When scientific understanding of a disease evolves, terminology must change to reflect new knowledge. The turfgrass disease formerly known as “Fusarium Patch” is a clear instance of this taxonomic shift, where research refined the identification of the true causal agent. This older term is now considered outdated. The disease known as Fusarium Patch is now correctly referred to as Microdochium Patch.
The Historical Context of the Nomenclature
Turfgrass managers historically applied the name “Fusarium Patch” to a disease appearing in cool, wet weather during autumn, winter, or early spring. Symptoms included small, circular patches of blighted grass, initially water-soaked before turning reddish-brown, tan, or gray. These patches typically ranged from 1 to 3 inches in diameter but could coalesce into larger damaged areas.
A distinguishing feature was the presence of a reddish or pinkish fungal growth visible around the patch perimeter, especially in the early morning. This observation led early researchers to attribute the disease to the Fusarium genus, as these fungi were frequently observed with the diseased tissue. The assumption was that a Fusarium species was the primary pathogen responsible for the damage.
The disease was common and damaging on fine turf, such as golf greens and tees, maintained at low mowing heights. Environmental conditions favoring its development included prolonged leaf wetness, high humidity, and cool temperatures between 32°F and 60°F. For many years, management strategies targeted a Fusarium species with chemical controls.
The Pathogen Reclassification
The shift in nomenclature began when mycologists conducted detailed microscopic and molecular analyses of the pathogen. The fungus initially identified as Fusarium nivale lacked the conidial foot cell, a specific microscopic feature characteristic of true Fusarium species. This discovery suggested the fungus causing the disease was incorrectly classified.
The first reclassification in 1980 changed the pathogen’s name to Gerlachia nivalis, and later to Microdochium nivale. This change was based on the fungus’s shared features with other species within the Microdochium genus. Fungi belonging to the Fusarium genus were found to be secondary invaders or saprophytes, colonizing tissue already damaged by the primary pathogen, Microdochium nivale.
The term “Fusarium Patch” persisted due to the long-standing use of the name and the difficulty in changing established industry language. However, the scientific community and turfgrass pathologists adopted the more accurate name to reflect the true identity of the fungal species.
The Modern Terminology: Microdochium Patch
The accepted modern name is Microdochium Patch, which directly references the fungal genus Microdochium. The same pathogen, Microdochium nivale, also causes Pink Snow Mold. This dual terminology is based on the presence or absence of snow cover, with Pink Snow Mold describing the disease’s appearance after the melting of a long-lasting snow layer.
Microdochium Patch is most destructive on cool-season turfgrass species, especially annual bluegrass (Poa annua) and creeping bentgrass (Agrostis stolonifera). The disease thrives when temperatures are cool, typically between 32°F and 46°F, and is exacerbated by prolonged periods of leaf wetness from fog, dew, or rain. Susceptibility is also increased by high nitrogen fertility in the fall, excessive thatch, and poor air circulation.
Symptoms include the formation of small, circular patches that may exhibit white or pinkish fungal growth, known as mycelium, visible on the leaf blades during wet conditions. The fungus attacks the leaf tissues but typically does not progress into the crown or roots. This means the turf often recovers when conditions become drier and warmer.