If you live in a cooler climate, you can probably grow currant shrubs (Ribes spp.) for their edible fruit as well as their attractive features. Growing in U.S. Department of Agriculture plant hardiness zones 3 through 5 or 7, these fast growers bear pale green to dark blue-green leaves and feature green or pink flowers that give way to white, black, pink or red berries. Currants are susceptible to several plant diseases, but powdery mildew is the most common problem.
Powdery mildew is a fungal disease that produces a visible, white to pale gray, powdery growth on infected leaves, stems and branch tips. If left untreated, the mildew fungi (Podosphaera spp.) can spread and infect the buds, flowers and berries. Infected currant shrubs often bear new growth that looks misshapen or stunted. The powdery coating on the leaves interrupts photosynthesis, which often makes the foliage turn brown or yellow and drop from the plant. After the leaves have fallen off, the exposed berries may get scorched by the sun, while infected fruit often cracks and rots.
The powdery mildew fungi overwinter in black protective structures, called chasmothecia, which release fungal spores the following spring in moist, warm weather. The spores quickly spread through insect activity, wind gusts and splashing water, and only need about 48 hours to germinate and infect new plant tissue. The pathogens thrive when the temperatures range from 60 to 80 degrees Fahrenheit and the humidity levels fall between 50 to 80 percent. Although the spores need standing water to germinate and infect, they don't need any moisture to become established and grow after infecting plant tissue.
Chemical treatments for mildew fall into one of two categories: preventative or eradicant. Preventatives help avoid powdery mildew infections, while the eradicants get rid of infections that have already started. Make a simple preventative fungicide by mixing 5 tablespoons of horticultural oil and 8 teaspoons of baking soda into 2 gallons of water. Spray the foliage thoroughly. Once your currant shrub shows powdery mildew symptoms, thoroughly spraying with a 1 to 2 percent horticultural oil spray -- 2 1/2 to 5 tablespoons of oil mixed in 1 gallon of water -- can help kill the infection and prevent further damage. Repeat applications every seven to 10 days until the weather conditions no longer favor powdery mildew growth.
Cultural Control Methods
Following proper cultural practices helps prevent and control mildew on currants. Plant your shrub in an area that receives full sun in the morning and partial shade in the afternoon. Currants tolerate various soil conditions, but prefer rich, well-draining, loamy soils. Although established shrubs are considered drought tolerant, they still appreciate some supplemental summer watering. Using an overhead sprinkler can help wash fungal spores off the foliage. Water until about the top 7 inches of soil are moist and allow the soil to dry to the touch before watering again. Reduce the relative humidity levels around your plant by selectively pruning out overcrowded interiors. This helps increase air circulation and sunlight penetration throughout the entire plant.