Soybeans: Powdery Mildew and Downy Mildew

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Incidence
2. Appearance
3. Disease Cycle
4. Management Strategies
5. Updates on Soybeans: Powdery Mildew and Downy Mildew
6. Related links...

Incidence

Both of these diseases occur in all regions of the province. They are most noticeable when conditions are wet or humid. Although powdery mildew and downy mildew occur in most fields, they are considered minor diseases and economically insignificant.

| Top of Page |

Appearance

Powdery mildew appears as a white powdery growth of the mildew fungus on the upper surface of the leaf. The seeds do not become infected. Downy mildew appears as yellow-to-brown spots on the leaves during late July through September. In moist weather, a pale blue-to-grey, downy growth of the mildew fungus appears on the lower leaf surface, directly under these spots. Severely affected leaves may drop prematurely. Whitish growth of the fungus may encrust the seeds, affecting even healthy pods. Planting infected seed may result in diseased seedlings.

Plate 68. Powdery mildew appears as a white powder on the upper surface of the leaf.

Plate 69. Downy mildew appears as yellow-to-brown spots on the leaf with pale blue-to-grey mildew on the lower leaf surface.

| Top of Page |

Disease Cycle

Powdery mildew develops on the leaves, usually in August and September. Outbreaks arise when disease symptoms begin in early July and the environmental conditions remain cool, cloudy and humid through to pod fill. Downy mildew survives in infected leaves and on seed. Air-borne spores blown into Ontario from the U.S. are the most common cause of infection.

Management Strategies

Removal of crop residue and rotation with non-host crops such as corn and wheat will help prevent both diseases. Fungicidal seed treatments will reduce seed-borne downy mildew. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for recommended products. (Order OMAFRA Publication 812)

| Top of Page |

Updates on Soybeans: Powdery Mildew and Downy Mildew

No updates available at this time.

| Top of Page |

Related links...

• Disease
• Soybeans
• Field Crop Protection Guide - Publication 812

| Top of Page |