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May 2002, No. 2
WHAT HAPPENED TO THE COLOR OF BENTGRASS?
I thought the putting greens were looking good; I even consulted my last Turfgrass Tip (Nitrogen Fertilization in Early Spring) prior to purchasing and applying an application of fertilizer.
I decided to use a synthetic organic product that contained readily available nitrate.
I was concerned with the cooler temperatures we faced and needed something that would not be temperature-release dependent.
Soon thereafter, the plants responded.
Leaf color darkened and the growth rate increased.
I was pleased; after all, there were still many ball marks from last fall that needed to heal.
As the temperatures rose, the growth rate of my greens did the same.
We broke high temperature records that second week in April and it seemed that we were going to miss spring and jump into summer.
However, reality hit soon thereafter.
It cooled off drastically, rains came and the sun went away.
So did the color of the bentgrass on my putting greens!
The annual bluegrass didn't seem to mind the swing in temperatures, but the bentgrass looked like a Monet up close, mottled and variable in color.
I looked for signs of a pathogen, was it a leaf spot of some sort? I thought it could be possible until I consulted Dr. Randy Kane of CDGA fame.
(click image to enlarge)
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(click image to enlarge)
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Most of us are aware of the response putting green grass plants have to cold temperatures in the fall.
There is often a dramatic color change from green to red and purples on our putting greens.
The turfgrass leaves change color just as the anticipated leaves on trees do in the fall.
WHERE DOES THE COLOR COME FROM?
The response stems from the shifting carbohydrates in the plant.
When growing conditions are optimal, sugars or carbohydrates, are produced through photosynthesis in the leaves and sheaths.
At night, these carbohydrates are redistributed or translocated to the crowns of the plant.
Most of the carbohydrates are stored in the crown and can be used for other physiological processes when needed.
However, when the plant experiences very cool or cold nights, the process of translocation is slowed and some of the carbohydrates remain in the leaf and stems of the plant.
One of the carbohydrates or sugars produced is glucose.
Bonded to glucose molecules are plant pigments called anthocyanins.
Not much is known of these pigments other than they provide the blue-red-purple color in plants.
It is known that chlorophyll, the pigment that colors turf green, reaches a maximum content in creeping bentgrass when temperatures are between 75 and 85°F.
Most of the time the chlorophyll masks the presence of anthocyanins and the grass appears green.
When cooler temperatures prevail the anthocyanin pigments remain in the leaf blade and stem.
The cooler temperature also causes the chlorophyll to become denatured in the leaf blade and the masking effect is lessened.
Some of the blue-red-purple pigments become more prevalent in the turfgrass blade and color the plant.
This shift of anthocyanin / chlorophyll concentration in the leaf blade can also occur in the spring.
Weather is the deciding factor.
Uncommon warm temperatures in early spring can cause bentgrass and annual bluegrass to green up quickly.
When followed by a drop in temperature, the concentration of anthocyanin can increase and a reduction in chlorophyll can occur.
This shift in "seeable" pigmentation can cause bentgrass to turn reddish brown, brick red or purple colors in the spring.
This condition can be aggravated by cultural practices including:
- Topdressing
- Brushing
- Core cultivation
- Foot traffic
- Vertical grooming
- Use of plant growth regulators
- Dry and windy weather
Perhaps, I failed to mention that I did get a topdressing application down just prior to the onset of the cold weather.
I did set my vertical groomers to "aggressive" and used them in two different directions on the same warm windy day.
Perhaps, I forgot to mention that I also used a brush on the putting greens to take care of some stubborn horizontal growth and then one Monday, I even rolled them.
I did notice the color change of the bentgrass days after the cold temperature swing.
Then a few wet and cold days later I noticed spot like lesions on my bentgrass leaf blades.
"Leaf spot", I thought was the culprit.
However after my talk with Dr. Kane, I was informed it wasn't leaf spot but the weather and a couple of other applied cultural factors.
The cold snap turned my bentgrass blue, okay maybe red-brown but blue has a nice sound.
The cultural practices I applied may have injured the turf further, but the lesions I saw were not the cause of my altered turf color.
The lesions or "leaf spots" that were present on the bentgrass blades were most likely due to opportunistic saprophytes taking advantage of my weather-weakened bentgrass.
It should be noted, that an application of a fungicide may help to control these weak saprophytes, but an application of fungicide to control the weather will not work.
No need to panic.
The best remedy for this problem is around the corner.
Warm weather, some sunshine and a little bit of plant available nitrogen will ease the distress caused by the cold weather.
LEAF SPOT . . . . . A REVIEW
There are many "leaf spot" diseases that can affect turfgrasses.
"Leaf spot" on turfgrasses were once referred to as generic Helminthosporium diseases.
Helminthosporium fungi, (now generally referred to species within the genera Bipolaris and Drechslera) can be responsible for the gradual browning and thinning (melting out phase) of many susceptible turfgrass cultivars.
RED LEAF SPOT ON CREEPING BENTGRASS
Drechslera erythrospila is the casual agent of red leaf spot on creeping bentgrass.
The symptoms are individual leaf lesions that are circular to ovate, usually straw colored and surrounded by a reddish-brown border that can be variable in width.
The centers of individual lesions are very small, lighter in color or absent altogether.
When conditions are optimum for disease advancement, lesions can overlap and coalesce to form uneven patterns giving the affected areas a reddish color.
Drechslera erythrospila is a warm-wet weather disease.
Primary infections of the plant crown and leaf tissue usually occur in late spring.
Take note that the evidence of the leaf lesion phase of this disease usually occurs after colonization of the crown, root, and tillers.
Disease incidence and severity can increase as air temperatures rise in June and July in a normal growing season.
This disease can be controlled by the preventative use of fungicides where red leaf spot has become problematic on an annual basis.
A preventative schedule should be used at the onset of warm humid weather in late spring.
Fortunately, red leaf spot is very uncommon in our area on the cultivars of bentgrass we manage.
However this pathogen is often mistaken for the cold injury we saw this spring because of the reddish-brown cast exhibited by our putting greens.
Also, if you do see copper to red colored patches (not spots on leaves) on your turf this spring, Microdochium nivale or pink snow mold, could be the culprit.
Remember this is a cool wet weather disease that first appears as round, water-soaked patches 1-3 inches in diameter that quickly turn yellow to orange-brown or reddish brown.
THE REAL LEAF SPOTS TO BE CONCERNED ABOUT . . . .
MELTING OUT ON KENTUCKY BLUEGRASS AND PERENNIAL RYEGRASS
(click on image to enlarge)
When Kentucky bluegrass (Poa pratensis) and perennial ryegrass (Lolium perenne) are severely infected with leaf spot or melting out (Drechslera poae and D. siccans), large irregular areas of turf turn yellow, then brown-straw in color and death of the plant can occur.
The disease can occur on all plant parts and can first be observed as small water soaked lesions.
Prime climatic conditions for these lesions to occur are when air temperatures linger between 65 to 75° F with high humidity and overcast weather.
These lesions can enlarge quickly into dark purple-red areas with the centers turning brown and ultimately a dull white color.
The sheath of the plant is also affected with similar lesions, although the lighter colored center is usually missing.
When colonization of the sheath tissue is extensive, the leaf becomes girdled and drops from the plant.
The leaf-dropping phase of this infection is what is known as "melting out".
Another leaf spot pathogen that causes concern on Kentucky bluegrass and perennial ryegrass is Bipolaris sorokiniana, which occasionally infects bentgrasses as well.
The leaf lesions of this disease are very similar to Drechslera leaf spots.
They first appear as small purplish spots and as they increase in size the centers turn brown, and finally fade to a light tan with purplish brown borders.
If allowed to persist, B. sorokiniana can blight the entire leaf blade in a matter of 4-5 days when conditions are optimum.
At this stage, irregularly shaped patches of browning turf will appear across a turf.
In conjunction, severe crown and root rot can develop that will adversely affect the plants' vigor and drought tolerance.
Look for leaf lesions as temperatures begin to rise this spring.
These fungi are favored by warm moist air, most surely to come.
Outbreaks of Bipolaris and Drechslera may be incited by the following conditions:
- High nitrogen fertility
- High atmospheric humidity
- Late spring to early summer temperatures (65 to 75°F for D. poae and 75 to 85° for B. sorokiniana)
Cultural practices to control these two leaf spot diseases include:
- Judicious use of nitrogen fertilizer.
(Apply amounts to support adequate growth especially in early spring and late fall)
- Avoid high rates of fertilizer from June through September
- Control the thatch layer (< ½ inch)
- Remove clippings if practical
- Mow between 1 ½ - 2" if use of turf allows (Kentucky bluegrass / Perennial ryegrass)
- In cases of new plantings, research resistant species and cultivars
Symptoms of these diseases can be especially evident in a turf that has a history of developing leaf spot (the varieties/species obviously are not resistant).
Some systemic fungicides can give adequate curative control of these diseases, however most fungicides are more effective when applied early in the development of the disease.
For additional information on bentgrass discoloration caused by extreme temperatures, check the May 21 Pest Update in the diseases section.
(Luke Cella, CGCS, Editor; Randy Kane, CDGA Turfgrass Pathologist)
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