Are your greens yellow?

During this time of year, many golf course superintendents report yellowing of creeping bentgrass greens in spots, patches, or irregular areas. While there are a few different diseases and conditions than can cause these symptoms, many assume that “yellow tuft” is to blame. In fact, yellow tuft is not very common in the Southeastern US, and typically only occurs on poorly drained greens or during very wet weather.

A misdiagnosis of yellow tuft can be costly. Few fungicides are labeled for this disease, and those that are have no activity on the other problems that cause yellowing in bentgrass greens. Here’s a description of a few of these “yellow” problems seen on bentgrass greens and recommendations for their management:

Yellow Tuft

Above-ground symptoms of yellow tuft appear in small spots, typically 1 inch in diameter or less. In severe cases, the spots may merge to create irregular areas of yellow turf. Close examination of these spots will reveal that the turf is slightly raised above the canopy. Carefully remove one of these spots from the turf using a knife or ballmark tool, and you will see a massive number of tillers emanating from a single node. This “witch’s broom” symptom is a key characteristic of yellow tuft.

Yellow tuft can cause damage to the turf if left unmanaged. Most of the damage is a result of scalping injury as the infected tillers are pushed upward above the height of cut.

Yellow tuft is caused by a Pythium-like organism called Sclerophthora macrospora. This pathogen infected the crowns and leaf sheaths of the turf. Once established, the pathogen induces excessive production of hormones, which leads causes the excessive tillering.

At this time, the Pythium fungicides mefanoxam and fosetyl-Al are the only fungicides labeled for yellow tuft control. Based on our experience, fosetyl-Al is not very effective, so stick with mefanoxam for preventive or curative applications.

If you have chronic problems with yellow tuft, poor drainage is likely to blame. Taking steps to improve surface or subsurface drainage will eliminate or reduce the problem and save lots of money in fungicides over the long term.

Yellow Spot

In my experience, this is the most common cause of yellow symptoms on bentgrass greens in the Southeast. Symptoms of yellow spot are very similar to yellow tuft, but the spots can be much larger, up to 4 inches in diameter. In addition, the growth habit of the turf is completely normal: it will not be raised above the canopy and no excessive tillering is observed.

Unlike yellow tuft, yellow spot never actually causes damage to the turf. However, due to its aesthetic effect and perceived effect on the consistency of the playing surface, it still warrants management.

We are still not sure what causes yellow spot. We have consistently seen certain types of blue-green algae (cyanobacteria) in the spots, and we theorize that they are producing toxins that induce the yellowing symptom. Other scientists have not been able to establish this same relationship, so the true cause remains unknown.

Nevertheless, we have seen very effective control of yellow spot from applications of chlorothalonil. Applications of 2.7 oz a.i. per 1000 ft2 every 14 days have typically provided good prevention, but higher rates and shorter intervals are needed for curative suppression. In addition, yellow spot tends to be more severe when fertility levels are low and the turf is subjected to frequent drought stress.

Etiolated Tiller Syndrome (ETS)

This is the least well understood of all of the problems that cause yellowing, but has been observed with increasing frequency over the last five years. ETS is easily distinguished from yellow spot and yellow tuft. ETS typically develops in irregular areas, but sometimes it can appear in patches coinciding with bentgrass clones. The key characteristic is that the youngest leaves in affected areas will be etiolated, or elongated and yellow in color. At first the rest of the plant will appear normal, and no increased tillering is evident as with yellow tuft. Some turf decline can occur if turf affected by ETS is subjected to heat or drought stress.

We still have no idea what causes ETS. There are plenty of theories, but no hard facts. Most cases we’ve observed have been associated with combinations of plant growth regulators and various biostimulants containing giberellic acid or cytokinins. Then again, there are a few cases where neither was applied.

If you are having problems with ETS, take a look at what you are applying to the greens. If you are applying plant growth regulators and biostimulants, try deleting one or the other out of your program. This may alleviate or eliminate the problem.

Nitrogen Deficiency

The most frequent cause of yellowing in putting greens is nitrogen deficiency. Typically, nitrogen deficiency appears uniformly across entire putting greens, but sometimes it can have a patchy appearance, especially on older greens, due to the responses of different bentgrass genotypes in the population. Any time you see yellowing, take a look at your recent fertility practices and have a tissue sample analyzed for nutrient content to determine if nitrogen or other nutrients are limiting.