I’m a gardener who remember when zucchini was a new vegetable. On the farm we planted a pretty long row of this new squash and learned very quickly that it’s easy to be overrun with zucchini. They wouldn’t stop producing in upstate NY.
But in southern PA it’s a different story. My experience is that the zucchini plants only produce fruit for about 2-3 weeks and then they die. The problem has always been bacterial wilt. This disease is carried by cucumber beetles. When these beetles chew the leaves of zucchini, they leave behind droppings that have the bacteria Erwinia tracheiphila in them. This bacteria get into the plant, multiplies, clog up the xylem (the plant tissue that carries water from the roots to the stems and leaves) and the next thing you know, the zucchini plants are wilted and then dead. If you break the stem of a zucchini with bacterial wilt, you’ll see a sticky mucus-like substance in it – that’s the bacteria.
I’ve been dealing with this for years… until this year. For the first time the zucchini plants didn’t die from bacterial wilt. I’m not sure why, though I have two possible reasons. This year I planted a different variety – Zucchini Elite – which I purchased from Harris Seeds. It might be that this variety is somewhat resistant to bacterial wilt. The other possible reason is that last year I had hung a cucumber beetle trap in the garden. I know a lot of beetles were caught on the trap and, while I saw beetles this year, I didn’t see as many. Like I said, I’m not sure what made the difference this year.
While the zucchini didn’t die of bacterial wilt, they still died but they produced for about twice as long as other years. The killer this year wasn’t bacteria wilt but the squash vine borer (Melitta curcurbitae.)
The adult squash vine borer is a moth that looks like a wasp. It has two clear wings and two that are bright metallic green. The body is orange and black. The pupae of this insect overwinter in the soil and, here in PA, the adults emerge in mid to late June. The females lay eggs at the base of squash and in about 10 days, the eggs hatch and the larvae tunnel into the stem of the plant. The larvae then feed on the inside of the stem and disrupt the nutrient and water transfer within the plant. After feeding from 4-6 weeks, the larvae leave the stems and pupate in the soil, waiting to emerge ten months later.
The borer damage causes the squash to wilt and then die. When I pulled the dead zucchini out of the ground this year I noticed that there wasn’t any mucus-like bacteria so I knew they didn’t have bacterial wilt. But what I did see was a white grub-like caterpillar inside of each stem – the squash vine borer larva.
Pesticides don’t work very well on squash vine borers because, unless you’re able to kill the adults, once the larvae are in the stem, insecticides won’t touch them. Row covering is suggested to exclude the adults but I’ve never had very good luck with row covers. I also read that you can try planting a second crop later in the season. In northern areas this moth only produces one generation of offspring. Zucchini planted in July will mature after the borers have finished laying their eggs.
I destroyed the plants that had squash vine borers to make sure the larvae didn’t get a chance to pupate. I also recently planted some more zucchini to see if a late planting will make a difference.
While the borers killed the zucchini, I harvested more squash than I ever have in the current garden. If I had to make a choice, I’ll take squash vine borers over bacterial wilt any day!