(News article for March 13, 2021)
In Extension, we often get questions about plants problems when they’re already present. Sometimes, once plant diseases are noticed, it’s too late to effectively treat them, so prevention is key.
Some of the most important steps in disease management must be taken before planting.
In vegetable gardens, it’s important to rotate where we grow certain plants. Plant pathogens (disease-causing organisms) differ a great deal with respect to the range of plants they affect, but plants that are more closely related to each other are more likely to be susceptible to the same diseases than ones that are more distantly related.
Therefore, we often use plant families to group plants for the purpose of crop rotation.
Vegetables in the same family with each other include the following: tomatoes, peppers, eggplants, and Irish potatoes (Solanaceae); cucumbers, squash, pumpkins, and melons (Cucurbitaceae); beans and peas (Fabaceae); cabbage, broccoli, cauliflower, Brussels sprouts, collards, mustards, turnips, and kale (Brassicaceae); onions, garlic, shallots, and chives (Amaryllidaceae or Liliaceae); spinach, beets, and Swiss chard (Chenopodiaceae); lettuce and artichokes (Asteraceae); carrots, celery, and various herbs (parsley, cilantro, dill, etc.; Apiaceae). Sweet corn is the only common vegetable crop that’s a grass (Poaceae). Likewise, while okra is in the same family as cotton and various hibiscus species, it’s the only common vegetable in the mallow family (Malvaceae). Sweet potatoes are in the morning glory family (Convolvulaceae).
Pathogens also differ with respect to how long they survive in the soil, but a longer rotation is generally better than a shorter one. As a rule of thumb, try to avoid planting vegetables of the same family in a given area more than once every three to four years.
For example, if you planted tomatoes in a certain bed last year, you might plant cucumbers there this year, green beans there next year, sweet corn there the year after that, and then plant tomatoes there once again.
Besides crop rotation, a second key to disease management in vegetable gardens is planting varieties resistant to common diseases. Resistance isn’t available for all diseases, but when it is, it’s generally one of the most efficient ways to manage disease.
For example, many modern tomato varieties have resistance to root knot nematodes (microscopic worms that affect roots of many vegetables) and one or more races of Fusarium wilt. Some also have resistance to tomato spotted wilt virus. Likewise, we can choose cucumber and squash varieties that are resistant to powdery mildew.
Information on disease resistance of specific varieties is available in the Louisiana Vegetable Planting Guide and from seed companies.
When purchasing seeds or plants, buy from a reputable source. Check transplants to make sure they’re free of disease symptoms.
Soil pH affects plant disease incidence in some cases. Excessively acidic soil favors two of the most problematic disease of tomatoes, southern bacterial wilt and southern blight. On the other hand, having acidic soil reduces the severity of potato scab. Getting a soil test and adjusting soil pH, if needed, can be a part of managing these diseases.
Good drainage is important for preventing many soilborne diseases. This is one reason that raised beds are often a good choice for home gardeners, particularly in areas with heavy soil, in low areas, and in locations with a water table (where soil stays saturated for some part of the year) close to the surface.
Finally, when you water plants, try to direct water to the soil rather than getting it onto the leaves, and if you water with a sprinkler, do this early in the day instead of in the late afternoon or evening.
Many pathogens need water to be present for a certain length of time in order to infect a plant. Watering from overhead late in the day allows water to stay on the leaves for a longer period of time and increases the risk of foliar diseases, like early blight of tomato. Watering early in the morning allows leaves to dry more quickly on sunny days.
Let me know if you have questions.
Contact Mary Helen Ferguson.
Rotating where certain types of vegetables are planted is an important part of preventing soilborne diseases like southern bacterial wilt of tomato. (Photo source: Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org)
Cucumber varieties resistant to powdery mildew can be selected to avoid problems with this disease. (Photo source: Rebecca A. Melanson, Mississippi State University Extension, Bugwood.org)
Most foliar (leaf) diseases, like early blight of tomato, are favored by longer periods of leaf wetness. If you water in such a way that water gets on leaves (e.g., with a sprinkler), do this in the early morning rather than late afternoon or evening. Watering early in the morning allows leaves to dry more quickly on sunny days. (Photo source: Rebecca A. Melanson, Mississippi State University Extension, Bugwood.org)