PEST BULLETIN Cool Weather Brown Patch & Related Diseases November 11, 2002 Nov. 11: Now that we are in full "prewinter" mode with the temperatures fluctuating between freezing and unseasonably mild, we may start seeing some of the early spring diseases reoccurring at this time. This is especially true of today's weather: cloudy, drizzly, and mid '50s. These conditions are ripe for activity of CW Brown Patch or other cool weather pathogens. With winter right around the corner, a targeted fungicide application for CW Brown Patch is probably not necessary, and may be best applied if it becomes a problem again next spring. The recent cool-down in air and soil temperatures is conducive to development of some Rhizoctonia diseases, such as Yellow Patch (a.k.a Cool Weather Brown Patch) caused by R. cerealis, and yellow rings or patches caused by R. zeae.   Rhizoctonia cerealis is the most commonly found Rhizoctonia fungus infecting turf during the colder temperatures (<50 F) of mid to late fall and early spring.   R. cerealis causes a distinctive thinning, "yellow patch" that looks very similar to a summer brown patch caused by R. solani (see photo #1).   This cool temperature brown patch can occur on both creeping bentgrass and mixed bent/Poa greens when conditions are favorable.   Once air and soil temperatures rise into the 50 - 60F range (and turf growth increases), R. cerealis Yellow Patch usually disappears. Perhaps more common on Poa annua or mixed bent/Poa greens are the distinctive, expanding yellow rings caused by Rhizoctonia zeae (photos 2,3).   These rings are usually observed in mid to late spring, when air and soil temperatures are in the 55 - 65 F range.   This disease has no official name at the moment, but is considered another form of cool temperature brown patch. R. zeae infection appears to be limited to the Poa in the stand of turf; bentgrasses in and around the rings or patches do not show yellowing symptoms.   This can cause the yellow ring to appear broken, or as a discontinuous arc or semi-circle, and may cause the ring to expand at different rates or to overlap with other rings.   In any case, the shape of the ring is usually suggestive of a Rhizoctonia infection, because of its "wavy" or scalloped appearance.   Photo 1 Photo 2 Photo 3 (click on an image to enlarge it.) Once air and soil temperatures warm further in late spring to early summer, R. zeae rings will disappear, similar to the R. cerealis patches.   However, it is interesting to note that R. zeae is also regarded as a serious high temperature pathogen of turf in summer in the southeastern US, and has been observed to cause more typical Brown Patch symptoms (on creeping bent as well as Poa) in Illinois during very hot and humid weather. Both of these cool weather Rhizoctonia diseases will subside when warmer air and soil temperatures stimulate green-up and faster growth of turf.   Therefore, in most cases, we do not recommend a fungicide treatment for cool temperature Rhizoctonia patches.   Also, we know from experience that many of the fungicides that control summer brown patch have little to no beneficial effect on cool weather brown patch diseases.   On the other hand, if symptoms are severe and you feel that a treatment is warranted, Rhizoctonia specific fungicides such as flutolanil (ProStar), or the broad spectrum QoI fungicides azoxystrobin (Heritage) and trifloxystrobin (Compass) may provide some relief. Photo #1.   Rhizoctonia cerealis causing a large patch in mild temperatures (40 - 55 F). Photo #2. Rhizoctonia zeae rings affect Poa stands in turf, causing rings to appear distorted or broken.. Photo #3.   Rings caused by Rhizoctonia zeae have been widespread on fairways and tees in spring 2002. .