Fusarium Patch Disease
Early symptoms of fusarium patch disease.
This is probably the most common of all cool-season turfgrass diseases.
It is caused by the fungus Microdochium nivale (formerly Fusarium nivale).
The fungus is favoured by cool, wet conditions and often causes disease on turfgrass stands that are weakened by unbalanced nutrition or stressed due to problems within the rootzone (generally a water-retentive rootzone and thatch).
All grasses can be affected by M. nivale but the symptoms of disease are generally first seen on Poa annua, if it is present in the sward. Due to its relatively shallow rooting, P. annua reacts quickly to stresses and changes in nutrient availability and it is likely that this growth habit makes it more susceptible to attack by this fungus. Alkaline turf is likely to be more susceptible to infection as the fungus grows closer to its optimum under conditions of high pH. Thatch and wet rootzones will also encourage this disease as they will tend to keep the base of the sward damp.
Initial signs of infection are small (1-2cm dia.) spots of dark brown, watersoaked* turf which rapidly enlarge under cool, wet conditions in to patches of affected turf up to 10-12cm dia. As patches increase in size they may coalesce to form irregular-shaped areas of infection. These enlarged patches tend to have a dark brown border indicating active infection in to the leaf tissues of the plants at the edge of the patch. The centre of these patches tends to become paler and straw-coloured as the disease progresses and as the fungus takes nutrients away from the plant. The fungal mycelium may grow out on to the turf surface under conditions of high relative humidity and masses of spores will be liberated from these affected tissues. The plants may take on a slight pale pink appearance due to the colour of the fungal mycelium. The fungus can kill not only the leaf tissues but also the crown and therefore, if this disease is left unchecked, scars will develop across the sward.
Frosts will not kill the fungus that causes this disease but they will halt its activity.
Under snow cover, M. nivale will grow close to its optimal rate and can therefore cause extensive damage to turf under these conditions. In this situation the disease is called Pink Snow Mould as a reflection of the symptoms that are seen on the turf following snow melt.
*Watersoaked This is a term generally used to describe the appearance of leaf tissue that has become wet-looking, generally darkened and translucent as a result of fungal infection breaking down the internal structure of the leaf.