   Antitranspirants as Least Toxic Fungicides - March 1, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Many gardeners are looking for alternatives to traditional chemical pesticides to control damaging insects, fungal pathogens, weeds, and other garden pests. To respond to this market niche, many new products are being developed and promoted. One area which has received attention is that of using antitranspirants as fungicides. As usual, you must tolerate some plant science lessons on our circuitous route to this topic. Bear with me, and we'll get there soon enough. Plants have natural mechanisms that protect them from excessive water loss and disease in their native environments. All plants need water in their leaves to conduct photosynthesis. Transpiration is the process that plants use to transport water from the soil, through the roots, up the stem, to the leaves. Leaves have small openings, called stomata, that provide exit holes for evaporated water to be released to the atmosphere. When soils water is available, each water molecule that evaporates is replaced by another that is waiting in line behind it. This also provides evaporative cooling to the leaves on hot days. When soil water is unavailable, the stomata close to decrease water use. However, plant leaves are not perfectly sealed like Tupperware. Rather, leaves are porous somewhat like a paper bag and some water escapes through the leaf surface even when stomata are closed. End of science lesson number one. To further understand how leaves protect themselves, we need to know more about leaf anatomy. The outermost layer of leaf consists of clear, window-like cells known as the epidermis. On the outside surface of the epidermis is a coating of waxy material (cutin) known as the cuticle. Just think of the last time you picked a ripe apple and felt that slightly sticky wax cuticle on the surface of the fruit. The thickness of the cuticle varies greatly by plant species. Many native plants, such as pinyon pine and juniper, have a naturally thick cuticle. This helps them tolerate our arid environment. Introduced plants, such as fruit trees, garden shrubs, annual flowers, and vegetable crops, are adapted to less arid locations and have a much thinner cuticle. End of science lesson number two. Back to the topic of this article: antitranspirants (ATs). ATs were originally developed to increase survival of transplanted trees and shrubs by reducing water use in various horticultural settings. The polymers used as ATs vary in chemical composition, but all function by increasing cuticle thickness and integrity. Some brand names are Cloud Cover, Wilt Pruf, Vapor Gard, and Safe Pack. Many plant pathologists also believe the cuticle protects plants from diseases caused by bacteria and fungi. Researchers have extracted cuticle waxes from apple leaves and applied them to the leaves of bean plants. They found the bean plants had greatly increased their resistance to Botrytis gray mold, a common pathogen. Other research has shown that removing the cuticle increased the incidence of foliar disease in many plants. The practical landscape applications in ornamental landscapes are numerous. ATs can provide up to 90% control of powdery mildew on roses, Euonymous, Zinnia, and other ornamentals. It can also control gray mold on tomatoes and downy mildew on melons, cucumbers and squash. In several research projects looking at these crops, the use of ATs has resulted in higher crop yields. I would especially encourage rose growers to try ATs as a least toxic control for powdery mildew and possibly black spot. Before using these products, try them on a small portion of the plant to test them for harmful effects. Also check the label to see if they are recommended for edible crops. On the down side, ATs can limit photosynthesis and reduce the evaporative cooling effect. In our sunny climate, the limited photosynthesis is less of a problem than the loss of evaporative cooling. For more information about least toxic pest management alternatives, visit the UC-IPM site at the Horticulture Links section of the Yavapai County Cooperative Extension web site: http://ag.arizona.edu/yavapai/. 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. |