Yellowing of Kentucky Bluegrass

Yellowing of Kentucky bluegrass

Similar to previous summers with adequate rainfall, Kentucky bluegrass has turned an off-color yellow in the last few weeks (Figs. 1 and 2).

Figure 1. Irrigated Kentucky bluegrass lawn showing severe yellowing on Aug. 6, 2011. Darker green areas are patches of tall fescue or different cultivars of Kentucky bluegrass.

Figure 2. Irrigated Kentucky bluegrass baseball field showing severe yellowing on Aug. 6, 2011. Darker green areas are patches of perennial ryegrass or different cultivars of Kentucky bluegrass. Rectangle of obvious yellow is a sod replaced in the last year.

We are still unsure of what could be causing this. What we do know:

•  Only the young leaves are yellow (Figs. 3 and 4), so it is probably not related to nitrogen or other nutrients mobile in the plant. However, iron deficiency could explain the symptoms because Fe is relatively immobile in the plant.
• There are no obvious lesions present so it is not mediated by above-ground diseases.
• We do not see it in tall fescue, perennial ryegrass, and some cultivars of Kentucky bluegrass (Figs. 1 and 2).
• We see it when soil temperatures are at their seasonal highs.
• We see it almost exclusively on irrigated turf and most frequently during wet summers.

Figure 3. Close-up picture showing the lower, older leaves of Kentucky bluegrass farther down in the canopy are retaining their color while the new leaves are chlorotic.

Figure 4. Close-up picture showing the lower, older leaves of Kentucky bluegrass retaining most of their color.

We know from past experience that this a visual effect and long-term health of the plant is not an issue. Therefore, immediate action is probably not needed other than to improve aesthetics. Reducing irrigation in the short should help. Increasing drainage and reducing compaction with more frequent aerification should also help in the long-term. We would not recommend a fungicide, insecticide, or > 0.5 lbs N/1000 sq ft (avoid almost any N in August except on greens or sports turf in use). We often see symptoms similar to this in the spring, but those are usually attributed to denitrification and Fe applications do not help then. However, a low rate of iron may be effective now. Since most the chlorotic leaves are the youngest and higher in the canopy, mowing should remove much of the yellowing.

Zac Reicher, Professor of Turfgrass Science, zreicher2@unl.edu

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