Fungal leaf spots, which include brown, yellow, or black spots, are common on many vegetable plants. Experience and regular monitoring will alert gardeners as to the seriousness of the problem. Foliar diseases are frequently weather-dependent and vary in severity from season to season according to rainfall and temperature. In some cases, leaf spots will not spread or cause much damage, but in other cases management actions should be taken promptly because the disease will continue to spread, perhaps defoliating the plant and reducing yields and eating quality. The first step is to learn about the common vegetable crop diseases in your area and, when disease symptoms are observed, identify the disease and determine the least-toxic management strategy.
Early blight of tomato is an example of a foliar disease that appears with great frequency in the northeast and mid-Atlantic states and has the potential to defoliate plants during the growing season. Powdery mildew, on the other hand, is a fungal disease that usually infects pumpkins and squash late in the growing season and is not typically a serious problem for backyard gardeners.
Bacteria are single-celled, microscopic organisms, bounded by a cell wall, that cause plant diseases. Bacteria are much smaller than fungi but can cause severe symptoms. Bacterial pathogens can cause soft rots of fruits, vascular wilts (e.g. bacterial wilt of cucumber and muskmelon), and leaf spots and blights (e.g. bacterial spot of pepper, bacterial blight of peas).
Monitoring for diseases
Regular plant inspection, especially on lower and inner leaves, will alert gardeners to foliar problems. Foliar diseases are progressive- they begin as small spots on a few leaves. Lesions grow and coalesce and may cause leaves to yellow and die. Identify problems early on to determine the cause of the problem. Monitor affected plants through the season.
Management of bacterial, fungal, and viruses that attack vegetables
Many bacterial, fungal, and viral diseases attack vegetable crops in Maryland home gardens. Most of these are not serious and in very few cases is spraying a fungicide recommended. Remember, when disease symptoms are noticed it is usually to late to spray a fungicide. Below are twelve tips that can help you prevent disease problems:
- Select disease-resistant varieties, particularly for those diseases that appear in your garden each year.
- Purchase certified, disease-free potato tubers, garlic bulbs, and asparagus and rhubarb crowns.
- Avoid planting on wet, poorly drained sites.
- Pull soil up into raised beds if drainage is not very good.
- Dig or till compost into the soil each year.
- Grow healthy plants by providing adequate light, water, and nutrients. Give each plant adequate space to ensure good air circulation and add organic matter to your garden each year.
- Keep bare ground covered with an organic mulch. Newspaper covered with straw works very well.
- Avoid watering foliage in the evening. It is best to direct irrigation water around the plant base where it can quickly reach the root zone.
- Avoid handling wet foliage.
Harvest your vegetables before they become over-ripe.
- Cut off and discard leaves and pull up and discard entire plants that are badly infected by disease.
- Pick-off and remove diseased fruits and clear your garden at the end of the season of all plant debris. This should be composted or tilled into the soil. Plant parts infected with especially damaging diseases, like late blight of tomato and potato, southern blight, and white rot should be bagged and put out with your trash.
- Keep weeds to a minimum and control those insect pests like thrips, aphids, flea beetles, and cucumber beetles that are most likely to spread diseases.
When disease symptoms are observed it is often too late to apply a fungicide, although fungicide treatments can help to protect new or un-infected foliage. Fixed copper, sulfur, and horticultural oil are some organic fungicides used by home gardeners. Always, carefully read and follow all pesticide label information and test the spray on a small part of the crop to check for signs of leaf injury (phytotoxicity.)