How to Find Out What's Ailing Your Plant

Master gardener Paul James shares tips on how to diagnose sick plants.

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If you see obvious signs of insect attack — such as holes in the leaves or evidence of chewing along the margins — you're on your way to diagnosing the problem.

Thankfully, most landscape plants grow up healthy and strong. But every now and then — often for no apparent reason — a plant starts to look sickly. Sometimes the condition comes on slowly, and sometimes the conditions appear to happen overnight.

"It's much easier to diagnose a plant's problem by getting up close and personal with the plant, but even then it can be extremely difficult to zero in on the actual cause of the problem because of the many factors and symptoms to consider," says master gardener Paul James. "In some cases, the only way to know with absolute certainty the cause of the problem is to send a tissue sample of the plant to a plant physiologist for examination and testing."

According to Paul, the first thing gardeners seem to do is panic, particularly if the poor plant is a prized one. "Most often their knee-jerk reaction is to dump lots of fertilizer on the plant and douse it with several gallons of water. But doing so is the absolute worst thing you can do," he says. "Instead, evaluate the symptoms and — through a process of elimination — establish the root cause of the problem."

The first thing James does is determine whether the problem is the result of insect damage. "I look for an actual pest and examine the leaves for signs of attack, such as holes in the leaves or evidence of chewing along the margins." He also examines the undersides of the leaves where a number of insects like to hide during the day. Finally, he examines the bark and limbs of woody plants for holes.

However, not all insects leave such obvious signs of attack. Insects such as aphids and spider mites that don't chew leaves but rather suck the juices from them make diagnosis far more difficult. And the damage caused by some insects can look a whole lot like the damage caused by a host of other problems from nutrient deficiencies, poor soil conditions to even fungal diseases. So it's important that you determine whether the problem is the result of an insect invasion by positively identifying the bugs. Then, and only then, can you rule out other causes, and then and only then should you take steps to control the critters.

"After ruling out insects, I then try to determine whether disease is the culprit and cause of a plant problem." James does so by examining the leaves carefully, and in the case of trees and other woody plants, he inspects the trunk, branches and stems. He looks for a number of different things — wilting or premature yellowing of the leaves, browning along the leaf margin, rust or black spots, oozing sap, abnormal growth and anything that might appear unusual or might suggest a fungal, viral or bacterial disease. Assuming your plant is exhibiting one or more of the symptoms just described, your best bet may be to take a sample of the plant — including both leaf and stem portions of the plant — to a nursery, master gardeners' group or even a good gardening buddy. Although the symptom may well be the result of a disease, it may also be the result of something else all together different. And among diseases, the pathogen can be fungal, viral or bacterial and it can be extremely difficult to determine which is which. And without known the exact cause, it's all the more difficult to determine the plan of action or treatment.

Not all plant problems are the result of insects and disease or some combination of the two. The truth is that most plant problems are the result of poor gardening practices, which result in unhealthy plants. The unhealthy plants are more prone to attack by insects and diseases. And for that reason, the most important place to look for the source of the problem isn't above ground but below ground.

Most plant problems begin in the root system of the plant. For example, if your soil doesn't drain well, it can cause all kinds of plant problems. The same is true of compacted soil. If the pH is too high or too low, it can spell disaster for plants and so on. So if your soil isn't healthy, it's a pretty sure bet your plants won't be healthy either. When trying to determine the cause of a plant's problem, you also have to rule out such things as too much wind or sun, which can lead to leaf scorch and over-crowding. The latter can cause stunted growth and disease problems.

There are plenty of other things that can be the cause of plant problems: damage done by string trimmers and lawn mowers, gasoline and chemical spills, the improper use of garden chemicals and natural gas leaks, excess salt used for melting ice, etc. So the next time you come across a landscape plant that seems to be suffering and you're not sure what to do, don't overreact. Instead, sort through all the possibilities, and chances are you'll be able to figure out the exact cause of the problem and take the appropriate measures to deal with it. "If, despite your best efforts, the plant doesn't respond well to the treatment," James says, "rip it out and try something else."

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