A mysterious foe

Plant pathogens often are more difficult for grounds managers to deal with than other types of pests. This may be due to the fact that you often can't actually see the pathogen itself. You may only see the symptoms, which can be difficult to define and also may closely resemble several different diseases. (How many possible causes can you think of for a dead patch of grass? Or dieback in a tree?) In other words, you may have to work a little harder to diagnose the problem correctly.

“Diseases often seem to come out of nowhere. Pathologists know better.”

To me, diseases always seemed not just more difficult, but a little more sinister than other pests. You can see weeds, understand where they come from, pull them out with your hands. The same for insects. They seem more “real.”

Perhaps this relates to the human tendency to fear the mysterious. Diseases often seem to come out of nowhere or occur randomly. In fact, pathologists know better. If a disease shows up one year but not the next, there's usually a reason. Probably certain environmental conditions required by the pathogen occurred one year but not another.

And yet, we do periodically hear of a “new” disease causing havoc somewhere, on some type of plant — a disease that no one had to deal with before. Is there actually such a thing as a “new” disease? Yes and no. Hitherto nonexistent diseases can't simply pop into creation. They must have been previously undiscovered. Or maybe a new, more virulent genetic variant of a known pathogen has arisen. Currently, gray leaf spot is the bane of turf managers in much of the United States. This species of fungus is not really new. It is, however, causing new and sometimes catastrophic problems.

Often, new diseases can be counted among the increasingly common invasive pests that arrive in our ports. One of the most infamous “new” diseases was Dutch elm disease. Of course, it was not new at all; it just hadn't previously found its way to North America.

To give you a better grasp on new as well as ongoing disease problems, we're focusing this issue of Grounds Maintenance on combating plant diseases. Our cover feature, “New diseases,” looks at emerging disease problems. In this feature (page 12), the University of Illinois' Andy Hamblin discusses turfgrass diseases, and Grounds Maintenance's technical editor, James Houx, covers diseases of ornamentals.

One of the most challenging things you face when you come across a new or unusual disease is identifying it. You may have to use the services of a pathology laboratory. When you do, the accuracy of the diagnosis is only as good as the sample you send in. Find out how to perform this basic but critical task in this month's “How to: Submit a plant sample,” on page 44, by North Carolina State University's Dr. Tom Creswell.

Pathologists often refer to the “disease triangle” when discussing plant pathogens. This concept posits that three conditions must exist for a disease to occur. First, the pathogen must be present; second, a suitable host must be available; and finally, conditions must be favorable for infection and disease development. The last condition is frequently the only one that landscape managers can control. Fortunately, this alone is often enough to prevent disease. However, is it possible that you're encouraging turf diseases without knowing it? Occasionally, what seems to be best for turf health is actually creating conditions conducive for disease development. Learn more about this in “Culture Clash,” by Dr. Jon Powell of the University of Minnesota, on page 17.

And don't forget to look for this year's edition of the Turfgrass Fungicide Update, a cross-referenced guide to turf fungicides and the diseases they control, starting on page 38.

We all enjoy a little mystery now and then. But not the kind that destroys our turf and landscape ornamentals. I hope this issue helps shed some light on pathogens and takes the mystery out of plant diseases.

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