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Potassium Influence on Susceptibility of Bermudagrass to Helminthosporium cynodontis Toxin

Rice Research Station, Louisiana Agric. Exp. Stn., Louisiana State Univ. Agric. Center, P.O. Box 1429, Crowley, LA 70527-1429

^* Corresponding author.

Inadequate K fertilization of bermudagrass, Cynodon dactylon (L.) Pers., can lead to stand loss and increased disease symptoms. This study was conducted to determine the relationship between fungal leaf spot (Helminthosporium cynodontis Marig.) susceptibility and K status of hybrid bermudagrass ‘Grazer’ and ‘Tifton 78’ bermudagrass were grown in washed sand under six K fertilization rates (10, 30, 70, 100, 200, and 400 mg kg^–1) in a greenhouse. Plants were clipped three times at 28-d intervals and the K concentration in the tissue was determined. Prior to the final clipping, 4 mL of culture filtrate containing toxin (CFT) produced by H. cynodontis was sprayed in a fine mist on each plant. Plants were rated for disease severity according to leaf spot coverage after approximately 36 h. A curvilinear relationship was observed between disease severity and tissue K concentration. Leaf spotting increased when tissue K levels decreased below about 25 g/kg. Tifton 78 was more resistant than Grazer to CFT. Tifton 78 maintained higher tissue K levels at all fertilization rates due to lower biomass production and comparable amounts of K removed. Varietal differences should be considered when determining proper K fertilization rates from tissue analysis.

Received for publication September 8, 1988.