Downy Mildew – Prevention and Control
Q. This winter I am growing cabbage, cauliflower, and broccoli in the same raised bed with a row cover. The cabbage and broccoli are doing beautifully but the cauliflower develops mold(?) on the heads before they are fully grown. The leaves are large, green and healthy; and when I pull the plant out of the ground the roots also look healthy. Are the row covers keeping in too much moisture or are the plants developing a disease?
A. Yes to both questions, and the cabbage and broccoli won’t be far behind the cauliflower in showing symptoms. However, before I address the specific problem you describe, let me say for all group members’ benefit, that we encourage gardeners to minimize pests and disease by several important “cultural practices.” These include 1) maintaining a totally weed-free garden with wide, dry aisles, 2) pruning leaves off the ground, 3) watering only at the soil level (never sprinkling) and only in the actual root area, 4) growing seedlings in a protected environment and transplanting stocky, healthy seedlings into the garden, 5) feeding plants a complete, balanced natural mineral nutrient mix that encourages healthy, rapid growth, 6) harvesting when crops are mature, and 7) if using row covers or “mini greenhouses,” open the ends on cool days (50+), and set the covers to one side on warm days (65+), to maximize sunlight and circulation, and reduce excess humidity build-up. By following these procedures your problems with pests and disease will be rare.
You may have been doing all of these things, Stacie, and only the increased humidity and warmth of the row covers could have given the disease an opening. You are probably experiencing Downy Mildew, a fungus disease. General symptoms for all affected vegetable crops, and which usually happen under high-humidity conditions, include spots appearing on leaves and a downy white or grey mold developing in these spots or on the undersides of the leaves opposite these spots.
With broccoli, cauliflower, cabbage, and brussels sprouts, dark spots may develop on the heads as well as the leaves. Black streaks may be visible on stems and a white fuzzy growth may develop. Seedlings are especially affected.
The best solutions are preventative, and constitute physical controls, such as I have described above. Also, it’s important to rake up and burn fallen leaves.
Now that you have the problem, if it is not too widespread, I recommend you remove all affected plants immediately and improve the physical conditions as much as possible. Biological control is your last option, short of removing the entire crop. Several chemicals are sold to control downy mildew, including Benomyl, Copper, Folpet, Lime Sulphur, Sulphur and chlorothalonil. Counsel with the store from which you obtain any of these materials, and always, when using pesticides, read the entire label on the container and follow the directions. Because mildew will built up a resistance to fungicides over time, especially Benomyl, if the problem persists you will need to consider changing the materials used occasionally.