Control Plant Diseases
For Better Farming And Better Living Mimeo BP 9-11 March, 1958

COMPATIBILITY OF FUNGICIDES
Control of plant diseases and insect pests with modern weapons of chemical defense has become a complicated and difficult problem. Plant diseases rarely occur as sole predators of any particular crop, but generally work in conjunction with insect pests that may attack the same crop. In view of this and the necessity for maximum efficiency imposed by labor costs and maintenance and operation of the expensive machinery required for application of these chemicals, most agricultural operations require the use of combination sprays in which necessary pesticides are applied in one operation. The recent development of “general purpose mixtures", in which fungicides and insecticides are formulated together in order to simplify the problem of pest control for the home-owner, has imposed additional requirements upon the modern fungicides. The ability of a fungicide to mix with an insecticide or other chemical without impairment of the efficiency of either component, and without injury to the plant to which the mixture is applied, is referred to as “compatibility". When a fungicide cannot be mixed with safety with another pest control chemical, it is said to be “incompatible" with that chemical. Compatibility of a fungicide with insecticides, hormones, antibiotics and other chemicals may be of three types.

Chemical Compatibility

In some cases, organic fungicides, when combined with other chemicals in combination sprays, will react chemically to reduce, destroy or impair the effectiveness of one or both components. In many instances, the exact nature of the chemical reaction is not fully understood. Practical experience has established the "chemical incompatibility” of fungicides with other compounds.
Thus, phenyl mercuries cannot be used in combination with fused bentonite sulfurs, due to absorption of the phenyl mercury by the colloidal bentonite inactivating the fungicidal value of the former. Several fungicides, such as Captan, Karathane, glyodin, carbamates, and the wettable sulfurs, cannot be combined with dormant insecticide oils due to undersirable chemical reactions. Similarly, the insecticidal value of rotenone, pyrrethrum and DDT ls greatly reduced or destroyed when they are combined with lime or alkaline materials. Likewise, fixed nicotine, nicotine oil or nicotine bentonite will lose considerable insecticidal value when used in combination with alkaline materials. Most modern fungicides are carefully investigated as to their compatibility
insecticides before release for. general use, and precautions to their safety with other chemicals are usually defined on the label of the container.

PURDUE UNIVERSITY Agricultural Extension Service, Lafayette, Indiana Department of Botany and Plant Pathologv. Life Science Building