Preventative measures for downy mildew are only important if the quality of seed is imperitive. Proper tillage of infected crop residue can effectively control populations of downy mildew spores. Rotating in nonhost crops can prove to be an effective strategy as well.
Fungicide seed treatment has been known to be effective in combatting downy mildew. Note that commonly used fungicides for soybean are unable to control downy mildew. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
There are resistant varieties of soybeans, but it is important to consider resistant varieties for more urgent diseases and pests.