Timely Topic 96 - BY DON MEEHAN

UNDERSTANDING FUNGICIDES

This time of the year our environmental conditions are just right for plant disease to get a good start. Many of the plants have fungal spores resting on them over the winter months. As the temperature warms and humidity stays high, the fungal spores germinate and invade the plant tissue. Fungicides are commonly used during this period to prevent these types of invasions. In this column, I will address fungicides and how they work. Fungicides are generally classified by their active or basic element ingredient. Copper fungicides are among the oldest and Bordeaux mixture is one of the earliest. Its fungicidal value was discovered about one hundred years ago and is still used to some extent today. Bordeaux mixture illustrates how fungicides work. It is a mixture of copper sulfate and lime to form copper hydroxide. Copper sulfate alone is very soluble in water and if a solution is sprayed on a plant, it is very phytotoxic (toxic to the plant). Copper hydroxide, however, is practically insoluble in water and will not cause damage to sprayed plants. But enough copper goes into the solution from copper hydroxide to kill fungus spores and so will control plant disease organisms without harming the plant. Elemental sulfur is actually the oldest of the fungicides on record, and reference to the "pest-averting sulfur" occurred as early as 1000 BC. It is still used today, mainly for the control of powdery mildew. Sulfur can also cause plant injury, especially at warm temperatures. Polysulfides, particularly lime-sulfur, are used both as protectants and eradicants. The mercury fungicides were developed in the early 1900s and have been widely used; the inorganic mercuries primarily act as disinfectants and the organic mercuries as protectants and eradicants. However, their high toxicity to animals has led to their disuse, and mercury fungicides will probably not be available before too long. The carbamate fungicides are all built on a carbamic acid base, primarily as ethylene dithiocarbamates. They vary depending on the metallic element associated with them. Ferbam is the iron salt, zineb and ziram are zinc salts, maneb contains manganese, and nabam is the sodium slat. Thiram is a nonmetallic carbamate fungicide. As a rule, fungicides work primarily as protectants, which means application must be made prior to fungal germination and invasion of the plant tissue begins. This is why it is so important to follow our recommendations about timing of sprays. Most of our spray programs for disease control start in the dormant stage of growth and are aimed at protecting the newly emerging leaf and flower parts. If you are in doubt about the timing of your spray program or the proper material to use for controlling a particular disease, call us for information. We have a number of spray guides for various home orchard plants. In the future I will spend time focusing on ways you can reduce or eliminate disease in your yard by means other than fungicides. LONG-TERM RAINFALL DATA NEEDED For several years how I have been collecting weather data on Whidbey Island to help get a better understanding of the various micro-climates present. Now there is a need to add the concern about ground water to the list as a reason to collect data. I am interested in gathering rainfall data from the past. If you have collected this kind of information in the past, or know someone who has, please contact me. This kind of information can be very useful in trying to understand patterns of climate for our area. TENT CATERPILLAR UPDATE At this point in time I have not had any calls on tent caterpillars, and no one has reported seeing tent development. I would like to point out that those of you pruning at this time of the year can reduce your problems with the caterpillar by closely observing egg cases on limbs and pulling them off when found. This was something I failed to include in my last column on tent caterpillars.

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