Currently, the average 2-inch under-sod soil temperatures for much of Oklahoma are in the low-to-mid 80s according to the Oklahoma Mesonet weather station network. In Stillwater, we are averaging around 90 F. Thus, root growth on cool-season grasses is not occurring. Research has also shown that as air and soil temperatures rise above 90 F roots will start to die. With high temperatures yesterday (June 10) in the triple digits and low temperatures not falling below 70 F (except for the pan handle) throughout Oklahoma, root mortality on cool-season grasses is going to be common in the coming days. For some areas the situation has been made worse due to prolonged drought and early onset of heat. According to the Oklahoma Climatological Survey, the month of June was the 2nd warmest and 4th driest on record since 1895. We have an average statewide precipitation deficit of 3.08 inches.
Turfgrass plants that entered summer without a sufficient root system are more likely to not sustain growth, turn yellow or brown and go into general decline (see images). Symptoms of decline can resemble diseases caused by pathogens. Patch symptoms due to summer stress may look much like take-all patch. Turf can appear off color or golden brown and in areas where the plant stand is thin algae formation is common (see images). When roots are examined no evidence of the pathogens that cause these patch diseases are typically present. While evidence of root inhabiting fungi will often be noted, many of these fungi are simply naturally occurring root-inhabiting organisms that are not pathogenic. Also beware that while it may appear to the naked eye that the grass plant has deep or healthy roots this may not be the case. The vascular portion of the root is resistant to decay and may appear to be healthy when in fact it is dead.
Weather conditions in general have been too hot and dry for most turf pathogens. However, we did have one case of Pythium blight on a putting green where a severe case of scalping occurred and excessive irrigation was applied for multiple days. This resulted in an excessively wet environment that favored Pythium development. We also have observed brown patch on our research putting greens in Stillwater. Brown patch epidemics began this weekend and have increased over the last day or so.
Hopefully the heat will pass soon and we will have a nice cool August. Hey, stranger things have happened…