Turfgrass dying in patches
Thank you in advance for your help. I have a well-maintained lawn in Tonka Bay, MN that has been showing random patches of die-off in the last few days. The lawn is fertilized appropriately four times a year (avoiding the hot times during the summer), it is irrigated with an automated controller that accounts for soil type and drainage (I irrigate roughly every 3 days, pre-dawn, and for longer durations to encourage root growth). The patches do not correspond to any low areas, but do roughly correlate to areas that see compaction from snow load in the winter and from recent construction activity, although these areas grew very well earlier this year. I have treated for grubs/insects this year already due to a large presence of scarab beetles. Any help identifying what is causing the patches and how to remedy them would be greatly appreciated!
Hennepin County Minnesota
Thank you for the question and good description of your practices. There are many possible reasons for brown areas in a lawn. Insect, disease, and lawn care are a few possibilities. You obviously are trying very hard to do all the right things because you value a well manicured, green lawn. Your question contains clues that might point to reasons for the brown areas.
1. Fertilizing 4 times per year might be too much. The University of Minnesota suggests fertilizing once in the fall on their lawn care calendar. Here it is: https://extension.umn.edu/planting-and-growing-guides/lawn-care-calendar Too much fertilizer applied early in the growing season can cause in increase in fungal grass diseases. Turf grass diseases are particularly difficult to identify because the organisms that cause the problems are usually microscopic. You need to get inches away from grass blades and use a magnifying lens to find the problem and even then, diagnoses can be elusive. Any treatment is based on accurate diagnosis. Check your grass blades carefully at the intersection of healthy and brown grass because that's where you will get the most information. Completely dead grass blades don't provide many clues! This Penn State publication might be helpful to you should you find signs of disease. Read it before scouting your turf so you know what to look for: https://extension.psu.edu/diagnosing-turfgrass-problems
2. Irrigation schedule and areas of prior construction. Some areas of construction have only thin layers of soil over construction debris. Thin soils don't retain moisture well and irrigating every 3 days, even deeply, might not be enough for these areas. Treatment would consist of building up the soil and incorporation plenty of compost to help retain water. If there is no construction debris in the area, the soil may have been severely compacted by equipment or piles of materials. This would reduce oxygen to grass roots and prevent water from soaking in well. Core aeration would be helpful in this instance.
3. Treating for grubs. Grubs can damage and kill lawn grass if present in high enough numbers. The University of Minnesota recommends treating for grubs such as Japanese beetles only if grub populations reach a certain threshold. Grub populations between 7 and 15 per square foot can cause significant damage to non-irrigated turf. Irrigated turf can withstand a higher grub count because the increase in water compensates for the roots chewed off by the grub. Grub treatment timing is important and should be applied mid-July to early September. To confirm that white grubs are the reason for yellowing or dying patches of turfgrass, dig up some small sections, about 1 foot square on the edge of dying grass. If grass readily pulls up, with few roots, and grubs are apparent in the soil (7 to 15 grubs or more per square foot), then treatment may be justified. https://extension.umn.edu/yard-and-garden-insects/japanese-beetles#life-cycle-1057360
Consider sending in a soil test to find out exactly what is needed, if anything, to improve the fertility of your soil. Here's how to do it: http://soiltest.cfans.umn.edu/
If a soil test comes back showing nothing is needed, you can't find evidence of fungal disease, and you feel that all the fertilizer and grub control chemicals are not causal factors, consider sending a sample to the U of M plant disease clinic: https://pdc.umn.edu/
Good luck and thank you for contacting Extension.
Anita - I just realized I never replied to your answer. First off THANK YOU for the very detailed response. Your information helped me address the turf condition last year. Once the heat subsided I saw a slow return to green in the affected areas. We didn’t see active grubs when examining the turf but had significant Japanese beetle activity last year. We treated with granule insecticide and baited for adults away from the lawn which we hope will reduce that population.
The most notable impact came throigh aeration in the early fall. We saw an almost immediate improvement in the turf quality and strong growth and color until dormancy. I will replay back next spring with more info on green up to help others who might be experiencing the same problem. Thanks again!