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Here’s how to break the cycle of some common garden diseases

By ​​​​​​​Rosanne Loparco
Posted 8/11/19

Diseases can come to your garden due to weather conditions, passed by the wind, or even carried by a visiting insect. With all the wet weather the past few weeks, disease problems will probably be …

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Here’s how to break the cycle of some common garden diseases


Diseases can come to your garden due to weather conditions, passed by the wind, or even carried by a visiting insect.

With all the wet weather the past few weeks, disease problems will probably be more prevalent since excessive moisture is one of the major environmental factors causing plant disease.

There are three elements which must be present to have a plant disease: a pathogen, the right environment, and a host. The pathogen is the organism that finds a susceptible plant and enters its system though injury, the leaves, stems and/or roots. The right environment has to be present for the pathogen to grow and spread; excessive moisture is the most common environment. The pathogen then needs the host plant; a weak or stressed plant is most susceptible.

If you can remove one of the three elements, the disease can’t develop. However, it’s tough to control the environmental factor when Mother Nature doesn’t cooperate.

Here are the more common moisture-driven plant diseases:


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, but can come on at any time during the gardening season.

Rust will weaken the plant, reducing flower and fruit production. You can remove infected leaves and destroy them. Try to keep foliage dry by insuring there is plenty of room between the plants for air to circulate.


This disease of both ornamentals and edibles is more common after a cool, and wet spring. Symptoms: look for dark, irregular blotches on the leaves. Sometimes buds and branches can be infected, and severely infected plants can die. Many shade trees such as ash and maple are prone to anthracnose. Improved air circulation will help by pruning dense coverage.

Black spot

This problem is the worry of most rose growers, but it can infect other types of plants too. Black spot can occur on leaves and stems; it normally won’t kill a plant, but it will weaken it. Leaves eventually yellow and fall off, making plants ugly. This disease overwinters on infected leaves and stems.The next year, it will establish itself again during periods of wet weather and/or with overhead watering. Consider planting more resistant varieties.

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 won’t kill a plant, but makes it look ugly. Provide good air circulation and/or consider planting resistant varieties.

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 as well as branches are infected and can become distorted and die. Where powdery mildew impacts 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 these products work.

Fungicides will not cure existing diseases. They will only protect the parts of the plant without the disease (slowing the disease down or stopping it from spreading). Once the disease is present and attacking the plant, 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 insure the plant and the problem you are trying to control is listed on the product label.

Preventative measures

If you’ve struggled with these problems, look for more resistant varieties and remember that preventative measures work better than reacting to the disease. Know which plants might be susceptible and monitor your garden regularly.

Practice good garden cultural controls. For example, avoid overhead watering which can provide excessive moisture to foliage; water early in the morning so moisture has a chance to dry during the day. Keep garden tools clean so you won’t move diseases from plant to plant. Provide enough space between your plants to improve air circulation.

Many of these diseases can overwinter; therefore, it’s important to keep plants clean by removing all fallen debris.

For more information, visit our website at, visit Cornell’s Disease Diagnostic Clinic website at or call our horticulture hotline on Wednesday and Friday, between the hours of 9 a.m. to noon at 315-736-3394.


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