Abstract

Pathogenic fungi cause a substantial reduction in expected crop yields; further losses can result during storage of harvested crops. For those fungi that can seriously affect economically important plants, means have been sought to control these infections by crop rotation and husbandry, genetic manipulation of the plant species, and external treatment of plants using agricultural fungicides.

Agricultural fungicide application accounts for about 20% of all pesticide use. Agricultural fungicides can be applied to the soil to control fungi that are resident there, to the seed or foliage of the plant to be protected, or to harvested produce to prevent storage losses. Those applied to the soil are in many instances nonselective, volatile soil sterilants, such as formaldehyde, which kill all soil organisms, including fungi. Seed and foliar-applied agricultural fungicides, are discussed also. Fungicides are classified herein as being nonsystemic or systemic. The nonsystemic fungicides have a protectant mode of action and must be applied to the surface of a plant generally before infection takes place. These do not translocate from the site of application. The systemic fungicides can penetrate the seed or plant and are then redistributed within to unsprayed parts or subsequent new growth, rendering protection from fungal attack or eradicating a fungus already present. Well over 100 plant pathogens have become resistant to various fungicides under field conditions. Failure of the acyl alanines, benzimidazoles, thiophanates, carboxanilides, dicarboximides, hydroxypyrimidines, some organophosphates, and most of the antibiotics has occurred. In other cases, a moderate decrease in sensitivity without a rapid loss of disease control has been observed as in the case of sterol biosynthesis inhibitors and organophosphates. The most effective approach is to use fungicides having different modes of action in combination, either as mixtures or in alternation, possibly utilizing both specific site and multisite inhibitors. Because of resistance problems great importance is attached to chemistries that inhibit novel fungal enzyme targets.

Newer fungicides, in order to retain cost-effectiveness, need to be very highly active, which also serves to achieve efficacy in the field at low dose rates thus keeping environmental pollution problems as small as possible. Recombinant DNA technologies are expected to play an escalating role in the validation of such biorational targets.