Downy Mildew Now Appearing in Brunswick County
A silent menace may be lurking in your garden, waiting for the perfect moment to attack your cucumbers, squash, pumpkin, and other related plants. Downy mildew was reported last week in Horry County, SC and a few gardeners and farmers are already combating it here in Brunswick County. A highly destructive foliar disease of plants in the cucurbit family, downy mildew is normally a late-season problem, easily managed with fungicides; however, with all of the rainfall that we have had this summer and the relatively high humidity that is common in the southeast, downy mildew and other foliar diseases have run rampant, leaving farmers and gardeners scrambling to find some answer on how to combat the problem. Unfortunately, once you begin to see symptoms on your plants, there is not much you can do for control.
Biology of downy mildew
For the most part, downy mildew was an annual late-season problem on squash and pumpkin production, but since 2004 it has become one of the most prevalent fungal diseases of cucumber production in the United States. To understand why, first you need to understand its biology. Downy mildew is an obligate parasite of plants, meaning that it requires living plant tissues to survive and reproduce. Since it requires a host plant, downy mildew is confined to areas that do not receive hard frost such as Georgia, South Carolina, and the piedmont and coastal regions of North Carolina. Reproduction of the disease takes place inside the plant tissue and will mature and produce spores and disperse to neighboring plants via wind. Development of disease is favored in cool temperatures, from 60 to 75 degrees and in areas of high humidity, so even when daytime temperatures are not favorable, cooler night temperatures will still encourage the spread of downy mildew.
Symptoms of downy mildew
Once plants are infected, symptoms will begin to appear about 4-12 days later. Signs of downy mildew include mottling and yellowing of leaves, which may also resemble nutritional deficiencies. However, downy mildew also causes angular lesions on leaf surfaces that appear yellowish or brown. This type of damage is what makes downy mildew different from all plant pathogens. You should inspect the underside of the leaves, as it may reveal gray-brown to purplish-black fuzz, which is the many spores produced by the disease. If the disease progresses further, leaves will become brown (necrotic) and begin to curl upwards. Downy mildew only affects the leaves and will not directly harm the fruit, however, if allowed to progress it will reduce your harvest and cause your fruit to become sunscalded due to a lack of shade (especially in watermelon and winter squash).
Fungicidal sprays are available to help slow the spread of downy mildew and should be applied to the undersideof leaves and subsequently reapplied every 5-7 days. It is best to avoid spraying in the middle part of the day (10a-3p). Always read the label on the bottle and observe any cautionary statements. You should inspect plant material every couple of days and remove infected plants away from non-infected ones at the first sign of downy mildew. Infected plant material must be placed in a bag and moved away from healthy plants. If you have overhead irrigation, try to avoid excessive leaf wetness, as this will help to lower disease spread and occurrence. Because fungicidal sprays only slow the spread of downy mildew, the best method of control is to plant resistant cultivars. Squash varieties that are resistant include Super Select and Zucchini Select; cucumbers like Poinsett and Galaxie are also resistant to downy mildew.
Unfortunately, downy mildew is here to stay and something that you will have to deal with if you have plants in the squash and cucumber family. However, if you are vigilant and take certain precautionary measures, you can successfully control the spread of downy mildew and prevent a yield loss for your garden plants.
For assistance identifying and managing plant problems, visit http://ces.ncsu.edu, where you can post your questions via the ‘Ask an Expert’ link, or contact your local Extension office. If you live in Pender County, call 259-1235. In New Hanover County, call 798-7660. In Brunswick County call 253-2610.
.Above: downy mildew on a squash leaf. Notice the “angled” appearance of the lesions.