Potassium Fertilization Affects Psychrophilic Pathogen Susceptibility of Annual Bluegrass.

Typhula incarnata Fr. (TI) and Microdochium nivale Fr. (MN) are important psychrophilic pathogens of cool-season turfgrasses. Existing field data suggests that K fertilization may affect disease severity, warranting additional experimentation under controlled conditions. In a greenhouse, annual bluegrass [Poa annua var. reptans (Hauskn) Timm.] was seeded into 120 – 30 x 10 cm diam. sand filled columns. Nitrogen (0.5 g m² ), K (0.5 g m²), and all other plant essential nutrients (PENS) were applied weekly for 90 d. Following establishment, weekly application rates of N and PENS remained constant, yet 5 different K treatments (0, 0.25, 0.5, 2, 3 g m²) were imposed for 90 d. Columns were then moved to a refrigerated room, maintained under a photosynthetically active radiation flux of ~300 υmol m^-2 s^-1, and day/night air temperature incrementally decreased every 7 d over 28 d (10/4°C, 4/-2°C, 2/-4°C, -2/-6°C). Plants were then buried under 10 cm of snow and kept under darkness at -4°C for 28 d. After thawing at 2°C, 8 replicates of each K TRT were inoculated with a 10 mm agar disc taken from TI, MN, or sterile cultures. Columns were incubated at 2°C (40 d) then 4°C (45 d) under periodic misting and evaluated for % necrotic turf every 10 d. Survival analysis of days to 50% necrosis (Day₅₀) was used to quantify disease progression. The experiment was conducted twice and data was pooled. Potassium treatment significantly affected Day₅₀ in TI (Pr>χ² =0.007) but not MN (Pr>χ² =0.277) inoculated turf. Regression of Day₅₀ by tissue K content at time of inoculation revealed an inverse linear relationship in both TI (Pr>F=0.0009, r²=0.98) and MN (Pr>F=0.08, r²=0.69) inoculated turf. Additional experimental data suggests that K fertilization affects psychrophilic pathogen susceptibility by modifying metabolic rate and/or fate of metabolic intermediates.