With the cool and wet conditions our region has experienced this growing season, disease problems in the home garden can be expected to be seen soon or later. The number and severity of diseases varies greatly from year to year. There are five major groups of plant pathogens: fungi, water molds, bacteria, viruses, and nematodes. All plants are vulnerable to disease. Diseases occur when environmental conditions are favorable for pathogens to develop into disease on susceptible plants and when there is an abundance of the pathogen present in the environment. Some pathogens attack a wide variety of plants and others attack only specific plants or plant families. Also, some pathogens can attack all plant parts, while others attack only specific plant parts such as leaves, stems, or fruit.

As always, successful disease management begins with an accurate diagnosis. However, the key to disease control in the garden centers on good cultural practices which aid in preventing many problems to begin with. By applying the following practices gardeners can greatly improve their chances for a healthy, bountiful garden:

Site Selection Site should have a minimum of 6 hours of direct sunlight each day. Also avoid planting within the dripline of very large trees. Site should also provide adequate soil drainage and good airflow.

Soil Tillage Done early enough prior to planting, to allow decomposition of organic matter. Organic matter that has not decomposed can be a source of disease organisms and can promote development of certain diseases such as root and stem rots.

Crop Rotation helps prevent the build-up of disease causing organisms in the soil. Closely related crops from the same family should not be planted in the same area for two to three years. Try dividing your garden into quarters and rotating plants into different quarters each year.

Common Vegetable Families:

Alliums (Onion Family): Onions, leeks, garlic, shallots

Brassicas/Crucifers (Mustard Family): Broccoli, cabbage, cauliflower, kale, brussel sprouts

Cucurbits (Gourd Family): Cucumbers, melons, pumpkins, squash, gourds

Fabaceae (Legume Family): Beans and peas

Solonaceous (Nightshade Family): Tomato, potato, pepper, eggplant

Umbelliferae (Carrot Family): Carrots, parsnips, parsley, celery, dill

Sanitation Remove or destroy all dead and diseased plant material in the fall. Many disease-causing organisms can survive the winter in plant debris left in the garden. Healthy crop residues can be composted and returned to the garden. Tilling under any remaining garden stubble can also help destroy overwintering populations of disease organisms. Since some disease causing pathogens are capable of overwintering on contaminated equipment or containers used on diseased plants, these items should be disinfected before being used again.

Resistant Varieties Select disease resistant varieties where available, but keep in mind resistant varieties are not completely immune to disease challenges, just less likely to be seriously infected or killed by a disease. Also some plant varieties may be resistant to one disease but susceptible to another. Do not save seed from diseased plants.

Proper Plant Spacing Allows for good airflow. Crowded plants are more susceptible to disease organisms requiring moisture to infect plants as foliage is unable to dry as quickly. Staking or trellising plants such as tomatoes, pole beans, and cucumbers helps promote better air circulation and also minimizes soil contact of the foliage and fruit.

Weed Control Weeds can be a source for several viral diseases and can also harbor insects which can spread viruses.

Insect Control Control insects to reduce the spread of diseases in the garden. In some situations, beneficial insects will control insect pests so gardeners should distinguish insect pests from beneficial insects before applying insecticides.

Watering Avoid overhead watering to prevent foliar diseases. If this cannot be done, water in the morning so that leaves and foliage can dry before evening. Avoid splashing water up onto the leaves and don’t work in the garden when the foliage is wet to prevent spreading disease pathogens. Maintain an even water supply and avoid wet-dry fluctuations.

Proper Fertilization Fertilize regularly and according to soil test results. Take care not to over fertilize as too much fertilizer, especially nitrogen, can predispose plants to disease.

Tobacco Use Avoid using tobacco while working in the garden. Tobacco products can carry tobacco mosaic virus, which is easily transmitted from workers’ hands to susceptible vegetable crops, such as tomatoes and peppers. Wash hands thoroughly in soap and water after handling tobacco or smoking before working with tobacco mosaic susceptible plants.

Despite our best efforts, diseases can and do occur. If the above mentioned strategies haven’t successfully reduced disease pressures in previous years, then it might be time to consider the use of chemicals to control disease problems before they start. Under the current growing conditions fungal and bacterial diseases are most common.

The biggest mistake gardeners make is that they begin a spray program after the disease problem has become severe. It is important to understand that fungicides work to prevent disease-similar to a vaccine in a person. They do not work like a drug, or silver bullet, to “cure” a problem after it has become established. In order to effectively use a fungicide, you need to make sure the fungicide is applied to plants before disease signs appear or at first sign of disease. Additionally, bacterial diseases such as some leaf spots, blights, and wilts can resemble fungal diseases. Keep in mind fungicides are designed to kill fungus not bacteria. However some copper containing fungicides may be effective on some bacterial diseases.

When deciding to use chemical management to control disease problems in the vegetable garden there are a few key things to keep in mind:

Is your diagnosis correct? Incorrect diagnosis is the leading cause of failed chemical management.

When spraying, focus on disease prevention rather than “curing” infected plants. Improper timing is the second most common reason fungicides fail to control disease problems.

Always read the label before applying any pesticide. Follow the instructions and restrictions. Do not use any chemical when temperatures are above 85 degrees F, or under windy conditions. Also be sure to note any restrictions and the time interval between application and harvest.