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