Copper spot

Small (2-5 cm) copper to salmon-pink spots that may coalesce; gelatinous spore masses form on leaves under humidity.

Copper spot presents as scattered patches, 3-8 cm across, with an orange to copper tone. Affected leaves within these patches show small, water-soaked lesions that range from red to brown, which can join together to cover entire leaves. Unlike the sharp boundaries seen in dollar spot, copper spot patches have blurred margins. During wet conditions, the disease can spread following mowing or rolling paths, similarly to Microdochium patch. Despite potential co-occurrence with dollar spot, differences in lesion color and patch edges can distinguish copper spot.

In warm and wet conditions, mycelium and sporodochia develop on the leaves. The sporodochia contain salmon-colored spores and take on a crown-shaped appearance when dry, turning orange to red. Small, black microsclerotia can be found embedded in infected leaves.

The fungus prefers hot and humid conditions, and the disease tends to thrive in the summer months. The fungi produce spores, which are spread by wind, rain, or human activity. These spores germinate and infect the leaf blades when the environmental conditions are favorable. The symptoms start as small, reddish-brown or coppery spots on the leaves that can coalesce into larger patches.

Copper spot occurs during warm, wet weather with optimal air temperatures for infection ranging from 20-30°C (68-86°F). It is particularly damaging to turfgrass that has been fertilized with excessive nitrogen or is grown in highly acidic soil, especially in Agrostis canina. The pathogen, Microdochium sorghi, survives adverse weather conditions and overwinters as sclerotia or mycelia in thatch and infested leaves. When soil temperatures reach a sustained level above 17°C (62°F), the sclerotia germinate to form sporodochia, which produce conidia during warm and moist periods. Conidia can infect leaves through direct penetration or stomata, causing leaf necrosis and the formation of more sporodochia with conidia. Disease development can be rapid, especially when conditions are conducive to infection and prolonged leaf wetness occurs.