Plant Pathology Infobytes
November 19, 1996
Organically Approved Fungicides for Home Gardeners
Interested in giving organic disease control a try in your vegetable garden next season? Many gardeners question the effectiveness of this approach to handling some of the blights, leaf spots and other diseases which annually cause vegetable production problems.
Organic disease control really does work and provides satisfactory protection from many vegetable diseases which commonly occur in Mississippi each season. This approach is based on sanitation, cultural, physical, and biological disease management procedures, as well as application of organically approved fungicides.
Alternatives to synthetic (non-organic) fungicides such as maneb, mancozeb, benomyl, and chlorothalonil for control of vegetable diseases include such materials as sulfur and copper containing fungicides.
Spraying or dusting with sulfur is an old remedy that still works on rusts, powdery mildews, and leaf spots on tomato, and other vegetable crops. Mix finely ground sulfur with water at the rate of 3 ounces per 3 gallons of water, or dust in an even coat over plant surfaces.
Be aware that sulfur is easily removed from plants during rain and frequent application may be needed to provide an adequate level of disease protection. To avoid plant injury, do not use when temperature is 90 degrees or above.
One of the most popular organically approved fungicides is Bordeaux Mixture. This fungicide can be "homemade" and is effective for a number of common fungus and bacterial blights and leaf spots of vegetable crops. It's also effective as a dormant season spray to help control black rot of grapes as well as other fruit diseases.
The mixture can be prepared by combining hydrated lime or any type of finely ground lime with powdered copper sulfate (sometimes referred to as "bluestone"). Both materials should be available at farm and garden supply stores, but some checking around may be necessary.
While Bordeaux Mixture can be prepared in several strengths, the most popular and effective for general home garden and orchard use is the 4-4-50 formulation. The numbers refer to the ratio of copper sulfate and lime in 50 gallons of water. For example, 4 pounds each of these materials would be combined in 50 gallons of water. Since this amount of spray material is considerably more than would be needed by most gardeners, the ratio can be whittled down to a more useable amount.
To prepare a gallon amount of a 4-4-50 Bordeaux Mixture spray, measure out 6 1/2 teaspoons of copper sulfate and 3 tablespoons of hydrated lime. The lime should be mixed with a pint of water to make a "milk of lime" suspension. The copper sulfate should also be dissolved in a pint of water. The latter process may take several minutes.
Before mixing the lime and copper components of the mixture, one additional step (but a very important one!) must be carried out. Each container of these materials should be strained through a cheesecloth filter. If cheesecloth isn't available, use cloth of a similar loose weave. The filtering is necessary to remove small pieces of lime or copper sulfate that won't dissolve; otherwise, you'll find these tiny pieces in the end of your garden sprayer tip, clogging up the works.
To make the filter, simply place the cheesecloth loosely over the top of another container and fix securely in place with a string or rubber band.
The filtered copper sulfate solution is added to a one-gallon container, followed by addition of the filtered lime solution. Enough water (about 3 quarts) is then added to the container to bring the total volume up to one-gallon.
You now have one gallon of Bordeaux Mixture ready for use. For two gallons, simply double the proportion of materials and prepare as before.
For best results, use your Bordeaux Mixture the same day of preparation and keep the sprayer agitated. Insecticides should not be added with the Bordeaux Mixture without first checking the insecticide label for possible compatibility problems.
Bordeaux mixture may used to control early and late blight on tomatoes and Irish potatoes. On leafy greens, this fungicide controls most of the leaf spots that commonly occur on mustard and turnips.
For addtional information on organic pest control measures, please feel free to check with us at your county Extension office.
Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.