Leaf senescence as a factor in the competitive and synergistic interactions of Drechslera sorokiniana and Curvularia geniculata on Poa pratensis

Abstract

The competitive and synergistic interactions of Drechslera sorokiniana and Curvularia geniculata on pathogenesis of leaf spot was evaluated on sequentially senescent leaves of Poa pratensis. Leaf age had no effect on the mean number of lesions produced from inoculations with D. sorokiniana or C. geniculata alone. The combination of D. sorokiniana and C. geniculata produced a significant increase in the mean number of lesions between the youngest and oldest leaves at 20, 25, and 30 °C. Disease severity on leaves of all ages resulting from inoculations at 20 and 25 °C was greatest in response to D. sorokiniana alone, followed by the combination of organisms, and by C. geniculata alone, in descending order. No difference in disease severity occurred on leaves of different ages in reponse to the inoculations at 20 °C, but a significant increase in disease occurred on the two oldest leaves at 25 °C in response to inoculations with D. sorokiniana and to the combination of organisms. At 30 and 35 °C, disease severity on leaves of all ages was greatest in response to inoculations with the combination of organisms, D. sorokiniana alone, and C. geniculata alone, in descending order. Disease severity increased from the youngest to the oldest leaves at 30 °C in response to inoculations with the combination of organisms or with D. sorokiniana alone. Reisolations established that inoculations with D. sorokiniana alone resulted in successful establishment of the pathogen in the tissue of leaves of all ages at 20, 25, and 30 °C. At these temperatures, inoculations with C. geniculata alone produced a significantly higher level of reisolation from the youngest to the oldest leaves of the shoot. Reisolation of the individual organisms from leaves inoculated with both organisms showed that C. geniculata established more successfully than D. sorokiniana in leaves of all ages as temperatures were increased. It was concluded that the synergistic increase in disease produced by the combination of D. sorokiniana and C. geniculata at high temperatures is the result of increased high-temperature growth of C. geniculata which is probably more saprophytic than parasitic on senescing, heat-stressed leaves and does not reflect a high-temperature increase in virulence of this organism.