How to prevent gray leaf spot?

Blog Dr. Irene Vroegop:
11 tips for a healthy summer pitch

Find out all about Gray Leaf Spot and more importantly, how to prevent it? Dr. Irene Vroegop has some valuable tips.

SGL agronomist and turfgrass disease expert Dr Irene Vroegop

Extreme droughts, extreme temperatures and a short transition period can cause a lot of stress to the plant (and the groundsmen). These are conditions in which diseases thrive, creating a situation that keeps us up at night. But luckily there is a lot we can do to put our mind and our pitch at ease. Naturally, some practices vary depending on the type of disease. I would like to start with diving into a fungus that can infect an entire grass playing surface within 48 hours; the most problematic and most feared disease on the list: Gray leaf spot. In order to fully understand how to control it, I’d like to tell a little bit about the disease itself before moving into prevention and cure. I hope it answers most of your questions, but of course feel free to get in touch with me for more information any time.

So, what is gray leaf spot?

Gray leaf spot is caused by the fungus Pyricularia grisea and is a very destructive disease of Lolium perenne (Perennial ryegrass). Furthermore, it can cause severe damage to the turfgrass species Festuca (Fescue grasses), Cynodon (Bermudagrass), Eremochloa (Centipedegrass) and Paspalum. The pathogen overwinters as spores and dormant mycelium in the lower leaves of plants and in thatch. When temperatures rise and humidity is high, the fungus produces spores on necrotic tissue. From that moment on the disease can spread rapidly. All in all: a disease that is very hard to control and should be prevented, rather than cured.

When and where are you at risk for gray leaf spot?

When there is a combination of a continuous period of leaf wetness and high temperatures the risk of infection is high. The temperature range in which gray leaf spot can occur is 15-39°C. At suboptimal temperatures (15-22°C or 32-39°C), a period of 21 to 36 hours of leaf wetness is typically required for infection. At optimal temperatures (22-32°C), peak infection can occur with as little as 9 hours of continuous leaf moisture.

Until recently, gray leaf spot was a grass disease only found in Southern Europe and other places with warm climates. However, because of global warming and the disease’s ability to adapt to new environments, we see that gray leaf spot is also becoming more prevalent in Northern Europe. We also see the disease develop earlier in the year than usual. Where gray leaf spot used to occur from June to August in the southern countries, in 2020 several cases of gray leaf spot were already found in spring in the UK and in Germany.

How can my grass get infected?

Initially, the fungal spores must be present on the grass in order to be able to infect. Spores often come from previous infections, from water, machines and from people. Avoiding fungal spores on your grass is nearly impossible. The spore itself does not pose a real threat to your grass at first; the plant’s immune system is the main factor. The immune system of a grass plant can easily be compared to that of a human being, both of which are highly developed. If a person is too cold or too hot, he or she automatically goes into shock and becomes more susceptible to disease. The same applies to plants. Extremely dry, wet, warm or very cold conditions force the grass plant to go into “survival mode”. As a result, the grass draws nutrients from its defense system in order to survive, and therefore becomes more vulnerable. This is also known as “grass stress” and is regularly part of the cause for infection of the plant.

11 tips to prevent gray leaf spot on your grass

Because of the destructive nature of the disease, it’s best to always try to prevent it. With the 11 tips below you have a good base for a healthy summer pitch.

  1. Balance is key: A good balance between light, water, temperature, CO2 and nutrition strengthens the immune system and makes the plant less susceptible to diseases.
  2. Limit drought: A plant needs water to keep itself cool during higher temperatures and to transport nutrients from the roots to the shoots. Without water, the plant dries out, causing grass stress. I recommend watering your grass heavily in the middle of the day rather than multiple short irrigation sessions (read below why).
  3. Limit extended periods of leaf wetness: Irrigating in the evenings and mornings can extend leaf wetness, which may turn into grass stress when temperatures are high. Using the Turf Fan on your TC50/TF50 will dry the plant and reduce the period of leaf wetness after irrigation or at times when humidity is relatively high.
  4. Limit applications of nitrogen: Balance nitrogen applications just enough for plant growth, but do not use it excessively. Excessive nitrogen applications will increase disease severity. In hot, humid weather, restrict the use of nitrogen, particularly quick-release, water-soluble sources. Water-soluble sources of nitrogen that promote rapid growth can enhance the development of gray leaf spot compared to slow-release sources, which often do not stimulate disease development.
  5. Limit soil compaction: Decrease compaction by aerating. Less compaction ensures better root growth, less stress and therefore less susceptibility to gray leaf spot.
  6. Reduce mowing length: A higher mowing height also positively relates to disease severity. A lower mowing length helps you removing excess gray leaf spot tissue. Keep the canopy open and dry and remove developing gray leaf spot lesions.
  7. Reduce temperature: In the middle of summer you will not reduce the temperature below the temperature range of gray leaf spot (15-39°C, with an optimum of 20°C). However, you will reduce plant stress by lowering the temperature. With the cooling mechanism of the TC50, temperature can be lowered approximately 6°C. The SGL Portal provides calculated cooling advice based on weather data, in order to use the TC50’s cooling mechanism as efficiently as possible.
  8. UV-C light treatment: I always advise customers to use their UVC180 as much as possible from mid-spring to mid-autumn. Preventative treatment with UV-C light will reduce disease incidence. When biological organisms are exposed to UVC light in the range of 200 nm to 300 nm, the light penetrates through the cell wall and disrupts the DNA molecules. Disrupted DNA cannot replicate and therefore the fungi dies before it causes any damage to your turfgrass.
  9. Disease forecast: Based on weather forecasts and data collected by our monitoring tools, we provide accurate 7-day disease forecasts. Based on this forecast, the disease pressure per disease is determined so you can make proactive decisions and act in time.
  10. Register grass disease cases: Register your grass diseases and build your own database. This way you can identify grass disease patterns and anticipate it. It is now also possible to register your grass disease cases on the SGL Portal. This option may not have been activated in your Portal account yet. If this is the case, please contact us and we will activate it for you.
  11. Renovation: Fungal spores survive most easily in the thatch. Therefore, it is important to keep the thatch layer of your grass as thin as possible through renovation.

But what if, despite all your efforts, you think see gray leaf spot?

First of all: make sure to identify the disease with certainty as quickly as possible. Especially since gray leaf spot can infect a large area of the grass surface within 1-2 days. Below you’ll find the symptoms of gray leaf spot. I would also recommend you to have a sample checked by a turfgrass disease specialist if you are not 100% certain. A wrong diagnoses can lead to incorrect treatment, with the risk of making the situation worse.

What are the symptoms of gray leaf spot?

In cool-season turfgrasses symptoms first appear as small, water-soaked lesions, which quickly become necrotic. The leaf spots can vary considerably in color, size and shape, but they are often:

  • Oblong
  • Gray to light brown
  • Surrounded by purple to dark brown borders

In warm-season turfgrasses, tiny, brown lesions on leaves and stems enlarge rapidly into round to oblong spots. The largest spots may extend almost across the entire leaf. In severe cases, the entire planting dies, leaving behind resistant grass species or weeds. Leaf spots are tan to gray and have purple to brown borders.

In both cool seasons and warm season turfgrasses, Blighted leaf tips often have a conspicuous twisted or fishhook shape and thick masses of grayish spores may give leaves a feltlike appearance. Lesions are also found on the leaf sheaths, spikes and stems. Under a microscope gray leaf spot spores are easily to recognize, since they have a distinctive pyriform shape and a glassy appearance.

Treat gray leaf spot

The most effective control method is the use of the UVC180. If it turns out your grass is infected with gray leaf spot, curative treatment with the UVC180 will help to reduce disease spread. As mentioned earlier, UV-C light has the ability to disrupt the DNA of fungal spores, such as gray leaf spot, preventing the fungus from reproducing. Because grass cells are much more resistant to UV-C light than fungal cells, the amount of UV-C light radiated by the UVC180 is high enough to kill the fungi and to combat further outbreaks, while the grass plant is not affected. Another option is to slightly reduce the height of cut in the areas where the first signs of gray leaf spot are visible.

Dr. Irene Vroegop
Irene is agronomist and turfgrass disease expert at SGL. Her degree in biology with specialization in Environmental Biology, and PhD research on Plant-Microbe Interactions, form a profound basis for her research and advisory role within the SGL agronomy department. As part of the SGL Assist team she supports groundsmen on a daily basis to achieve a top quality pitch throughout the year.