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Powdery mildew on phlox

Note to media editors:

Got gardening questions? Contact the Hortline at (515) 294-3108 (Monday - Friday; 10 a.m. -12 noon and 1 - 4:30 p.m.) or send an e-mail to hortline@iastate.edu. For more gardening information visit us at Yard and Garden Online at www.yardandgarden.extension.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 the 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 phlox. To be effective, fungicides should be applied at the first sign of the disease and repeated on a regular basis. 


Selection of powdery mildew resistant varieties is another option. ‘David’ (white flowers), ‘Katherine’ (lavender-blue blossoms), and ‘Robert Poore’ (reddish purple flowers) possess good resistance to powdery mildew. 


When should I stop harvesting my asparagus? 

Stop harvesting well established asparagus plantings in early June in southern Iowa and mid-June in northern areas of the state. If harvested over a longer period, the asparagus plants may be weakened and less productive the following year. 


What is the correct mowing height for a lawn in summer? 

Kentucky bluegrass and other cool-season grasses thrive in the cool weather of spring and fall.  Hot, dry conditions in summer are stressful for cool-season grasses. Kentucky bluegrass lawns should be mowed at a height of 3 to 3 1/2 inches during the summer months. Bluegrass lawns should be mowed at a height of 2 1/2 to 3 inches in spring and fall. The additional leaf area during summer shades and cools the crowns of the turfgrass plants. The higher mowing height also provides more food-producing foliage and promotes deeper root growth. 


When mowing the lawn, never remove more than one-third of the total leaf area at any one time.  Removing more than one-third of the leaf area severely injures the turfgrass plants and reduces their ability to withstand additional environmental stresses. 



Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

There are two high-resolution photos available for use with this week's column:

 Asparagus5-24-06,  1 MB