Copper Spot (Zonate Leaf Spot)

Copper spot, also known as zonate leaf spot, primarily affects Agrostis spp. It is particularly common in A. canina (velvet bentgrass) and A. capillaris (colonial bentgrass) in the northeastern United States. However, the pathogen responsible for copper spot can also cause zonate leaf spot in other grass genera such as Cynodon, Zoysia, and other Poaceae species.

 

Causal agents

Microdochium sorghi fungus

 

Primary Hosts

Agrostis spp. It is particularly prevalent in A. canina and A. capillaris. In warm-season grasses, the disease caused is referred to as zonate leaf spot.

 

Disease Cycle

The causal agent of copper spot is Microdochium sorghi (formerly known as Gloeocercospora sorghi). It produces sporodochia, which are fruiting bodies that emerge through stomata in the leaf epidermis. The conidia, which are the reproductive spores, are hyaline (transparent), filiform (needle-shaped), and have one to seven septa. They are 1.4 to 3.2 by 20 to 95 um in size, with an average size of 2.4 by 83 um. The conidia are produced in a gelatinous matrix and have a salmon color when seen in large numbers. Additionally, small, black sclerotia, measuring 0.1 to 0.2 mm in diameter, can develop in necrotic leaf lesions. No sexual stage has been observed for this pathogen.

 

Epidemiology

Copper spot occurs during warm, wet weather with optimal air temperatures for infection ranging from 20 to 30°C (68 to 86°F). Copper spot 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.

 

Symptoms

Copper spot exhibits scattered circular patches, ranging from 3 to 8 cm in diameter, with an orange to copper color. Individual leaves within the patches display small, red to brown, water-soaked lesions that can merge to affect entire leaves. Copper spot patches have more diffuse margins compared to the well-defined margins of dollar spot patches, allowing for differentiation between the two diseases. During wet weather, copper spot infection centers may spread in the direction of mowing or rolling, similar to the spread pattern observed in Microdochium patch. Co-occurrence with dollar spot is possible, but differences in lesion color and patch margins help distinguish copper spot from dollar spot.

 

Credit: John Kaminski/Penn State University

Figure 13.4.1: Symptoms of copper spot on bentgrass include (a-d) small reddish-brown spots that may coalesce under severe pressure.
 
 

Signs

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.

 

Credit: John Kaminski/Penn State University

Figure 13.4.2: Signs of the pathogen include (a&b) black, lenticular to spherical microsclerotia, 0.1-0.2 mm in diameter. (c) Conidia, borne on the sporodochia, are (d) needle-shaped and average 1.4 x 83 um. 
 
 

Management

Copper spot is not a common disease, but it is one of the few diseases that is enhanced by lower pH soils. Improving growing conditions for the turfgrass, judicious use of nitrogen, and fungicides can help to reduce disease severity.

 

Cultural

To manage copper spot, it is recommended to follow practices that are effective in controlling dollar spot. Dew should be removed in the morning through mowing, rolling, or dragging. Air circulation can be increased by pruning trees or installing fans to reduce the leaf wetness period. During periods of active copper spot, limit applications of water-soluble nitrogen sources, typically not exceeding 0.25 lb per 1,000 ft² (12 kg N/ha) per application. Avoid mowing infested turf when the foliage is wet. It is important to maintain a soil pH of 6 to 7. Lime applications may be beneficial in reducing disease severity in soils with pH levels below 5.5. New cultivars of Agrostis canina with improved resistance to copper spot are available, and consulting turfgrass specialists or the National Turfgrass Evaluation ProgramLinks to an external site. can provide information on cultivars with increased tolerance.

 

Chemical

Effective fungicides for controlling copper spot include:

  • azoxystrobin
  • chlorothalonil
  • fluazinam
  • azoxystrobin + chlorothalonil
  • propiconazole + chlorothalonil
  • iprodione + thiophanate-methyl
  • DMI fungicides

 

Be cautious with the use of sequential applications of SDHI fungicides like boscalid. Studies have shown that such applications can actually increase the incidence and severity of copper spot. Therefore, when dealing with high disease pressure, it’s advisable to avoid using SDHI fungicides alone or to tank mix them with copper spot fungicides to mitigate the negative effects on disease control.