Spring Dead Spot Research Targets Better Control in Bermuda

Spring dead spot (SDS) is a major disease that affects bermudagrass in the United States and worldwide. Within the United States, the disease is most prevalent in the northern range of bermudagrass adaptation. Oklahoma State University and Kansas State University researchers are focusing their efforts on gaining a better understanding of the way bermudagrass is infected, with the ultimate goal of developing improved control options. Today we know the disease is caused by three root-rotting fungi: Ophiosphaerella herpotricha, Ophiosphaerella korrae, and Ophiosphaerella narmari (Tisserat et al., 1989). All three fungal species are found in the United States with O. herpotricha being the most abundant causal agent in the Midwest. O. korrae has been located throughout the United States and Australia. O. narmari has been isolated in California, Oklahoma and Kansas, and is a major pathogen in New Zealand and Australia (Wetzel et al., 1999). The fungus usually takes from two to three years to become fully established. Symptoms of the disease include circular bleached and depressed thatch areas from 6 inches to 3 feet in diameter. Once established, the below-ground roots and rhizomes are typically covered with dark brown to black fungal hyphae. Like many root-rotting fungi, this fungus is most active in the early fall and spring when temperatures and moisture favor fungal growth and when bermudagrass growth slows down. In the fall, infection weakens the bermudagrass root system and predisposes it to winter injury. For this reason, the disease is more common in Northern colder climatic areas and during years of severe winter. Researchers have shown there is a close association between resistance to SDS and resistance to cold temperatures (Baird et al, 1998). In other words, bermudagrass varieties that resist the cold also resist SDS infection. Since freezing temperatures tend to increase damage, it stands to reason that cold-resistant varieties would show less damage than nonresistant varieties and would be less susceptible to attack by opportunistic fungi. Nus and Shashikumar (1993) showed that infection with O. herpotricha and O. korrae reduced the ability of a single bermudagrass line to adapt to cold temperatures, and that this lack was possibly related to changes in cell membrane properties. With the coming of spring and warmer temperatures, bermudagrass breaks dormancy and spring growth continues. In diseased areas, damaged tissue often fails to regrow, leaving Continued on page 52 We know Spring Dead