Yard and Garden: Battling Common Summer Problems

Summer is a great time to work in the garden or yard to make it look as beautiful as possible. However, problems can crop up and limit a yard’s overall potential. Here are some tips from Iowa State University Extension and Outreach on ways to avoid those issues or correct them when they occur, with help from ISU Extension horticulturists. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

How can I control powdery mildew on my garden phlox?

Powdery mildew is a common disease of garden phlox (Phlox paniculata). The fungal disease produces a grayish white coating on the leaves. Infected leaves eventually turn yellow and then brown. Initial symptoms appear on the lower leaves with the disease progressing upward.

Powdery mildew is most commonly found on plants growing in shady areas and in crowded plantings with poor air circulation. Optimal conditions for powdery mildew are cool nights followed by warm days.

Cultural practices can reduce the severity of powdery mildew on garden phlox. The amount of disease inoculum can be reduced by cutting off and removing diseased plant debris in fall. Plants growing in shady locations should be moved to a sunny site. In overcrowded plantings, improve air circulation by digging and dividing perennials.  

While cultural practices are helpful, fungicides may be necessary to control powdery mildew on garden phlox. To be effective, fungicides should be applied at the first sign of the disease and repeated on a regular basis.

The best way for home gardeners to avoid powdery mildew on garden phlox is to select and plant mildew resistant cultivars.  ‘Shortwood’ (rosy pink flowers), ‘David’ (white flowers), ‘Katherine’ (lavender blossoms), and ‘Robert Poore’ (reddish purple flowers) possess good resistance to powdery mildew. (The resistance or susceptibility of garden phlox cultivars to powdery mildew varies within the United States. A cultivar that possesses good resistance to powdery mildew in the Midwest may be susceptible to powdery mildew in other regions of the country.)

How do I control peony leaf blotch?

Peony leaf blotch is caused by the fungus Cladosporium paeoniae. The disease is also known as red spot or measles. Typical symptoms include glossy purple to brown spots or blotches on the upper surfaces of the leaves. The disease may cause slight distortion of the leaves as they continue growth. Leaf symptoms are sometimes most apparent on the edges of older leaves. On stems, symptoms appear as long, reddish brown streaks.

Peony leaf blotch is best managed through sanitation. The fungus survives the winter in infected plant debris.  Diseased plant material should be removed in fall or early spring (before new shoots emerge).  Cut off the stems at ground level. Remove the plant debris from the area and destroy it.  Proper spacing and watering can help to minimize the severity of the disease. Space peonies three to four feet apart.  When watering is necessary, avoid wetting the peony foliage. Fungicides can be used as a supplement to sanitation and good cultural practices.

How can I control black spot on my roses?

Black spot is a common fungal disease of roses. Black spot is caused by the fungus Diplocarpon rosae.  Symptoms of black spot are circular black spots on the leaves. Infected leaves turn yellow and drop prematurely.  Initially, symptoms develop on the lower leaves and gradually move upward.  By late summer, severely infected plants may be nearly defoliated.

The black spot fungus overwinters on fallen leaves and infected canes. Spores are splashed onto newly emerging foliage in spring.  Black spot development is favored by warm, wet weather. 

Careful rose selection, cultural practices, and fungicide treatments can be used to control black spot on roses. Rose cultivars differ widely in their susceptibility to black spot.

When purchasing roses, select rose cultivars that are resistant to black spot. When selecting a planting site, choose an area that receives six or more hours of direct sun each day and provides good air movement. Sunny locations and good air movement promote drying of rose foliage and discourage black spot infections. Reduce the amount of overwintering fungi by carefully cleaning up the leaf debris in fall.  When watering roses, apply water directly to the ground around the plants. Do not wet the foliage. Fungicide applications must begin at the first sign of disease symptoms.