Algae

Algae are simple photosynthetic organisms, not fungi, and in horticultural and turf situations they range from single-celled planktonic forms suspended in water to filamentous mat-forming types and blue-green algae (cyanobacteria). On turf and bare soil, algae typically appear as a thin, slimy, dark green to black film that sits on the surface, often in low, poorly drained or compacted areas. When it dries out it can form a crusty black or grey-green scale that sheds water and prevents seed germination or grass recovery. In ponds, tanks and irrigation lines, algae appear as free-floating green or brown suspensions, stringy hair-like filaments, or pink to black slimy deposits on wet surfaces such as pot bases, benches, shade cloth, drippers and the inside of pipework.

Growth is entirely dependent on light, moisture and available nutrients rather than infecting plant tissue directly, so algae are considered a cultural and environmental problem more than a classic disease pathogen. They proliferate wherever water sits on a surface for extended periods, especially in warm weather, under shade, in areas with poor air movement, or where nutrient runoff (particularly nitrogen and phosphorus) accumulates in irrigation water, retention ponds or on saturated growing media.

In Australian nursery, turf and landscape settings algae are common year-round in irrigated situations but peak over the warmer months when light and temperature both favour rapid growth. Typical sites include waterlogged or compacted turf areas, greenhouse benches and floors, capillary matting, pot surfaces on drip-irrigated stock, dams and irrigation storages, and biofilm build-up inside irrigation lines and filters.

Management focuses on reducing surface moisture and nutrient availability: improving drainage and aeration, avoiding overwatering, increasing light penetration and air movement, correcting compaction, and reducing nutrient runoff into water bodies. Physical removal (scraping, brushing, flushing lines) and periodic cleaning of tanks, benches and irrigation infrastructure help prevent build-up, alongside routine monitoring of irrigation water quality.