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THE FUNDAMENTALS OF FUNGICIDES
by Nanette Londeree, Consulting Rosarian

Fungicides are chemicals used to control the fungi that cause mold, rot and plant diseases. They are pesticides – defined as any chemical that is used by man to control pests, be they insects, plant diseases, fungi, weeds etc. Fungicides may be of natural origin like botanicals, inorganic chemicals like copper or sulfur, or synthetic organic chemicals like Funginex. They are generally considered to be either a protectant, something that stops disease organisms from infecting a plant, or an eradicant – something that can destroy disease organisms or inhibit their development after initial infection.

Fungicides can be further defined as being a contact material – it stays where it is applied or a systemic – one that moves throughout the plant. Many of the newer organic chemical products are considered locally systemic. They do move through the plant, but since roses grow so rapidly, the systemic becomes diluted by the expanding cell tissue and must be reapplied every week or two, depending on the rate of growth.

The majority of commercially available fungicides have some level of toxicity as defined by the U.S. Environmental Protection Agency (EPA). Toxicity is a measure of how poisonous a material is. The commonly used measure of oral and dermal toxicity is LD50 – the lethal dose required to kill 50% of the study population. The lower the LD50, the more poisonous the chemical. In order to provide a rapid identification of the dangers of a chemical there are warnings on all product labels. There is a numeric category and a signal word used to describe the toxicity level. The information below includes a general indication of the probable oral lethal dose for a 150-pound man:

  • I Danger – highly toxic; a few drops
  • II Warning – moderately toxic; a teaspoon to an ounce
  • III Caution – slightly toxic; more than an ounce, less than a pint
  • IV Caution – toxic; over a pint

    There are many products available for you to choose from. Before you go shopping, you need to decide what you are trying to achieve (prevention vs cure) and what type of treatment you want to use, as well as the general level of toxicity. Whatever material you choose to use, always remember to READ THE LABEL; don’t try to second-guess the manufacturer’s instructions. The label provides information on the materials safe use, hazards, EPA registration, dilution levels plants it is registered for use on and the type of diseases it is effective against.

    Antitranspirants (Wilt-Pruf, Cloudcover) - used as a preventive measure – it will not kill any disease. These products form a thin, waxy layer on the surface of the leave

    Surfactants (Safer Fungicide) – used as a protectant; composed of fatty acid salts that work by disrupting the water balance inside ungerminated spores, by blocking certain fungal metabolic functions and increasing the “wettability” of the leaf surface

    Inorganic chemicals (copper, sulfur) – generally used as a preventive measure, applied as a dormant spray in combination with horticultural oil

    Bicarbonate and oil – either sodium bicarbonate (baking powder) or commercially available potassium bicarbonate (Kaligreen, Remedy) – 1 tablespoon per gallon of water mixed with ultrafine horticultural oil. This mixture changes the pH on the plants surface making it less hospitable to the fungus; repeated usage can also raise the pH of the soil

    Neem oil (Rose Defense, Shield-All, Tri-Act) – botanical treatment with low toxicity that is also an insecticide and not harmful to most beneficial insects

    Contact fungicides - Broad spectrum, multi-site surface protectants that do not enter the leaf; when sprayed on a plat, the active ingredient remains on the leaf surface. Their mode of action is to attack or damage more than one site in a fungus.

  • Chlorthalinol (Daconil 2787, Fungi-gard)
  • Mancozeb (Fore)

    Systemic fungicides – single site, mostly locally systemic, enters the leaf and the active ingredient penetrates to the interior of the leaf. They are designed to enter the plant because their single target site of fungal action is unique to the fungus without a similar site existing in the plant.

  • Benomyl (Benlate)
  • Fenarimol (Rubigan)
  • Myclobutanil (Eagle, Immunox)
  • Propiconazole (Banner Max, Tilt)
  • Thiophanates (Clearys 3336, Halts Systemic, Greenlight Systemic)
  • Triforine (Funginex)
  • Triadimefon (Bayleton, Strike, Fung-Away, Intercept)


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    Last Modified: 01/29/2016