SLIME MOLDS

• Pathogens:  Mucilago spongiosa, Physarum cinereum
• Grasses Affected:  All commonly cultivated turf grasses, as well as the grass weed species found in association with them, are colonized by these two species of slime molds.
• Season of Occurrence:  Spring, summer and early fall.
• Symptoms and Signs:  All above-ground plant parts as well as the surface of the thatch or soil may be covered with a creamy white to translucent, slimy growth.  With time, this slimy overgrowth changes to distinctive, ash-gray fruiting structures.  The affected areas of turf grass assume a dull gray appearance due to the high population of fruiting bodies on the leaves.  The shape of these areas ranges from circular to serpentine streaks, and they vary in width and length from a few inches (5-10 cm) to several feet (3-5 m).
• Conditions Favoring Disease Development:  Slime mold development is favored by prolonged periods of wet weather.  Under cool, humid conditions, the spores of the pathogens absorb water, the cell walls crack open, and a single, motile swarm spore emerges from each.  These motile swarm spores ingest microorganisms and decaying organic matter, and leave behind the undigested debris.  With time, they divide several times and then change their form by retracting their flagella and becoming more rounded.  Ultimately, the spores united in pairs, nuclear fusion results, and zygotes are formed.  The zygotes are non-flagellate and continue their existence as naked, amoeboid cells which ingest food, increase in size, and become multinucleate due to a series of mitotic nuclear divisions.  This growth form of the organism is call the plasmodium, and is the stage of growth that creates the slimy appearance on the leaves of turf grass plants.  Eventually, the plasmodium undergoes changes that lead to the extensive production of fructifications.  The process by which the fruiting structures are formed is a complex one, and it varies among species.  Spread of the pathogens is accomplished primarily by wind-borne spores.  With the accumulation of the plasmodium on the surface of the leaves, there is an exclusion of light and thus an interference with respiration and transpiration processes.  This in turn leads to a disturbance of the metabolic activity of the underlying leaf cells.  As the fruiting structures of the slime molds are formed, even more light is excluded from the leaves.  In time, the impaired physiological activity of the leaf cells results in overall leaf chlorosis.  Consequently, the leaves are predisposed to invasion, and in some cases, complete destruction by secondary bacteria, yeasts and fungi.