Cereals: Powdery Mildew

Excerpt from Agronomy Guide for Field Crops (Chapter 6)

Order OMAFRA Publication 811: Agronomy Guide for Field Crop

Table of Contents

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

Incidence

Powdery mildew is a common plant disease that can cause damage when present in wheat and barley fields. Wheat cultivars will vary in their susceptibility to the disease. The yield impact of powdery mildew infections is hard to predict. The disease robs the plant of nutrients and reduces the photosynthetic ability of the leaf. Yield losses are generally minimal from early infections unless the weather remains cool and humid. Mildew infections that attack the flag leaf and the second leaf are more serious. The health of the top two leaves determines the kernel size, test weight and yield. Losses due to powdery mildew have been stated anywhere from 2%-30% of total yield. Very rarely in Ontario have losses been greater than 10%-15%.

| Top of Page |

Appearance

The characteristic symptom of the disease is the production of a fluffy white-to-grey fungal growth that often begins on the lower leaves.

Plate 117. Powdery mildew produces a white-to-grey fungal growth on the lower leaves and moves up the plant.

Infection can move rapidly up the plant on leaves, sheaths, stems and heads under favourable conditions. Leaves develop elongated yellow streaks or areas that may turn brown and die prematurely. Severely diseased plants may lodge or result in poor grain fill. Older, light-grey areas of fungal growth often have small black spots. The white-to-light grey fungal growth is most noticeable in the early morning while the plants are still wet. The infection is superficial, and the fungal growth can be easily removed with your fingers or a knife.

| Top of Page |

Disease Cycle

The fungus survives on crop residues, such as straw or stubble, fall-planted winter wheat seedlings, volunteer cereals and wheat. Spores that are released are primarily spread by the wind. The spores require near 100% relative humidity and temperatures between 15°C and 21°C (59°F and 70°F). A dense stand and vigorously growing crop can lead to poor leaf-drying conditions which are favourable conditions for powdery mildew. The disease is very susceptible to weather conditions that promote drying of the crop environment such as hot, dry, sunny weather. Powdery mildew also thrives in fields where high rates of nitrogen have been used. Nitrogen not only increases tiller formation, causing dense stands, but also increases the susceptibility of the crop. Watch for mildew in fields that have had more than 78 kg N/ha (70 lb N/ac). Powdery mildew growth stops when temperatures are above 25°C (77°F).

Management Strategies

In most cases, powdery mildew has little impact on rye or oats since these crops are very resistant to the disease. In areas prone to severe mildew, use resistant (tolerant) winter wheat varieties. Removal of crop residue through tillage in conjunction with a crop rotation that limits wheat or other susceptible cereals from being planted in the field for a minimum of 2 years may lower disease risk. Foliar fungicide applications are necessary when disease levels will result in yield losses. Thresholds for fungicide applications differ depending on the age of the crop. Early-season powdery mildew control is warranted when 5%-10% of the lower leaves are infected, which may limit later infection. Later in the season, powdery mildew symptoms on the flag leaf (1% of leaf) and the second leaf (3%-5% of the leaf) require immediate attention, especially if prolonged wet, humid weather is forecast. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for further information on fungicide products. (Order OMAFRA Publication 812)

| Top of Page |

Updates on Cereals: Powdery Mildew

• Field Crop Protection Guide - Publication 812
  • Cereals: Wheat: Powdery Mildew
  • Cereals: Rye: Powdery Mildew

Related links...

• Diseases
• Cereals

| Top of Page |