Get Down Defoamer is an excellent defoamer that reduces spray tank foam from fertilizer/pesticide/adjuvant sprays but also effectively prevents foam from ever forming when used properly.
To prevent foam before it forms, use rate of 1 ounce per 100 gallons of solution. To control existing foam use rate of 1oz. to 3 oz. per 100 gallons squirted into foam on surface of spray tank. Amount used will vary with tank size, type of agitation, chemicals being used and water hardness.