   Plant Disease Diagnosis - September 10, 2008 Jeff Schalau, Associate Agent, Agriculture & Natural Resources University of 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 use. On most plants, we can agree on normal structure and function. However, under some conditions, "normal" can be difficult to determine – especially if the plant is not widely grown in the area. 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, etc. Plant diseases caused by living organisms generally become more widespread within the plant, can move from plant to plant and have a random or scattered pattern with symptoms that 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 does not spread. In plant disease diagnosis, damage patterns and rate of development are both useful indicators when identifying potential causes. In addition, disease symptoms are often brought on by a series of events and could be combinations of biotic and abiotic factors as well as other conditions which could also include insect or animal damage. In diagnosing plant diseases, the gardener must assume the role of detective searching for clues. First, you must be familiar with the plant species being affected. Then, you follow the trail of evidence consisting of “symptoms” (changes in the plant from its normal appearance in response to the disease organism) and “signs” (actual vegetative and/or reproductive structures created by the plant disease organism). These are often documented in gardening literature. Define the problem, look for patterns, interpret the rate of spread, and then narrow it down to a list of potential causes. Symptoms can be similar for many diseases, but if and when signs appear, most diseases can be accurately diagnosed. Unfortunately, when actual signs appear it can often be too late to treat many diseases. In addition to major problems with curly top virus in tomatoes and other vegetable crops (see August 20, 2008 Backyard Gardener), I have seen three other diseases this summer: Phymatotrichum root rot (also called Texas root rot or cotton root rot), slime flux (also known as wetwood), and fungal diseases in cool season turf. All three are usually present in the summer months. Phymatotrichum root rot is a fungal disease affects a broad range of host species and in present throughout the Verde Valley. It is always fatal to infected plants. There are no effective treatments and the only plants totally resistant are grasses, bamboos, palms, agaves, yuccas, bulbs (lily family), and other monocots. Slime flux or wetwood affects mesquite, cottonwood, ash, elm, mulberry, willow, poplar, apple, firs, maples, pine, sycamore, and other trees. The disease is caused by bacteria and/or yeast in the plant xylem (sap wood). It is characterized by a watery liquid that flows from branch crotches, cracks, pruning cuts and other wounds. The liquid is often foul smelling and can attract insects. There is no recommended treatment and infected trees often live for many years. Fungal diseases in cool season turf are just beginning to appear. Admittedly, I am no expert in this area. However, the likely suspect is Pythium blight. Symptoms are small dead patches in the tall fescue, Kentucky bluegrass, and ryegrass. Fungal diseases in cool season turf are favored by warm temperatures, high humidity, poor drainage, overwatering, and excessive nitrogen fertilization. Cultural practices that mitigate the above listed factors and preventative treatments with broad spectrum fungicides may provide some benefit. The University of Arizona Cooperative Extension has publications and information on disease and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |