A steady stream of plant samples, ranging from chunks of turf to dead tree limbs, crosses our desks each day during the summer. "What's wrong with my plant?" gardeners want to know. It's easy to blame an insect or the neighbor's weed-killer spray drifting into his yard. Sometimes these are the culprits, oftentimes not.
To correctly determine which disease (anything that disrupts the normal growth of a plant) is causing it to become "sick," horticulturists use a diagnostic process, just as doctors use a process to determine what's wrong when people are sick.
To begin, determine if the problem is abiotic (non-living) or biotic (living), and, if biotic, possibly identify whether it is an insect, virus, bacterium, or fungus. Why is this important? Control methods are successful only if the correct problem is identified and the solution targets that specific problem.
Start by asking, "What is this plant and what does a normal one of these plants look like?" By comparing the affected plant to a healthy one, symptoms (the visible response of a plant to a disease) can be noted.
Correct identification is important because many insects and diseases can be eliminated if the species and sometimes variety are known. For example, fire blight, caused by the bacterium Erwinia amylovora, only attacks certain species in the rose family.
Next, examine the plant and the environment around the plant. How much light, water and fertilizer is it getting? What is the soil condition - heavy clay, or has organic material been added to increase oxygen movement and nutrients? Any recent construction around the plant? Any unusual weather within the last year? When was the problem first observed and how quickly is it spreading?
What is the distribution of symptoms? Is it on one plant or many, on only one species of plants or several different species? Is the entire plant affected or only a portion? Make notes of your observations; the more detailed the better.
While further research is often needed to identify the specific disease causing the problem, many questions can be answered based on the above observations. Abiotic problems include mechanical, physical, chemical, and nutritional.
Symptoms can be very obvious, such as lawn-mower damage around the trunk of a tree (mechanical), frost (physical), or new leaves yellowing on silver maples due to an iron deficiency (nutritional). Sometimes the problem is not as obvious, such as an underground natural gas leak killing a tree (chemical).
Generally, abiotic problems affect all species of plants simultaneously, whereas diseases usually affect one species, start in one area and then spread. Abiotic problems can often kill quickly (within a few days) without a yellowing stage.
Diseases kill much more slowly and often will cause a yellowing of the leaves before they fall off. If the pattern of the disease is regular, such as all the leaves of a certain age affected, it is most likely an abiotic cause.
Living organisms generally don't produce uniform, repeated damage; typically, their damage is much more irregular. An entire failure of the plant, including leaves, stem, flower, and root, is usually a symptom of an abiotic problem. Biotic diseases progressively attack only portions such as the leaves or flowers, not all portions simultaneously.
Some symptoms could be telling of either biotic or abiotic disease, so look at the characteristics carefully. Curled or distorted leaves could be symptoms of an abiotic problem, such as herbicide or low temperatures, but if you see aphids, eriophyid mites, or powdery mildew, it's a biotic problem.
Untimely leaf drop could be low nutrition (abiotic) or insects (biotic). Some symptoms are almost always going to be abiotic, such as leaves that are yellow-green color (low nitrogen, excessive moisture, planted too deep); yellow leaves with green veins most likely means nutrient deficiency or drought; purple cast is most often due to excess phosphorus.
Some symptoms, such as skeletonized leaf (insect), tunnels (insect), a grayish cast (spider mites or thrips), or galls (insect, fungus), almost always indicate a biotic problem. If all symptoms are pointing toward a biotic disease, then you might be able to narrow it down to insect, virus, bacterium, fungus, or gall.
Look not only at the symptoms but also for signs, such as visible parts of the pathogen on the plant like the insect body or fungi fruiting structure.
Viruses are too small to see but the symptoms are unique. Look for mosaic (yellow and green mottling of the foliage), ringed coloration, stunting, distortion of leaves or flowers, or necrotic areas (localized death of living tissue, but not the entire leaf surface). Bacteria are also too small to see, but again the symptoms are unique. Infested areas will look water-soaked, have a slimy texture, and often have a rotting smell. If the disease persists, the plant tissue will totally disintegrate.
Fungus symptoms include dry texture, concentric rings and discoloration spots. Leaf spots generally have distinct margins, are circular with concentric rings. The center of the ring is usually tan (old dead tissue), then brown (newly dead tissue), then light yellow (edge of fungal infection).
Galls can be caused by either insect (hackberry nipple gall) or fungus (juniper-hawthorn rust). Galls by insects are usually species specific, so Cooley spruce gall aphids create galls only on spruce trees, and poplar twiggall flies attack only aspen and some poplars. Galls are irregular growths of irregularly organized tissue, and the symptoms are usually very obvious.
Insects are usually categorized by orders, such as Coleoptera (beetles), Orthoptera (grasshoppers) and Homoptera (aphids).
In the diagnostic process, it's more important to correctly identify the type of damage than type of insect that caused the damage. If the leaves have been chewed on, then the list of culprits can be narrowed down to insects with chewing mouthparts. This could include leafcutter bee, caterpillars, beetles, sawflies, grasshoppers and slugs.
If you have a plant problem, and are unable to determine the cause of it, you can take it to the experts. The Plant Diagnostic Clinic is a non-profit laboratory/consulting service supported by Colorado stae University Cooperative Extension and Jefferson County. The clinic aids in the identification and treatment of plant problems caused by disease, insect and environmental conditions. There is a $10.00 charge for diagnosis ($7.00 for residents of Jefferson County).
They will also identify plants, weeds, and insects for you. There is a $10.00 charge for insect ID ($7.00 for residents of Jefferson County). The first two plant/weed ID's are free and there is a $5.00 charge for each one after that. Virus testing for INSV and TSWV is $5, Microscope exam (for spores, root rots) $10, a culture is $15 and serological testing for sclerotinia, pythium and rhizoctonia is $45. The clinic is open Mon-Fri, 8:30am to 4:30pm, May through September.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010