With turf lawns, fungal diseases cause the most serious diseases. Disease problems can be exacerbated by poor turf grass maintenance but they can also be improved by the introduction of a sound lawn care maintenance regime.
How fungi grow
Some types of fungi live only on dead organic matter (saprophytes), but others have the ability to attack living plants (pathogens). These disease-producing organisms also often have the ability to survive and develop on dead organic matter in the soil. When moisture and temperature conditions (high humidity and moisture at the plant base) become favourable, they attack the living tissue of the grass, causing severe injury or death. Most known fungi originate from spores which often require warmth and moisture for germination. The result of germination is normally the emergence of the vegetative part of the fungus. In some outbreaks this can even be seen as a fine, cobweb-like substance on the lawn first thing in the morning. Each individual filament of the mycelium is termed a hypha. There is a wide range of variation in the types of spores produced by fungi and this assists in their identification. Many fungi produce both tough resting spores for survival as well as spores for widespread dispersal in wind or water.
Factors for fungal disease development
There are many factors that can lead to the development of fungal disease in your lawn:
Thatch is the layer of dead and living shoots, stems and roots that develops on the surface of a root zone below the green tops. A thatch layer of 6-8 mm is beneficial, any more than that and problems become more likely – particularly when thatch layers exceeding 20 mm. Thatch can prevent good air and water movement and where there is an excess of thatch, the turf tends to produce roots in the organic layer, the condition of which can fluctuate from saturation to drought. When thatch is wet it remains damp for long periods and this favours for the growth of fungi as well as increasing the difficulty of securing good control with fungicides.
Diseases known to be favoured by high thatch levels include: dollar spot, fusarium blights, pythium blight and helminthosporium leaf spot. Dethatching using a metal rake or dethatching machine can assist in controlling these diseases.
Adequate soil moisture levels are important for the health of the grass and must be provided to keep grass growing actively.
Good drainage must be maintained since over-watering and excessive periods of waterlogging can encourage fungal disease development.
Humid air and heavy dews can also contribute to disease development by keeping the foliage wet.
An essential requirement for good turf growth is an open, well-drained soil that is not hard and compacted. This need has to be considered prior to planting. Most turf grass species do not grow optimally in heavy clay soils since roots cannot absorb oxygen properly in prolonged wet periods. In these conditions, the air pockets between small soil particles fill with water and take an excessive time to drain away, suffocating the root system.
Compacted areas of soil need to be broken up and treated with gypsum and organic matter to improve soil structure. Core aeration followed by loam topdressing can be helpful over broader areas.
Infertile soils, including those with unfavourable pH levels, tend to support only unhealthy and unthrifty plants and are indirectly responsible for disease development. The pH range tolerated varies with the turf varieties.
Fertilising can affect the frequency and severity of disease attack. Grass that has been weakened by starvation or is soft and succulent because of excessive nitrogen fertiliser, is often more susceptible to disease attack.
Excessive traffic may break, tear and injure grass plants. Broken or injured grass blades and stems are more susceptible to wilting and disease attack. Most diseases are favoured by mowing as it wounds leaves, disperses infected clippings and spores and removes leaf tissue that is required for photosynthesis. Keep mower blades sharp to reduce the amount of damage.
The circulation of air can reduce disease occurrence as movement of air as a breeze or wind produces a cooling and drying effect, which is less conducive to turf grass fungi. In a lawn the regular pruning of adjacent thick shrubs can improve turf grass vigour by increasing light levels and improving air circulation.
Fungal disease identification
Different controls required for each individual disease so correct disease identification is essential for effective control. With experience it is sometimes possible to recognise some diseases by the symptoms that are produced on the turf but as symptoms may vary from place to place and it can be difficult to determine accurately whether a disease is present or whether the turf is affected by other factors. Fungal disease often cannot be identified with certainty by sight. An accurate diagnosis depends on examination under a microscope and often culturing of the fungus.