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Beginner

Scientific Name: Sphaerotheca macularis

Identification

• Mainly affects red, purple and black raspberries.
• Affected plants exhibit a white, powdery growth on the underside of leaves and yellowing on the upper surface.
• Initially, the upper surface of the leaf will exhibit light green blotches that have a water-soaked appearance.
• Leaves may be dwarfed and mottled.
• Leaves may also have a twisted appearance as the leaf margins curl upward.
• Shoots of infected plants will appear long and spindly.
• Fruit developing from infected plants will become covered with mycelial growth.

Often Confused With
Raspberry Mosaic Virus

Period of Activity
Powdery mildew is favoured by warm, humid weather, but does not require free moisture or rainfall.  Raspberries grown in tunnel culture are especially prone to the disease because of the warm, humid conditions and protection from direct rainfall. The disease is most serious when air circulation is poor.

Scouting Notes
Search the underside of leaves for white, powdery growth. Pay particular attention to plants with abnormally small or mottled leaves. Scout for powdery mildew during warm, dry summers and on plants grown in tunnels.

Thresholds
None established.

 

Advanced

Scientific Name: Sphaerotheca macularis

Identification
Powdery mildew is caused by the fungal pathogen Sphaerotheca macularis and mainly affects red, purple and black raspberries. The fungus also attacks strawberry plants; however, cross-inoculations do not occur.

Affected plants exhibit a white, powdery growth on the underside of leaves and yellowing on the upper surface. In the initial stages, light green blotches with a water-soaked appearance are visible on the upper leaf surface.   The shoots of infected plants appear long and spindly. If the fruit buds become infected, the plants is less productive. Fruit developing from infected plants becomes covered with white powdery mycelial growth which make it unmarketable. The intensity of the disease varies from year to year.

Often Confused With

Raspberry Mosaic Virus
Symptoms of raspberry mosaic virus are very similar to those of powdery mildew. Leaves of infected plants range in colour from light green to dark green or yellow-green mottling. The distinct white, powdery growth typical of powdery mildew distinguishes between the two diseases.

Biology
The fungus overwinters as mycelium in dormant buds on shoot tips or as dark, globular cleistothecia on leaf surfaces in the fall. After a dormancy period, ascopspores in the cleistothecia are discharged and infect new growth in spring.  Infected leaves become covered with fungal mycelium and conidiophores give rise to conidia on the upper surface of these leaves. These are discharged when temperatures reach between 17°C and 22°C.  These airborne conidia are spread by wind and serve as the secondary inoculum and give rise to repeated infection cycles during the remainder of the growing season.

Period of Activity
Powdery mildew is favoured by warm, humid weather, but does not require free moisture or rainfall.  Raspberries grown in tunnel culture are especially prone to the disease because of the warm, humid conditions and protection from direct rainfall. The disease is most serious when air circulation is poor.

Scouting Notes
Search the underside of leaves for white, powdery growth. Pay particular attention to plants with abnormally small or mottled leaves. Scout for powdery mildew during warm, dry summers and on plants grown in tunnels.

Thresholds

None established.

Management Notes

• Improve air circulation by
• thinning excess canes in the row,
• keeping rows and between rows weed free,
• maintaining a narrow row width by repeatedly mowing small primocanes that have grown into the row middles. Rows should be spaced about 2.5 m apart and plant widths should be maintained between 30 and 45 cm.
• Remove and destroy infected plant tissue.
• When powdery mildew becomes an issue during pre-bloom and until blossoms open, spray with a registered fungicide when mildew is first observed. Repeat in 7 to 10 days. Reapply the fungicide after harvest. See the Raspberry Calendar in OMAFRA Publication 360: Fruit Crop Protection Guide (Chapter 3 Berry Crops).