Horticulture Principles and Practices

In terms of chemistry, fungicides may be classified as organic or inorganic.
Phytotoxicity
The immediate (acute) or
continuous low (chronic)
impact of a chemical on a
plant or its part.
1. Organic fungicides. Organic fungicides are more selective and pose less
environmental danger. The newer types are especially readily biodegradable
and less phytotoxic (damaging to plant tissue). The most widely known class of
organic fungicides is the dithiocarbamates, which include old and still useful
fungicides such as thiram, maneb, zineb, and mancozeb. Thiram is used in apple
and peach orchards, turf, and vegetable gardens. Other classes of organic
fungicides are the substituted aromatics, thiazoles, triazines, and dicarboximides.
2. Inorganic fungicides. The core elements in inorganic fungicides are sulfur, copper,
and mercury. Sulfur is available in one of several formulations: powder, colloidal
sulfur, or wettable powder. When applied, it may kill by direct contact at high
environmental temperatures (21°C [above 70˚F]) by fumigant action. Sulfur is used
in controlling powdery mildew. One of the most popular copper formulations is
Bordeaux mixture, the oldest fungicide (consisting of CuSO 4 and hydrated lime),
which is effective against downy mildew. It also repels insects such as flea beetles
and leaf hoppers. Inorganic copper fungicides are not water soluble.
8.2.3 NEMATICIDES
Nematicides are chemicals designed to penetrate the relatively impermeable cuticle of
nematodes. They are generally applied by professionals by injecting fumigants of
halogenated hydrocarbons under pressure into the soil.
8.2.4 RODENTICIDES
Rodenticides are pesticides designed to kill rodents. Rodents are most effectively controlled
by poisoning. The most widely used class of rodenticides are the coumarins. They
must be ingested repeatedly to kill the pest. Thus, they are safe in case of accidental
ingestion. An example is Warfam, which was developed by the University of Wisconsin.
Coumarins are anticoagulants.
8.2.5 MOLLUSCIDES
Chemicals that are designed to kill mollusks are called molluscides. These chemicals are
usually formulated as baits; an example is methiocarb, which is very effective against
snails and slugs in ornamental plantings. Metaldehydes are one of the oldest and most
successful molluscides.
8.2.6 MITICIDES
Miticides are pesticides designed to kill mites.
8.2.7 AVIACIDES
Aviacides are pesticides designed to kill birds. They are commonly included in grain and
used as bait. Strychnine is an aviacide.
8.3 GROWTH REGULATORS IN PEST CONTROL
Plant growth regulators are used to manipulate plant height, promote rooting, and reduce
fruiting, among other uses. A high concentration of certain plant hormones can reduce
infection by some pathogens. This effect has been observed in tomato with respect to
Fusarium and in potato with respect to Phytophthora. Viral and mycoplasma infections
are known to cause reduced vigor and stunting in plants. However, an application of
246 Chapter 8 Principles and Methods of Disease and Pest Control