Q: My Swiss chard has white blisters all over its leaves. It looks like there are little black worms squirming around inside the blisters. What is causing this problem and how can I deal with it?
Asked by Becky Evans, Cheyenne, WyomingSwiss chard belongs to the Chenopodiaceae family and is closely related to beets and spinach. All three of these vegetables are susceptible to spinach leaf miners (Pegomya hyoscyami). Understanding a little bit about this pest's life cycle makes controlling it easier.
You will most likely never see the adult leaf miners, which are a small, grey flies. In most parts of the United States the flies first come on the scene in late April or May. They lay eggs on the undersides of leaves. Soon after tiny maggots hatch and burrow their way between the surfaces of the leaf. The mines they make look like big, blotchy white or cream-colored blisters with dark colored larvae squiggling around inside (I think they are truly one of the grossest garden pests). The larvae feed for 2 weeks or more before they drop into the soil. They reemerge in about a month as adult flies and the cycle starts over again. Depending on where you live, there can be 3 or more generations of leaf miners per year.
The best way to deal with leaf miners is to prevent them from causing a problem with your plants. Construct a hoop house over your spinach, beets, and Swiss chard right after you sow or plant seedlings and cover it with a row cover, which is a lightweight fabric that lets air, water and light in but keeps pests out (row covers are available at any well-stocked nursery). The cover basically creates a barrier between your plants and the pests. Leave the hoop house and the row cover in place until late summer.
If you don't put up a hoop house and you notice leaf miner damage, the next best thing is to pick off all infested leaves---and I mean every single one. Don't be tempted to put the leaves in your compost pile! Stuff them into a plastic bag, tie it off, and toss it in the trash (or, if you have chickens, feed the leaves to them). Be vigilant. Check your plants every day and remove leaves as the mines appear. This typically radically reduces or even eliminates the problem.
Keeping up on weeding can also help prevent leaf miners, because they feed on chickweed and lambsquarters. Since the adults overwinter in the soil, it is important to rotate beets, chard, and spinach to different spots in the garden each year. This way when the flies emerge in spring their favorite plants aren't growing right there.
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Willi Galloway writes The Gardener column. She lives in Portland, Oregon and writes about her kitchen garden on her blog DigginFood. Her first book Grow. Cook. Eat. A Food-Lovers Guide To Kitchen Gardening will be published in January 2012.
(Images: All images by Willi Galloway)