Here is a Tift 85 pasture that has been cut twice so far and is showing symtpoms down on the leaves and lower down the stem. We are seeing both leaf blight (helminthosproium) and leaf rust. These issues we commonly see in late summer when weather is warm, usually between 75 degrees F and 90 degrees F, and with high relative humidity.
Bermudagrass leaf spot is caused by a fungus from the genus Helminthosporium, and the disease has been informally called Helminthosporium leaf spot, Helminthosporium leaf blotch, or Leaf Blight. Leaf lesions of helminthosporium are irregularly shaped and brownish green to black in color. We may see it in irregular patches. Leaf spots are more numerous near the collar of the leaf blade.
Leaf rust or Puccinia disease has similar impacts as Helminthosporium. We will also see red to orange lesions can on leaf and stem. Look for a raised area or blister which is the rust postules like we see in wheat and corn. Rubbing your finger over the leaf will leave a rusty color.
Management is strictly avoidance. Coastal, Tift 44, and Tift 85 have some level of resistance while Alicia is extremely susceptible. But even less susceptible varieties are infected with leaf spot when potassium is low. Most reported leaf spot cases are directly related to low soil potash. Nutrients are removed from bermuda hay fields in about a 4-1-3 ratio of N, P2O5, and K2O with harvest. We need 75 percent as much potash as nitrogen applied each season. Split applications of K are better in sandy soils. With helminthosporium, removing the inoculant is also recommended. In addition to tying up nutrients, thatch holds water and reduces air circulation. This is a conducive environment for inoculum. The only practical way to reduce thatch is burning in spring before green-up.
Visit Leafspot Diagnosis and Management in Bermudagrass Forages for more information.