   Diagnosing Plant Diseases - June 3, 1998 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County What is a plant disease? It seems like an easy enough question, but often the diagnosis can be difficult. The textbook definition of a plant disease: any alteration of a plant that interferes with its normal structure or function and renders it unfit for its normal use. On most plants, we can agree on normal structure and function. For instance, if all the leaves on a sycamore tree turn brown and die in May, we would assume that this is not normal. However, sometimes "normal" can be difficult to determine. For example, two of Yavapai County's native oak trees always lose their leaves in May. This normal event is followed by a flush of new growth. Naturally, Yavapai County Cooperative Extension informs a few new residents each May about Emory and Arizona White oaks. Plant disease can be divided into two broad categories: those caused by living organisms and those caused by abiotic (nonliving) factors. Living disease organisms include fungi, bacteria, viruses, nematodes, insects, mites, mollusks, rodents, etc. Plant diseases caused by living organisms generally become more widespread within the plant, move from plant to plant and have a random or scattered pattern. Living disease organisms also tend to increase with time. Abiotic factors include mechanical, physical (environmental extremes of temperature, light, moisture, etc.), and chemical injury. Disease symptoms from abiotic factors are usually recognizable and widespread. For instance if all the damage occurred on leaves of a certain age or in a particular pattern on the plant. Abiotic factors are usually not progressive: plant damage tends to stay confined to a given area and not spread. The plant damage patterns and rate of development are both useful indicators when identifying causes of plant disease. You are now a plant detective looking for clues. You are dispatched to case involving an ailing Bartlett pear tree. At the scene you inspect the victim: a well cared for tree that was correctly pruned (must have attended a Cooperative Extension pruning workshop). Inspecting the damage, you notice that only some branches are affected. On the affected branches, the leaves are dead, brownish-black and still attached to twigs. The twigs also have drooping tips. The next of kin of the victim (the owner) tells you that symptoms first appeared in the spring and the foliage progressively died back from the branch tips. What do these clues point to: living or nonliving factors? The symptoms spread with time and were irregular. Your answer should have been a living organism. You look for more clues. Out of the corner of your eye you notice a severely pruned Pyracantha bush. The owner tells you that before pruning the pear, they had to clear away the Pyracantha that had some dead branches in it from the previous year. You evaluate the case and determine that the culprit was fire blight: a bacterial infection that infects many species in the rose family. The infection was transferred to the pear from the Pyracantha by the pruning shears. It may not have happened had they cleaned the pruning shears in a 10% bleach solution. Fire blight is somewhat difficult to treat, but there are options to prevent its spread. Plant disease diagnosis is rarely as easy as the above example, but with enough information, it is usually possible. Define the problem, look for patterns, interpret the rate of development, and then narrow it down to a list of suspects. Follow these guidelines and you too can be a plant disease detective. The University of Arizona Cooperative Extension has publications and information on plant diseases and treatment. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |