Yellow spot recently was observed in the mid-Atlantic. Yellow spot is an undescribed malady, which mainly is seen in creeping bentgrass grown in sand-based root-zones on greens and tees. Symptoms appear in the summer and yellow spots generally range from one to three inches in diameter. Turf within spots does not die, and in most cases, does not even thin-out. Hence, yellow spot is a visual problem. The malady usually appears overnight in large numbers in the summer during periods of high humidity, rain and high night time temperatures (>70ºF). They develop in full sun and shaded environments. Maryland researches demonstrated that yellow spot can develop during dryer periods when there is a need for frequent syringing and irrigation. In that study, almost no yellow spot developed when turf was allowed to wilt prior to irrigation. Yellow spot usually disappears with the advent of cooler night temperatures; however, they can remain evident into early autumn.
The proposed causal agents of yellow spot are cyanobacteria (aka blue-green algae). The mechanism responsible for the chlorotic appearance induced by cyanobacteria is not by infection. According to researches, cyanobacteria filaments migrate at night from thatch to the base of plants, and glide up leaf sheaths and onto leaf blades. It has been suggested that cyanobacteria induce or elicit a chlorotic response either through the production of a toxin or by iron chelating compounds. Other researchers, however, failed to find cyanobacteria in association with yellow spot in some cases. The yellow spot symptoms do mimic the description given for yellow dwarf, a mollicute (i.e., a bacteria-like organism bound only by a membrane) disease of bentgrass golf greens in Japan.
Yellow spot is effectively controlled by Daconil (i.e., chorothalonil) and Fore (i.e., mancozeb) when preventively and fortuitously applied. These same fungicides control blue-green algae on greens and lend evidence that these organisms (i.e., cyanobacteria or mollicute) may be the cause of yellow spot. Curatively, two or three applications of the aforementioned fungicides on a seven day interval effectively reduce yellow spot.