Prior to Diagnosis, Get to Know Your Plants

When I first began to study horticulture, I took a class where the instructor walked us through campus examining plants for insect, disease and mineral deficiency problems. On one of these walks, a student remarked: “It seems that every plant has one problem or another.” The instructor noted how true this is.
The closer you look at a plant, the more imperfections you see in it. This does not mean that you should become alarmed and start spraying. Much better to seek an ex-plantation for the apparent flaws. In fact, it often turns out that an imperfection is not a problem, but rather an expression of the plant’s aging process or life cycle, or a sensitivity to an environmental condition that the species regularly displays.
Dennis Held of Simi Valley writes the following: “My plants (geraniums, sunflowers, beans, etc.) have some leaves that are brown, yellow or abnormal in some other way. How can I tell if these leaves have changed color because of disease (like fungus) or from some other cause, like a mineral deficiency?”
Take into account the weather we experienced this spring. Each day, it seemed, was hotter than the previous one. Heat speeds up all biological processes, and plants this spring went through their life cycles incredibly fast. Broccoli seedlings that I planted from a six-pack in mid-April had produced a healthy crop by the first week in June, just 45 days later.
The yellow leaves Held has observed, if they are lower down toward the base of the plant, have simply aged. Since sunflowers and beans grow faster than most plants, their aging process will begin that much earlier. When leaves age, their nitrogen is translocated to newer, developing leaves at the top of the plant. Nitrogen is a key component of chlorophyll, the pigment that makes leaves green; as a result of aging, and the nitrogen removal and chlorophyll disintegration processes that accompany it, a leaf turns yellow.
If yellow leaves are observed at the top of a plant, this is a sign of nitrogen deficiency in the plant. Again, though, this is not necessarily cause for alarm. It may be that every year, during a certain season, a plant is simply unable to take up all the nitrogen it needs to appear lush and green. A classic example of this is the blue potato bush (Lycianthes rantoneii), whose leaves periodically turn yellow, only to become lush and green when the seasons change.
If leaves are brown, as opposed to yellow this time of year, the problem is either soil or water salinity, mineral imbalance in the soil or excess heat. If the problem is salt-, or mineral-related, the browning will be confined to leaf tips or margins; sunburn is indicated by irregular brown patches anywhere on the leaf surface.
Leaf spots – whether yellow, brown, black or white – are generally a sign of fungus, but also could be caused by bacteria. Stippled or densely speckled leaves mean that spider mites are present.
None of the problems or conditions described above will necessarily have an adverse effect on the crop. Geraniums often have many discolored leaves but still show perfect flowers. Sunflowers and beans may have sunburned leaves and spider mites but still produce the seeds and the pods for which they were planted. The broccoli I planted in April soon was visited by the aphids and cabbage loopers that always find this plant, yet a nice broccoli crop still was produced.
Two excellent books are available to the backyard gardener on the subject of plant problems. One is “Pests of the Garden and Small Farm,” which focuses on vegetables and fruit trees, and the other is “Pests of Landscape Trees and Shrubs,” whose concentration is ornamental plants. Keep in mind that the category “pests,” for a horticulturist, includes insects, fungi, bacteria, viruses, gophers, snails, weeds and any living organisms that can interfere with proper plant growth. Published by the University of California, these volumes contain dozens of color photos illustrating past problems and mineral deficiencies. Emphasis is placed on least toxic solutions to garden problems. You can order these books by calling (800) 994-8849.
Don’t expect to have all of your questions answered by these, or any other books. It can be enormously frustrating to go through page after page of text and find that every plant problem under the sun is discussed – with the exception of your own. Upon perusal of these UC publications, though, there are some general conclusions that you soon will reach, and these, for the most part, will put you at ease.
Plant problems, you discover, ultimately are linked to inadequately prepared soil, improper exposure (too little or too much sun), poor air circulation around and/or through the plant (because plants are too crowded or inadequately pruned) or incorrect watering procedures. These problems are correctable since the fault is not in our plants but in ourselves.
Tip: To propagate your tomato plant, remove shoots growing from leaf nodes toward the base of the main stem. After detaching their lower leaves, half bury these shoots in pots or in the ground.

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