Diseases can come to your garden due to weather conditions, passed by the wind, or even carried by a visiting insect. There are three elements, which must be present to have a plant disease: a pathogen, the right environment, and a host. A pathogen is the organism that finds a susceptible host plant and enters its system though injury, the leaves, stems or roots. The right environment has to be present for the pathogen to grow and spread; excessive moisture is the most common environmental condition. If you remove one of the three elements, the disease can’t develop, however, it’s tough to control the environmental factor. Here are the more common moisture-driven plant diseases.

Rust: This disease will affect many ornamentals and edibles during humid conditions. Look for powdery, rusty orange or brown spots on the plant foliage. It is most common in summer or early fall. Rust will weaken the plant, reducing flower and fruit production. Remove infected leaves and destroy them. Provide plenty of room between plants for air to circulate.

Anthracnose: This disease of both ornamentals and edibles is more common after a cool, wet spring. Look for dark, irregular blotches on the leaves. Sometimes buds and branches can also be infected, and severe infection can mean death for the plant or tree. Many shade trees such as ash and maple are prone to this disease. Improved air circulation will help.

Black spot: A common problem for roses, black spot can also infect other plants. Black spot can affect leaves and stems. It normally won’t kill a plant, but will weaken it. Leaves turn yellow and fall off, making plants look unsightly. This disease overwinters and then establishes itself again during periods of wet weather or with overhead watering. Keeping the garden bed clean is critical to control.

Powdery mildew: Look for white granular patches resembling dust on the leaves or stems. It can be rubbed off with your fingers. The spores for this disease are moved by air; it rarely kills a plant, but makes it look ugly. Consider planting varieties that are more resistant.

Downy mildew: This problem is distinctly different from powdery mildew. Where powdery mildew appears on the top of leaves, downy mildew appears on the underside of the foliage and has a bluish tinge. On the foliage, small yellow spots develop while bluish-white fluffy growth appears on the underside. Leaves and branches are infected and can become distorted and die. Where powdery mildew affects the appearance of the plant, downy mildew can be systemic and kill the plant.

If you’re considering a fungicide, it’s critical to understand how they work. Fungicides may protect the parts of the plant without the disease (slowing the disease down or stopping it from spreading). Once the disease is present, it is usually too late to apply a fungicide. Fungicides work best as a preventative, applied before the disease shows itself. As with all pesticide use, be sure to read labels completely and ensure the plant and the problem you are trying to control is listed on the product label.

If you’ve struggled with these diseases, look for varieties that are more resistant and remember that preventative measures work better than reacting to the disease. Practice good garden cultural controls. For example, avoid overhead watering, water early in the morning, keep garden tools clean, and provide enough space between your plants. Many of these diseases can overwinter; therefore, it’s important to keep the garden area clean; remove all fallen debris and replace any mulch every season. Be sure to visit our website, cceoneida.com for more information regarding disease control.

Home and garden questions can be emailed to homeandgarden@cornell.edu or call 315-736-3394, press 1 and then Ext 333. Leave your question, name and phone number. Questions are answered weekdays, 8am to 4 pm. Also, visit our website at http://cceoneida.com/ or phone 315-736-3394, press 1 and then Ext 100.