Break the cycle of several common garden diseases

Published Aug 6, 2017 at 9:00am

Diseases can come to your garden due to weather conditions, passed by the wind, or even carried by a visiting insect. Some years, you can have a serious problem, and then the next year, it can be dormant, waiting for the right conditions to grow. With all the wet weather the past few weeks, disease problems will probably be more common.

So, what causes diseases to grow and spread? There are actually three things which must be present in order for a plant to have a disease. Remove one of these elements and the disease can’t develop. The three elements are a pathogen, the right environment, and a host.


This is the organism that finds a susceptible plant and enters its system. The organism can enter the plant via a pruning cut, a plant injury, or through the leaves, stems, or roots.


The right weather conditions have to be present for the pathogen to grow and spread. Depending on the disease, the right weather will vary.


The pathogen needs a susceptible plant host to grow. It could be a variety of plant which is prone to the disease; or it can be a stressed plant (i.e., a plant weakened by insect damage or drought stress).

With our wet weather, we have an environment which could be a factor in some key disease issues. Here are some of the more common diseases.


This disease spreads by wind and water and will affect many plants during humid conditions. Look for powdery rusty orange or brown spots on the plant foliage. The spots can be seen on either side of the leaf.

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 plants to increase air circulation. Consider a preventative fungicide.


This disease is more common after a cool, wet spring. Look for dark, irregular blotches on the leaves. Sometimes buds and branches can be infected. Ornamentals and edibles can get this disease. The fungal spores of this disease can overwinter, move through the air, and then settle in cool, wet weather. Many shade trees such as ash and maple are prone, as are many ornamentals such as dogwoods, and many fruiting vegetable plants will also suffer. Improved air circulation will help as well as a preventative fungicide.  

Black spot

This is a dreaded problem with rose growers. Black spot can occur on leaves and stems. Leaves eventually yellow and fall off, making plants ugly. This is another disease that overwinters and then establishes itself again during periods of wet weather or with overhead watering. Consider plant resistant varieties or use fungicides.  

Powdery Mildew

Look for white granular patches resembling dust on the leaves or stems that 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 or consider planting resistant varieties. Horticultural oils or a fungicide work best if applied before the disease shows itself.

If you can break the disease cycle, you can prevent the diseases. However, it’s tough to control the “environment” when Mother Nature doesn’t cooperate.

Look for more resistant varieties and remember that preventative measures work better than reacting to the disease after its present. Know which plants might be susceptible, keep an eye on your garden, and consider preventative fungicides. For more information, visit our website at or visit Cornell’s Disease Diagnostic Clinic website at