Experts tackle readers’ garden questions.
Q: I have a question that I don’t know why it happens or how to fix it. Just about all our turnips have worm holes every year, but it doesn’t affect carrots, potatoes, rutabagas, etc. Can you give me some idea of what to do?
Richard Croft, South Lake Tahoe
Master gardener Carmen Schindler: The damage to your turnips (Brassica rapa) is most likely caused by cabbage maggots (Delia radicum), also referred to as cabbage root maggots. Cabbage maggots are the larvae of the cabbage root fly. The legless fly larvae relish the opportunity to burrow through the large taproots of turnips.
Cabbage root flies prefer to lay their eggs in moist, organic-rich soil, so if you are using manure, be sure to let it age and incorporate it well into the soil. The maggots are active throughout the year and have several generations.
Young plants quickly die as a result of the maggots feeding on the roots. However, older plants can often tolerate cabbage maggot activity, making early detection difficult because damage is not visible until after harvest. Some signs of cabbage maggots might include wilting, lighter green foliage than normal, and overall poor growth.
All control measures are preventive as nothing practical can be done when maggots have already appeared in a crop. Elimination really begins long before planting, with the aim of discouraging the cabbage root fly from laying its eggs in the first place.
The ideal preventive measure is to add a floating cover, such as cheesecloth, over the crop row at the time the turnip seeds are planted. The cover should be held tightly down on all sides with something like heavy lumber so the cabbage root flies cannot access the plants. The cover can lay directly on the foliage if there is enough slack for plant growth; however, it is better to support the cloth with hoops made out of PVC pipe or something similar.
Turnips have been grown for more than 4,000 years and originated in Northern Europe. The bulb, as well as the green top, of this cool-weather root vegetable is edible.
Consistent water will ensure tasty and mild-flavored turnips. The drier the soil, the more robust the flavor will be. The turnips should never be allowed to get too big and should not be eaten when they are too old. While turnips are frost-hardy, they can be injured from extended periods of weather below 30 degrees.
Carmen Schindler is a UC Cooperative Extension Master Gardener for Sacramento County.
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