Downy mildew is caused by a funguslike oomycete (water mold). Peronospora sparsa also infects and damages pineapple guava and rose.
Identification
Downy mildew on caneberries first causes a pale green to yellow discoloration on the upper surface of leaves. The lesions later become brown, purple, or red. Mature lesions are commonly angular because their spread is limited by leaf veins.
When conditions are humid or wet, gray to whitish spore masses develop on the underside of infected leaves, but these can be sparse and difficult to see. Whitish mycelia mats of the pathogen also develop on the underside of leaves.
Primocanes (first-year stems) systemically infected with the pathogen commonly have reduced growth (stunting). Red streaks develop along the side of the cane that faces the sun, and terminal leaves on the canes turn reddish.
Fruit with downy mildew (called dry berry) are dull colored with dry, shrunken drupelets. Infection of young green fruit causes them to become hard, red, and shriveled. Fruit infected later in the season also dry and shrivel and sometimes crack and split into two parts. Downy mildew infected pedicels (flower and fruit stems) are dry and red.
Life Cycle
The pathogen overwinters as mycelia inside canes, root crowns, and roots. As new shoots emerge in the spring, the pathogen moves with the growing point and infests new leaves and stems.
Weed growth and dense canopies create humid conditions that favor the development of the downy mildew on caneberry suckers. The disease becomes common during the growing season when weather is wet and temperatures are 65 to 75°F. Spores develop on dense foliage near the cane or at the base of the plant. Spores are produced during cool, wet nights and are spread by splashing water and wind. Disease symptoms develop within about 10 days after infection.
Damage
Fruit yield amount and quality are reduced when caneberries develop downy mildew. The disease is especially damaging to blackberry and boysenberry. However, blackberry varieties with Native American names such as Apache, Arapahoe, Chester, and Navaho are resistant to downy mildew.
Solutions
Use pathogen-free planting stock if available. Plant varieties resistant to downy mildew. Avoid planting caneberries at sites with a history of this disease. Also avoid planting near roses or wild blackberries because the pathogen can spread from these alternative hosts.
Anything that can be done to improve air circulation around plants and hasten drying will reduce the abundance of downy mildew. For example, growing berries beneath plastic-covered, high-hoop tunnels virtually eliminates the disease because of a drying effect. Avoid overhead irrigation and instead use drip or furrow irrigation. Direct any hand watering to the soil around the root crown.
Once new caneberries become established, remove suckers to reduce humidity at the base of the plant. Control weeds because these impede air movement and increase humidity. Remove affected plants and destroy old-fruited canes after harvest. Where downy mildew has been an unacceptable problem and before foggy or rainy weather is expected, thoroughly spray flowers, foliage, and fruit with a preventive fungicide such as Bordeaux mixture, another copper fungicide, fosetyl-al, mancozeb, or phosphorous acid.
References
Adapted from Downy Mildew in Blackberries, University of California Agriculture and Natural Resources (UC ANR) and Pest Management Guidelines: Caneberries, University of California Statewide Integrated Pest Management Program (UC IPM).
