Photo credit: General symptoms of Gray Leaf spot infection (Photo: Roger Clark, AGS)
The first known UK case of Gray Leaf Spot disease on perennial ryegrass turf signals the arrival in Britain of potentially the most damaging fungal disease for this grass variety – and it’s a problem that can be extremely difficult to control
By Dr Kate Entwistle
The Turf Disease Centre
As a disease of amenity turf, Gray Leaf Spot initially developed as a problem on warm-season turf grasses (notably St Augustine grass, Stenotaphrum secundatum) in the USA but, in the early 1990s, it became a serious problem in Lolium perenne perennial ryegrass and tall fescue turf being maintained in sports facilities. The disease then began to appear in Europe and has, over the past 15 to 20 years, progressed through much of mainland Europe – most notably Spain, Portugal, France, Germany, Austria and Switzerland.
In August 2020, however, The Turf Disease Centre confirmed the development of Gray Leaf Spot disease in a UK sample of perennial ryegrass turf, with spores of the causal fungus abundant in the symptomatic plants. This is the first known occurrence of the disease in UK perennial ryegrass and the importance of this outbreak should not be underestimated. This is potentially the most damaging fungal disease that can develop on stands of perennial ryegrass turf and the disease can be extremely difficult to control.
Spores of Pyricularia grisea developing on a leaf lesion
Causes and signs
Gray Leaf Spot disease is caused by the fungus Pyricularia grisea (although some accounts refer to the fungus as P. oryzae) which produces uniquely shaped spores that allow for rapid and absolute confirmation of its presence on affected plants.
The sexual stage of the fungus has not been identified in amenity turf and therefore the disease develops through successive cycles of asexual infection, with each cycle producing a greater spore mass than the preceding one. Under high temperatures (28°C and above) and humidity, the fungus produces spores extremely rapidly and the disease can appear to ‘explode’ through previously unaffected turf.
The fungus produces a huge number of asexual spores (conidia) onto the surface of infected leaf tissue and, en masse, the development of spores gives the appearance of a grey velvet covering to the leaf – hence its common name of Gray Leaf Spot. These spores are very easily moved by wind, water or mechanical means to enable rapid spread of the infection and an increase in disease severity.
Leaves may show lesions or spots when the fungus has entered the plant and started the process of infection. More commonly, these small leaf lesions go unnoticed and the first observed symptom will be diffuse areas of tan-coloured turf that appear to increase rapidly in number and in size, can follow the line of mowing (through movement of spores, or water across the turf) and may resemble either drought or heat stress.
Turf breeders have, however, made huge steps in producing varieties of perennial ryegrass that show significantly reduced susceptibility to the disease (and smooth stalked meadowgrass – Poa pratensis – appears to be unaffected).
Visit www.theturfdiseasecentre.co.uk for details of how The Turf Disease Centre provides an independent turf disease service to turf managers and others in the amenity turf industry
Spores developing en masse giving a gray ‘velvet’ appearance to the leaf
Understanding Gray Leaf Spot
I have been working closely with Sabine Braitmaier (ProSementis GmbH, Germany) on Gray Leaf Spot for several years. Sabine, who first identified the disease in Germany in 2017, has helped many stadia to limit its occurrence and severity through her understanding of the fungus and how infection can progress. A more complete article on Gray Leaf Spot – the fungus, the disease and its management – will be published in the coming months but, in the meantime, additional information is available from myself (firstname.lastname@example.org) or Sabine (Sabine.Braitmaier@prosementis.de).
It is essential that we understand the fungi that cause disease problems in turf so that we can effectively predict occurrence and identify the best management options. To further our understanding, we will be working with Dr Colin Fleming, Dr Deborah Cox and Dr Thomas Fleming (Agri-Food and Biosciences Institute, Belfast) to complete molecular studies on the fungus present in the UK and hopefully explore its relationship with the fungi that cause Gray Leaf Spot in turf grasses across Europe and more widely.
NB: It is imperative that the disease is diagnosed correctly before any action is taken.