other tricks have been tried to beat the beetles. Traps baited with beetle attractants and insecticides can yield an impressive body count, but unfortunately many more beetles invade your crops rather than the traps. Another strategy is to plant trap crops – cucurbits that are especially alluring to cucumber beetles – and spray only the trap plants with insecticides. The fly in this ointment is that cucumber and muskmelon plants are two of the plants that cucumber beetles like best.

How about biological controls? Maybe if we sprayed a solution containing fungi or nematodes that attack insects? Nice try, but spraying a cucumber beetle with biologicals has about the same impact as spraying a Volkswagen Beetle. The beetles are just too tough to be taken down by wimpy microbes.

Row covers offer a more hopeful answer to the beetle-bacterium dilemma. Made of spunbonded polypropylene or similar materials, row covers are lightweight, translucent fabrics that form a barrier between cucumber beetles and vulnerable crops. They are stretched over the rows in the spring immediately after seeding or transplanting, preferably on wire hoops to form a tunnel over the plants. The edges and ends of the row cover strips are buried with soil. 

What about watering the young plants beneath their plastic canopy? The best way is to run soaker hoses or drip lines down the rows before installing the row covers.

It turns out that this tubular mini-greenhouse is ideal for young cucurbit plants. The cool springtime soil warms quickly inside the plastic cocoon, which makes these warmth-loving plants happy. They are also sheltered from the Iowa winds that often damage unprotected plants. Furthermore, the cucumber beetles are on the outside looking in. The result is larger and healthier plants. 

This cozy cucurbit kindergarten can’t continue forever. Once flowering starts, row covers are usually removed to allow bees and other insects to pollinate the flowers. Of course, cucumber beetles can invade now, too. But field trials at ISU have shown that protecting muskmelons until flowering starts can delay bacterial wilt and lessen its severity.

What if row covers could stay on a bit longer? Would we get even more protection? In a 2007 ISU field trial, that’s exactly what happened. We tried two different tricks to extend the row-covered period by 10 days – simply opening the row cover ends to allow access by pollinators, or inserting a bumble-bee box (which can be mail ordered) underneath the row covers.

Remarkably, there was far less bacterial wilt in these treatments than in either the no-cover rows or the treatment that removed row covers when flowering started.  It seemed that those extra 10 days made a big difference in cutting down on bacterial wilt risk.

Even though more research needs to be done (we always say that), row covers seem to be coming into their own for bacterial wilt control as an alternative to chemical warfare or watching your plants dry up. Row cover materials are available in garden supply stores and on the Internet. 

For cucumber beetle protection, it is advisable to use the spunbonded or mesh products rather than slitted clear plastic, because cucumber beetles can pass through these slits.  Row covers may be worth trying in your muskmelon or cucumber garden in 2008.

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