Japanese beetle damage on rose foliage. Photo credit: Lois Miklas
In any given gardening season, I may need to combat powdery mildew on phlox, rust on hollyhocks, aphids on milkweed, Japanese beetles on my roses and caterpillars chewing cabbage leaves. Daunting though the list may seem, you can use simple strategies to solve each problem.
Powdery mildew and rust
A fungal disease called powdery mildew produces white spots on the leaves of garden phlox and vegetables such as squash. This destructive disease appears as disfiguring, white, powdery patches on the foliage. The blotches, usually circular in shape, grow together into a continuous mat of mildew. By late summer the lower leaves may turn yellow and drop prematurely. The disease moves upwards until many of the top leaves are covered with fungus. Flower buds may have abnormal development or fail to open. The fungus removes nutrients from the plant during infection and may result in the plant losing vigor and declining.
Hollyhock rust, also a fungus, initially appears as light yellow-orange spots on the upper surface of leaves. These develop into brown pustules on the underside. Pustules may also develop on the upper side of the leaves (a different spore stage) and on stems and green flower parts.
Warm temperatures, high humidity and poor air circulation between plants create conditions favorable for fungal disease development. Spores spread the fungus. The wind blows the spores to other parts of the plant or to other plants. During winter the fungus can survive on plant parts and debris.
What can you do?
- Plant in a sunny spot, as susceptible plants need at least six hours of sunlight each day. Shading will increase the incidence of disease.
- Water regularly, but soak the soil instead of the foliage.
- In the fall, remove and destroy all infected plant debris to reduce the likelihood of disease the next season.
- Plant next season’s vegetables in a different location.
- You can use a fungicide such as wettable sulfur to control powdery mildew, beginning applications when the first white spots appear. To control rust begin applying wettable sulfur several weeks before rust normally appears. When using fungicide, spray the upper and lower surfaces of the leaves, covering them thoroughly. Repeat treatments every seven to fourteen days. Follow all directions on the label. Contact your county Penn State Extension office for more information.
- For the best long-term solution, choose cultivars resistant to fungal disease. Plant breeders have developed several resistant varieties. Resistant varieties of phlox, for example, include ‘David,’ ‘Starfire,’ ‘Bright Eyes’, ‘Eva Callum’ and ‘Franz Schubert’. This does not mean the plant will not become infected, but the disease will be less severe or spread more slowly.
Aphids are soft, pear-shaped, and very tiny. They reproduce prolifically and both adults and nymphs suck plant sap. Remove them by spraying with water from the hose, or with horticultural soap. The ladybug is the aphid’s natural predator.
Pick off the shiny beetles and drown them in a container of soapy water. Do not invest in beetle bag traps, as they tend to attract more beetles to the garden. The Japanese beetle larvae, white grubs that feed on organic matter and roots of grasses in the soil, can cause a great deal of damage to your lawn. Control them with bacterial milky spore. Apply milky spore to the lawn every year for three years as the spore count must build up to be very effective.
Caterpillars, like weevils, beetles and leaf miners, are insects that chew. You can best control them when they are young, so be vigilant. Control options include beneficial insects and mechanical removal, or you can use Bt (bacillus thuringiensis). Remember, however, that butterfly larvae are caterpillars. If you want butterflies, be careful how you use pesticides, including Bt. Be tolerant of small pest populations. Your ‘pests’ are providing food for beneficial insects. Next time you see a munched leaf take pride in providing habitat for another creature.