Downy Mildew of Soybean
Published: 01/27/2022
Downy mildew is caused by the oomycete Peronospora manshurica. It is common but seldom causes serious yield loss. Lesions occur on upper leaf surfaces as irregularly shaped, pale green to light yellow spots that enlarge into pale to bright yellow spots. Older lesions turn brown with yellow-green margins. Young leaves are more susceptible, so disease is often found in the upper canopy. On the leaf underside, fuzzy, gray tufts may be seen growing from lesions, particularly when humidity is high or leaves are wet (e.g., early in the morning). Downy mildew-infected pods show no external symptoms, but the inside of the pod and seed may be covered with a dried, whitish fungal mass that appears crusty and contains spores. Infected seed can be smaller, appear dull white and have cracks in the seed coat.
High humidity or extended leaf wetness and moderate temperatures favor disease development. The increased resistance of older leaves and higher temperatures midseason usually stop disease development before extensive damage occurs.
Many sources of resistance are available. Crop rotation and burial of infested crop residue can reduce pathogen levels.
Downy mildew symptoms on soybean leaf.
Image: D. Mueller
Fuzzy growth that can sometimes be observed on leaf underside.
Image: D. Mueller
Downy mildew lesions enlarge into pale to bright yellow spots over time.
Younger leaves are more susceptible to downy mildew.
Characteristic pale green to light yellow spots indicative of downy mildew.
Fuzzy growth characteristic of downy mildew on leaf underside.
Fuzzy growth characteristic of downy mildew on leaf underside.
Fuzzy growth characteristic of downy mildew on leaf underside.
Early downy mildew lesions.
Older downy mildew lesions beginning to turn brown.
Seed showing symptoms of downy mildew.
Characteristic early pale green to light yellow spots indicative of downy mildew.
Gallery Images: S. Markell, C. Grau, and D. Mueller
This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel.
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or a part of an individual's income is derived from any public assistance program. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA's TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination write to USDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410 or call (800) 795-3272 (voice) or (202) 720-6382 (TDD).
USDA is an equal opportunity provider and employer.
©2022 by the Crop Protection Network. All rights reserved.
