Fungal diseases of lawns: Predisposing factors
Fungi cause the most serious diseases of turf grasses. Disease problems can be exacerbated by poor turf grass maintenance. Conversely, they can also be improved by the introduction of sound cultural practices. It is therefore important to develop a disease management strategy.
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How fungi grow
Fungi must secure nutrients from dead or living tissues. Some types 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 become favourable, they attack the living tissue of the grass, causing severe injury or plant death.
Most known fungi originate from spores, which usually require warmth and moisture for germination. The result of germination is normally the emergence of mycelium, which is the vegetative part of the fungus. In some types of disease 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. This assists in their identification. Many fungi produce both tough resting spores for survival, and also spores for widespread dispersal in wind or water.
Factors that favour disease development
The factors that can favour disease development include high moisture levels, low air movement, thatch accumulation, poor soil condition and nutrition, wear, mowing and the turf variety.
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. Grass clippings do not significantly contribute to thatch accumulation. A thatch layer of 6-8 mm is beneficial, but, beyond that, problems become more likely - particularly with thatch layers exceeding 20 mm.
Thatch can prevent good air and water movement. 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. Once the thatch is wet, it remains damp for long periods. This favours for the growth of fungi and increases 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-tined rake or dethatching machine can assist in controlling these diseases.
Adequate, but not excessive, soil moisture levels are important for the health of the grass. Sufficient water must be provided to keep grass growing actively. Too much or too little water can weaken turfgrass and predispose it to disease.
Good drainage must be maintained. Over-watering and excessive periods of waterlogging makes turf susceptible to disease and favours fungal disease development. 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.
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, as 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.
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 turfgrass species. For example, the pH for green couch is approximately 6.0 to 8.0, however blue couch will tolerate acid soils with a pH of less than 5.5.
Fertilising may 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 more susceptible to disease attack. Exercise judgment when making applications of nitrogenous fertilisers, particularly in periods of the year when diseases are most likely to occur.
Excessive traffic may break, tear and wound the plants. Broken or wounded grass blades and stems are more susceptible to wilting and disease attack. Most diseases are favoured by mowing. Mowers wound leaves, disperse infected clippings and spores, and remove leaf tissue that is required for photosynthesis. Keep mower blades sharp, as the damage caused by dull mower blades can be extensive.
The circulation of air can reduce disease occurrence. The 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.
Grasses, even cultivars within a species, can vary in their susceptibility to certain diseases. Queensland blue couch (Digitaria didactyla) and seashore paspalum (Paspalum vaginatum) are more likely to be infected with dollar spot (Sclerotinia homeocarpa) than green couch (Cynodon dactylon). Floral smut (caused by Ustilago cynodontis) is found only on Cynodon sp. including green couch, and kikuyu yellows (Verrucalvus flavofaciens) is unique to kikuyu (Pennisetum clandestinum). Spring dead spot (Leptosphaeria spp.) is found on green couch, kikuyu, broadleaf carpetgrass (Axonopus compressus) and soft-leaf buffalograss (Stenotaphrum secundatum). For persistent problems, changing to a more resistant species is worthwhile.
Identification of disease
Because of the different controls required for each individual disease, 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. However, 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. Fungi causing disease cannot be identified with certainty in the field. An accurate diagnosis depends on examination under a microscope and often culturing of the fungus.
Tests are available to identify basic nutritional problems and likely fungal plant pathogens associated with disease symptoms. The cost of an accurate diagnosis is minimal compared to the economic loss from damaged turfgrass and wasted chemical and labour from using an inappropriate treatment method.
If in doubt about a turf grass disease, seek a professional disease diagnosis.