Question
My lawn develops a white powder in August that lasts into fall.  It seems similar to the powdery mildew that I also get on many of my flowers at this time of year.  What is it?  Can I do anything about it?  I live in Rockford, Illinois, zip 61114, nearest large city is Chicago just southeast of Rockford.  

Answer
It is powdery mildew (just like the roses, zinnias, and begonias). It is usually harmless to grass and the susceptibility vary with species and cultivars.

To reduce chance of occurance, water once per week really well (water deeply, but infrequently) if rain has been lacking. Draught stressed grass often develop powdery mildew. Start watering early summer and water through early fall unless a good weekly rain storm has occurred.

Fertilize your lawn in late spring, early fall and late fall. Powdery mildew is seldom a problem on a correctly, actively fertilized lawn. E.g. try to fertilize memorial day, labor day and halloween. Avoid fertilizing too early in spring or during hot summer months. Avoid using too much herbicide and insecticide on the lawn.

The last fertilizer application of the year ("halloween") should be done AFTER the top growth stops, but before the roots go dormant. The timing is not all that difficult, but critical. If you mow weekly in late fall, wait until you have not had to mow two weeks in a row and then fertilize. Usually between Halloween and Thanksgiving. The goal of this late season fertilizer application is to build up the storage reserves in roots, rhizomes and crowns of the roots for next spring.

Avoid fertilizing during hot summer months. Always apply fertilizer to a dry lawn and water in immediatly with plenty of water (or apply when rain is expected).

Finally, mow your grass high and mow frequently. Target a height (after cutting) of 2.5-3.5" tall and try to mow before it reaches 5"-6" tall. E.g. once or twice per week during the very fast growing periods of late spring and early fall, and at least once per week or every other week during the hot summer months.

Some flowers develop powdery mildew easily and can be very hard to control. Plox for example are almost sure to develop powdery mildew. Plant in full sun with plenty of wind circulation. Water faithfully and do not stress the plants. Zinnias likewise. They should have adequate spacing (no crowding), full sun and plenty of water on regular basis. Even so, the above two types of flowers are very susceptible to powdery mildew. Begonias can develop powdery mildew especially if draught stressed. plant in humus rich soil and water on consistent basis. Avoid crowding and prune nearby shurbs to allow proper air circulation. Roses are also very susceptible and for some cultivars you may have to accept some mildew or start a spraying program. If you spray your roses, spray begonias, pholx, and zinnas also.

Try Triadimefon (Bayleton, Strike) or Triforine (Funginex). Start spraying when first sign of mildew is detected (or before on preventiative basis). Continue consistently (every 7-14 days) especially during periods of cool, damp weather.