The past two years Delaware has had a greater incidence than usual of soybean downy mildew. In some fields incidence has been near 30 percent.
A few people have asked about the seriousness of this disease and the potential impacts it might have on the soybean crop.
Downy mildew is an extremely common disease of soybean that occurs wherever soybean is grown. Downy Mildew is caused by an oomycete, Peronospora manshurica. Oomycetes are not Fungi, but are more closely related to brown algae in many aspects.
The commonality with the oomycete pathogens is that they are water-loving, so they become more problematic under consistently humid or wet conditions. It overwinters in soybean residue as spores and can also enter fields through infested seedlots. During periods of cool, humid weather the organism produces sporangia, which can be dispersed long distances to soybean leaves. Younger leaves tend to be more susceptible than older leaves. Under the right environmental conditions sporangia germinate and infect the foliage and produce pale green /yellow spots. Often the underside of the leaf will be fuzzy and grey when viewed from below. As the spots age the spots turn brown and may resemble symptoms of other foliar diseases.
Pods can also be affected without presenting any obvious outward symptoms. Infection can result in pods or seed becoming encrusted in a white mass of the pathogen. Crusted seeds often appear white, may crack, and can be lighter than unaffected seeds. If favorable weather persists, the pathogen can continue to produce spores and spread. Moderate, humid temperatures favor downy mildew, and temperatures above 86F stop disease progress and sporulation. The downy mildew pathogen is highly diverse and over 33 races of the pathogen are known.
Impact and Management
Downy mildew seldom causes yield loss. In rare cases reports of up to 8% yield loss have been reported in some states. Variety plays a big role in susceptibility to downy mildew. Therefore, you should take note of highly symptomatic varieties and consider switching varieties if downy mildew is a concern to you.
Unfortunately seed companies seldom provide downy mildew resistance ratings: this is a further indication of the rarity in which this disease significantly impacts the crop. Rotation away from soybean for at least one year will help reduce primary inoculum and tillage, where practical, will further bury and destroy inoculum. Fungicide applications specifically targeted for downy mildew are not recommended. Furthermore, because downy mildew seldom appears at high levels, extension specialists do not have efficacy data for this pathogen.