Does your Bluegrass have the Yellows?

Lush, water- and fertilizer-fueled grass growth is often
light green or yellow. Clippings should be collected if
you can't mow often enough to prevent the accumulation
of clumps - which cause the grass underneath to turn
even brighter yellow!

Tony Koski, Extension Turf Specialist

You’re not the only one who has had enough! Have you noticed some yellowing in lawns in the past week or so? The precipitation and cool weather over the past couple of weeks have created some temporary (well, if the rain stops) problems for our cool-season lawns.

While there is plenty of glowing green turf out there, the persistent moisture has encouraged ample growth, especially on well-fertilized lawns - growth which may be lighter green or even yellow. Fast- growing grass leaves are often lighter green because leaf chlorophyll content is less concentrated or “diluted” by the rapid growth rate. Plus, those leaves lower in the canopy are normally more pale green – or even slightly yellow – than leaves higher up in the canopy, where light intensity is greater. When weather delays mowing and the grass gets tall, the darkest green leaves are mowed off – leaving the pale green and yellow leaves behind. When this rain stops, temperatures warm and you catch up on your mowing, growth will slow and more normal color will return.

Iron chlorosis develops in random patches. The youngest leaves
will be the brightest yellow, since iron doesn't move well
from older leaves to younger ones. Rapid growth and saturated
soil will encourage this temporary iron deficiency in turf.

The wet, cool conditions are also causing iron chlorosis to develop. The roots can’t get adequate iron from the soil to supply the rapidly growing shoots – so the turf becomes iron-deficient and chlorotic (yellow). With iron chlorosis, the YOUNGEST, newest leaves at the top of the grass plant will become deficient first. This is in contrast to the yellowing described above, which occurs on the OLDER leaves in the lower part of the turf canopy, with the youngest leaves at the top of the canopy being darker green. Persistent, severe iron chlorosis can cause the entire plant to become yellow – sometimes to the point that some plants die and the turf thins out. As soil temperatures warm and we get drier weather, the iron chlorosis we are seeing will diminish. 

Melting out disease can become a big problem in bluegrass
lawns during extended rainy, cool periods in the spring.

Melting out can quickly kill large patches of turf when
conditions are favorable: cool, moist and cloudy.

We are seeing melting out (Bipolaris sorokiniana) in bluegrass lawns now. Normally more of a problem in the Midwest and other wetter parts of the country, this fungus is turned on by continuous cool, wet conditions.  It causes small brown spots to form on leaves (similar to leaf spot disease seen on turf under warmer summer conditions – but those are caused by a different fungus, Drechslera poae) and leaf sheaths. Under ideal (cool, wet) conditions, the fungus invades crowns, rhizomes and roots of the grass plant – causing leaves to turn light green or yellow. Heavily infected plants die and turn brown. The turf yellows and thins out in patches – hence the name “melting out”. Severe melting out can cause large, irregularly shaped patches of dead turf. Advanced melting out is very difficult to control with fungicides. This disease is more common on older lawns; overseeding with newer, more resistant bluegrass cultivars is probably the most effective management tool. Also, heavy spring nitrogen use will increase the potential for and severity of this disease.

Roughstalk bluegrass (Poa trivialis) loves cool, rainy weather.

Rainy, cool weather also encourages the growth of two light green grasses in lawns: annual bluegrass (Poa annua) and roughstalk bluegrass (Poa trivialis). Poa annua is flowering profusely right now (an important allergen for many this time of the year), and Poa triv will spread rampantly in this kind of weather. Selective control of Poa trivialis in bluegrass lawns is not  possible; annual bluegrass can be controlled long term using a combination of strict cultural practices and use of Tenacity (mesotrione) herbicide.

Annual bluegrass (Poa annua) invasion is favored by cool,
rainy weather.

When things dry out and warm up, the severity of these problems should diminish – except that the Poas will never go away on their own. And I’m betting that this will be a banner year for necrotic ring spot – a root rot disease that is encouraged by overly moist soil conditions in the spring and summer. More to come on that…

And speaking of more to come….a look at the 10-day forecasts by both AccuWeather and Weather Underground are calling for rain on 9 of the next 10 days in Fort Collins.