Douglas A. Spilker, Ph.D., is a consulting ornamental plant pathologist and entomologist, garden writer and lecturer. Dr. Doug can be reached at

This article applies to:



Go Out & Look: Winter Scouting for Pests and Diseases
by Douglas A. Spilker, Ph.D.    

Which plants grew well this year? Which did poorly? Which now have diseases or insects? Now is the time to scout for insects and diseases in the landscape. End-of-the-year scouting is also a great excuse to enjoy a walk through the garden before cold weather sets in.

As gardeners, we never really want the gardening season to end, but we know that cold weather comes every year. However, the fall and early winter season is a wonderful time of year to meander throughout the yard assessing the landscape on how successful our yearly partnership was with nature. This is not only a time to add to your garden journal about your favorite plants and what varieties did well, but also to “scout” the landscape for potential pest and disease problems for next year.

Although you commonly hear about scouting during the season as the first step in integrated pest management, scouting can be equally important as the growing season winds down. Scouting can be as simple as looking closely at your lawn, flowers, shrubs and trees to see if pests or diseases are present. It can actually be easier as the leaves fall, revealing hidden culprits.

Write Right and Wrong


If you have not done so, record in your journal what problems occurred and most importantly the locations. Think about the pest or disease cycle of the organism and how the environment or other factors might have contributed to the issue. The “right plant in the right place” is a common adage in gardening for good reason. Did your lilac have powdery mildew because it is planted in a protected area with high humidity? Did your roses get overhead irrigation, which was conducive to black spot development? Were sun-loving plants stressed because they were planted in a shady area? Just like people, healthy plants are better able to both thwart and recover from pests and diseases.


Examine Trees and Shrubs

While scouting, look especially at trees and shrubs that appear weak and unhealthy, have sunken areas (cankers) on twigs or have branch dieback, which could indicate a pest problem. Examine plants closely for any signs of pests, especially the presence of egg masses and other indications of overwintering insects and sources of disease inoculum. A simple hand lens can help. One of my early gardening mentors used to say “prune when the knife is sharp.” Pruning out affected branches can often be an effective method to reduce pest populations.

In addition to fallen leaves, cane cankers on roses harbor the black spot fungus and should be pruned out and destroyed.

Some pest problems might easily be seen such as the cocoons of bagworms. Bagworms are very damaging to many species of trees and shrubs, including juniper, arborvitae, cedar, pine and spruce. Bagworm larvae feed on the foliage of the trees creating open areas of the canopy. The simplest way to manage bagworms is to harvest the bags. Be sure to discard the bags and do not just let them drop to the ground, or the larvae will just crawl back. When removal of bagworms is not feasible, record which plants need to be sprayed to control this devastating pest. Physical removal is also recommended for any gall or gall-like tissue found, such as the cedar-apple rust galls on junipers.


You might have to look closely to see some pests, or they can be missed. For example, populations of the tiny oystershell scale can be so copious that they totally encrust branches. Scale insects remove plant juices using their piercing-sucking mouthparts, causing twig and branch dieback. Pruning out infested branches might help, but make a note that spring spraying may be needed to control the vulnerable “crawler” stage on plants with heavy scale populations.

Oystershell scale

Time to Tidy

To suppress future disease problems, clean up landscape beds by removing and destroying diseased plant material. Rake and destroy diseased leaves from fruit trees and discard mummified fruit. This can reduce disease inoculum for next year’s garden.

Fallen leaves are usually the source of rose black spot infections, but do not underestimate the inoculum source from cane cankers. When scouting your roses, prune out any diseased canes. After leaf fall, tree limbs affected by fireblight can be difficult see. Give infected branches a spot of paint to indicate which ones need pruning during tree dormancy.


ID the Pest Perps

Correct identification of pests is important. Certain insect species require distinct control measures. You have all winter to learn about them in classes, on the Internet, in guide books or by taking samples to your local cooperative extension office. Most insects progress through a series of growth stages from egg to immature to adult and not all treatments or strategies are successful for all pests or pest life stages. By knowing a little bit about pests and their development, you can better develop pest control strategies.

End-of-the-year scouting is also a great excuse to enjoy one more walk through the garden before cold weather sets in and allows for reflection on the successes and failures of this year’s garden.

The cedar-apple rust fungus overwinters as a gall on juniper branches.

Write It Down

One of the most important tools in a gardener’s shed is the garden journal. It can be a small notebook, a bound diary or a digital recorder to gather your gardening thoughts and observations. Be sure to sketch out your vegetable garden, so you do not forget to rotate crops. Winter can be long and memories can be short. A gardening journal provides a resource to consult when planning changes to your garden, during plant selection and for developing pest control strategies for the coming year. Not only that, but it can be quite enjoyable reminiscing about past trials and tribulations experienced as an avid gardener.


A version of this article appeared in a November/December 2011 print edition of State-by-State Gardening.
Photography courtesy of Douglas A. Spilker, Ph.D.


Posted: 01/11/12   RSS | Print


Share this story on:
Facebook       Twitter            

Other People Are Reading