ARS | Publication request: Potassium influences forage bermudagrass yield and fungal leaf disease severity in Mississippi

Submitted to: Forage and Grazinglands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2012
Publication Date: July 25, 2012
Repository URL: http://handle.nal.usda.gov/10113/54319
Citation: Read, J.J., Pratt, R.G. 2012. Potassium influences forage bermudagrass yield and fungal leaf disease severity in Mississippi. Forage and Grazinglands. doi:10.1094/FG-2012-0725-01-RS.

Interpretive Summary: Potassium (K) deficiency of bermudagrass is associated with a fungal leaf spot disease of significant importance to forage and turf producers in the southeastern US. During 2010, a naturally occurring epidemic was observed in plots of ‘Tifton 44' bermudagrass in Mississippi that tested low in soil K and were fertilized with different rates and sources of K. The team of scientists evaluated the relationships of K in fertilizer and plant tissue to disease severity and forage yields. Results demonstrated a negative association between forage yield and disease severity (r = -0.60). Plants provided 103 and 201 kg K/ha had both increased forage yield and decreased severity of disease symptoms, as compared to the 0 or 33 kg K/ha treatment. The team also determined, for the first time, whether K fertilization affects the identity and frequency of individual species of fungal pathogens that comprise the disease complex. Six species of fungal pathogens were observed in symptomatic leaves. Frequencies of four, variable pathogens differed (P=0.01), but were not affected by K treatments, and the pathogen x K treatment interaction was not significant. Therefore, the reduced disease severity with K treatments did not result from changes in frequencies of pathogens. Since symptoms still occurred at the highest K levels, the beneficial effects of K fertilization may not be strong enough to warrant its use as a sole cultural control practice for fungal leaf spot pathogens in bermudagrass.

Technical Abstract: Leaf spot diseases are associated with K deficiency in forage bermudagrass. In 2010, a natural disease epiphytotic caused by six species of Bipolaris, Curvularia, and Exserohilum (dematiaceous hyphomycetes) was evaluated in 56 plots of ‘Tifton 44' bermudagrass in Mississippi. Pathogen occurrence, disease severity and forage dry matter (DM) were determined following a single spring application of K fertilizer (0, 33, 67, 134, and 201 kg K2O/ ha, 0-0-60), poultry litter (2.24 Mg/ha) and NPK fertilizer (15-5-10). Values for DM yield in August increased linearly across five K rates (r = 0.56, P < 0.01), and were negatively associated with disease severity (r = -0.60, n=14) and percent green tissue (r= 0.78). Frequencies of fungal pathogens differed significantly (P < 0.01) and ranged from 100% in B. hawaiiensis and E. rostratum to 3% in C. geniculata. Among four variable pathogens, K treatment and pathogen by K interaction effect was not significant (P > 0.50). These results suggest increased DM and percent green tissue, and decreased disease severity, observed at 134 and 201 kg K2O/ ha, did not result from changes in the frequency of pathogens in the disease complex. Consistent with research in other crops, K fertilization of K-deficient bermudagrass enhanced DM yield and reduced the severity of fungal disease. However, because K fertilization did not eliminate disease symptoms, it should not be expected to provide complete cultural control of leaf spot in Tifton 44 bermudagrass.