I have seen enough turf samples over the years to have noticed another potential cause for foliar discolouration, and the culprit is quite an unlikely one
red/purple discolouration to the turf will only affect bentgrasses and so, if the discolouration is seen on Poa annua, you should be alerted to the fact that it is more likely to be a physiological problem. The fungus responsible for bentgrass reddening is Drechslera erythrospila and the disease is given the descriptive common name of Red Leaf Spot.
Close inspection of the affected leaf tissues will show the presence of distinct red spots on the leaf blade. They may have a paler centre and a red/purple border but, in cases where the disease is very active, the extent of the foliar infection can make the whole leaf appear dark red/purple. Again, different cultivars will show a range of susceptibility to this disease and the affected turf can appear quite patchy or show as a general discolouration, particularly on younger swards. The fungal genus, Drechslera, contains several species that have the potential to cause leaf spot diseases and other problems on all turfgrasses. Historically, the fungi that are now called Drechslera, used to be associated with other fungi in a group referred to as ‘Helminthosporium’ and some text books may still use this old nomenclature. The Drechslera species of fungi
respond very quickly to the wetting of a dry turf and, following rainfall or irrigation, the fungi will rapidly produce spores, penetrate the leaf and initiate new area of infection. Therefore, the foliar discolouration caused by this fungus on bentgrass turf will develop rapidly under conditions of heavy dew following daytime temperatures that rapidly dry the sward. Typical symptoms on bentgrass turf can be seen in Figure 2. So, the observed patchy red/purple discolouration of turf that develops in the spring or autumn may be caused either by a natural physiological
process within the plant or by the fungal disease of Red Leaf Spot on bentgrass plants. Could there be any other reason for the discoloration? Well, generally, the thinking is no - but I have seen enough turf samples over the years to have noticed another potential cause for foliar
discolouration, and the culprit is quite an unlikely one. Generally, on bentgrass turf (and
frequently on creeping bent) during the spring months when we experience heavy dews or at times during the year when the sward may remain wet for extended periods, I have often seen one particular fungus within the leaf tissues in parts of the lamina that are showing a red discolouration. The fungus is Sclerophthora
macrospora and you may know this as the cause of the disease Downy Mildew or Yellow Tuft (a more descriptive common name). The fungus is what’s called an obligate parasite which means that, like the rust fungi, it will only live on a living host and will not cause death of the plant like, for example, Microdochium nivale will.
Once the plant is infected, the fungus S. macrospora will remain within the plant and increase activity and symptom development whenever the turf sits wet. Under extreme conditions, the fungus stimulates increased tillering of infected plants, causing a tufted appearance to the sward which becomes yellow or chlorotic due to the plant’s inability to take up sufficient nutrient to maintain colour in the quantity of leaf tissue that develops. The root development is also reduced in affected plants, further increasing the severity of chlorosis. However, the fungus can be found inside leaf tissues of plants that are not showing typical Yellow Tuft symptoms. In these cases, the affected leaves may be tan in colour from the tip down or they may show small areas of red pigmentation along
the lamina. When viewed under the microscope,
the large oospores of the fungus are readily apparent within the leaf tissue and, although they resemble the oospores of Pythium in general appearance, their overall size confirms their identity. Figure 3 shows a bentgrass turf that has Sclerophthora macrospora in the leaf but is not showing typical symptoms of Yellow Tuft.
How frequently is this infection being overlooked, considered to be purely an effect of the prevailing weather conditions or identified as Red Leaf Spot? When the quantity of the S. macrospora in the leaf is high, the red pigmentation that develops in the plant cells can closely resemble a Drechslera infection. What we don’t see however, is the pale centre to the leaf spot and the darker circle of pigmentation that characterises Red Leaf Spot disease. Physiological colour changes due to ambient weather conditions require no plant protection product application. The turf will recover as soon as the night-time temperatures begin to increase. Red Leaf Spot disease may grow out untreated with fungicides but, if the disease is considered to be active and extensive and the weather likely to remain favourable for infection, an application of a suitable fungicide may be appropriate. As for the fungus S. macrospora, it is generally considered unnecessary to apply any fungicide to manage these infections, but increasing surface drainage is generally all that is required to reduce the expression of any symptoms. As always, identification of the cause of the symptoms is crucial for effective management.
Dr. Kate Entwistle, The Turf Disease Centre Email:
kate@theturfdiseasecentre.co.uk www.theturfdiseasecentre.co.uk
Figure 1 -D
response to warm days and cool nights
Discolouration of Poa annua in
turf (note: the Poa in the sward is unaffected)
Figure 2 -R
Red Leaf Spot infection on bentgrass
discolouration associated with the presence of Sclerophthora macrospora
Figure 3 -B
Bentgrass that is showing a foliar
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