|Scientific Name||Physarum cinereum|
|Description||A grey or purple-grey pall suddenly develops on an otherwise healthy lawn. The overall appearance resembles the scattering of ash from a fire. The substance, sometimes referred to as ‘UFO ash’, coats not only the turfgrass, but also any broadleaf or grass weeds in the lawn. Patches range from a few centimetres in diameter to (in extreme cases) in excess of a metre in diameter.
On closer examination, grey sporangia (structures) may be may be seen on both the top and bottom of leaves. These range in size from 0.5 to 7.0 mm (if they have grown together). As they mature and rupture they release masses of dark brown spores. As the organism dries off, an ash-coloured and calcium-rich powder is formed. This transfers readily onto other objects if brushed.
|Cause||These symptoms are caused by Physarum cinereum, a type of slime mould. Spores are distributed by equipment (such as mowers), on grass clippings, by rain splash, in wind currents and by animals. They remain dormant in the soil until they are activated by the right combination of high temperatures, cooler nights, wet weather and humidity, usually from spring to autumn. The phenomena can recur in the same area every year, under suitable climatic conditions. However, due to robust resting stages, the organism may seemingly vanish for a few years, reappearing in the same spot when the conditions are right.|
|Hosts||Slime moulds are not highly host specific. They merely use low-growing vegetation as an anchor point for a phase of their lifecycle. Physarum cinereum has been recorded on many types of warm season turfgrass including: blue couch (Digitaria didactyla), green couch (Cynodon dactylon), soft leaf buffalo grass (Stenotaphrum secundatum) and Zoysia species. Often the problem is first seen on broadleaved weeds in lawns, such as catsear (Hypochoeris radicata), cudweed (Gnaphalium spp.) and creeping indigo (Indigofera spicata), with the leaf presentation and colour contrast highlighting the presence of the organism.|
|Lifecycle||Although slime moulds look like fungi, they have mobile (amoeboid and flagellate) stages, making them animal-like in nature. They are classified as being in the Kingdom Protozoa. In cultures, slime moulds will ´crawl´ out of the petri dish.
In lawns, the mobile stage occurs in damp conditions within thatch (the layer of organic material that accumulates between the soil surface and the green shoots). The organism is initially a single cell which engulfs its food sources: bacteria, fungi and decomposing organic matter. These single cells merge to form a larger organism known as a plasmodium. The presence of plasmodia can give the lawn an ´oily´ appearance.
Under stress conditions, such as water-logging, the organism reproduces, moving to an exposed leaf surface and forming the visible sporangia. These quickly become powdery as they mature, dry and release spores (gametes), which unite with other compatible gametes to start a new amoeboid stage. The organism is persistent, with a number of resting stages. This ensures its survival under adverse conditions.
|Damage||This is a minor, but aesthetically disruptive disease of plants. Plants should recover quickly. The sporangia of grey slime mould have a broad base and adhere loosely to the leaf surface. The organism may be brushed off to reveal a healthy leaf.
If the problem is persistent, the leaf may yellow due to a shading effect.
|Control||Grey slime mould is short-lived on lawns and the issue will normally resolve naturally in one or two weeks. The organism may be mowed, broomed or lightly raked off. After it dries out, it can be washed off. A mild detergent solution (4 mL per litre of water) may assist.
Dethatching with a steel-tined rake or dethatching machine can be helpful in reducing food sources and humidity. The problem may be exacerbated by poor drainage. Measures to increase root zone aeration (such as spiking) can also assist in control.
|Similar diseases||Rhizoctonia sp. can produce greying of foliage, however there are no obvious structures on the leaf surface and the plant becomes weakened and may die.|
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