Evaluation of Resistance to Ergot, Caused by Claviceps purpurea, in Kentucky Bluegrass, Based on Incidence and Severity Estimates

Ergot, caused by Claviceps purpurea (Fr.:Fr.) Tul., is an important disease of grasses grown for seed. Ergot can be especially severe in Kentucky bluegrass (Poa pratensis L.) seed production, with yield losses estimated as high as 25% (2). Claviceps purpurea invades the ovary, replacing the potential seed with a hard black sclerotium. Sclerotia are typically one to several times longer than the host seed and black in color, making visual detection relatively easy. Spread of conidia of C. purpurea, which ooze from infected florets in a mixture of conidia and a sugary syrup derived from the plant sap (honeydew), can occur through seed head to seed head contact, rain splashing, or insect movements. The flower is the only organ infected (3). Ergot control through fungicide application at flowering is possible but expensive. Host resistance is the preferred approach to disease control. However, few quantitative studies of resistance to ergot in Kentucky bluegrass are available, and there is little information concerning variability associated with different methods of ergot assessment. In the cereal grains, ergot has been assessed preharvest, as percentage of spikes with sclerotia (13), or percentage of florets with sclerotia (4,7,10) or as postharvest estimates of percent sclerotia among seed by weight (9). Expression of ergot in Kentucky bluegrass as percentage of flowers infected is difficult, since individual flowers are small and clustered into numerous tightly packed spikelets on panicles. Methods of disease assessment have included number of sclerotia per panicle (6) or per spikelet (5), sclerotial weight per panicle (6), percent seed replaced by sclerotia (2), or disease ratings of sclerotia or honeydew production (12). Cultivars vary in number of flowers per panicle and in timing and duration of flowering, potentially confounding disease resistance trials. Considerable yearto-year variation can occur, with wet conditions during flowering favoring disease development (1), although quantitative measures of variability are lacking. Information on year-to-year variability and within-year variability is needed for effective evaluation of commercial or experimental cultivars for resistance to ergot. In addition, an understanding of advantages and disadvantages of the various assessment methods appropriate for determining host resistance to ergot would facilitate selection of assessment methods for both reliability and efficiency. The objectives of this study were to determine if ranking of cultivars or experimental selections in terms of susceptibility to ergot varies depending on methods of disease assessment, to determine to what extent year-to-year variability can be accounted for by rainfall data, and to evaluate resistance among cultivars of Kentucky bluegrass to C. purpurea.