This rule of thumb is inaccurate, costly, and hazardous to crop health. The physical properties of wettable powders from various sources can differ considerably. For example:
1 ounce of Brand X Captan 80WP = 2.4 tablespoons
1 ounce of Brand Y Captan 80W = 5.2 tablespoons
Using weight, 1 lb/100 gives a concentration equal to 0.16 oz per gallon
1 tablespoon of Brand X Captan 80WP would make a 0.42 oz per gallon mix
1 tablespoon of Brand Y Captan 80W would make a 0.19 oz per gallon mix
In other words, 1 tablespoon per gal of Brand X Captan 80WP would be equal to more than 2 lb/100 gal and 1 tablespoon Brand Y Captan 80W would be slightly more than 1 lb/100 gal. Using volume to estimate the weight of Captan 80WP could result in crop damage.
The lesson is clear, weigh it before you spray it. Considering the cost of pesticides, accuracy can save money. Considering the damage that can be done by pesticide overdoses, accuracy can save crops.
Purchase a weighing scale. Even a good letter scale will work well.
- Powell, C. C. 1981. Small batch WP fungicide guide and sample problems. Plant Pathology Notes Ohio State University Cooperative Extension Service. Department of Plant Pathology Newsletter. July 27, 1981.
- Simone, G. W. Extension Plant Pathology Report No. 31. Department of Plant Pathology, University of Florida.