Leaf spot and crabgrass

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

High temperatures, lack of rain in many places and high humidity, have put the hurt on turfgrass in many areas across the state. The early season sightings of diseases on lawns, such as red thread, have now been replaced by sightings of leaf spot. Leaf spot is generally active during warm summer weather. The symptoms are water-soaked spots on leaf blades that turn purple/brown/black. Turf infested with leaf spot will appear yellowish to reddish-brown, and a general thinning of the turf with no distinct pattern may occur if conditions are ideal. The best management strategy to alleviate damage is to irrigate lightly and frequently, preferably around mid-day (noon) and apply a light fertilizer application (0.5 lb. N/1000 ft.2 or less). Avoid irrigating at night because it results in the leaf blades remaining moist throughout the night and, therefore, more susceptible to disease infection. Another cultural management option is to raise the mowing height to reduce turfgrass stress. Fungicide applications to home lawns are generally not recommended, as they can be expensive and often give unpredictable results.

One grass that is currently thriving in the summer heat is crabgrass. In most areas, crabgrass is really cutting loose and control with Dimension, active ingrediant dithiopyr, which has early postemergence activity is probably past. Probably the most common postemergence crabgrass killer in the homeowner market contains the active ingredient MSMA, usually sold as a product that has the phrase “crabgrass killer” somewhere in the title. If you are going to use products that contain MSMA, beware that at high temperatures (greater than 80º F) these products might burn the turf a little, so you might need to decide how important it is for you to eliminate the crabgrass from the turf. Remember that sometimes green is better than brown even if it is a weed! Another active ingredient that is now available in homeowner products and is very effective for postemergence control of crabgrass is quinclorac. Please remember that when using any pesticide to read and follow all label directions.

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