Response to leaf inoculations with Macrophomina phaseolina in white clover

Summers in the southeastern USA produce a harsh environment for survival of white clover (Trifolium repens L.) stolons. Long periods of drought and ho t temperatures are interspersed with rain showers that create ideal conditi ons for fungal pathogenesis. Previous studies indicated that Macrophomina p haseolina (Tassi) Goidanich may be an important pathogen that limits surviv al of white clover stolons in the summer. The objective of this study was t o determine the range in response of 20 white clover cultivars, germplasms, and breeding and naturalized populations for resistance to M. phaseolina u sing a leaf tissue assay. Discs were cut from leaves excised from 50 plants of each entry and inoculated with an agar plug cut from the margin of a M. phaseolina colony. Leaf discs were scored according to the rate of necrosi s induced by the pathogen. The experiment was conducted as a randomized com plete block with four replicates and was repeated with 50 additional plants from each entry. Differences in responses of entries to inoculation with M . phaseolina were observed in each run of the experiment. Brown Loam Syn. 2 germplasm and North GA population had the least disease and the greatest n umber of plants selected as resistant to M. phaseolina. Large-leaf plants s elected for resistance gave highly consistent responses when retested, with 35% of the plants having no leaf necrosis following inoculation with M. ph aseolina. The leaf tissue assay was not as reliable for selecting consisten t resistant phenotypes among small-leaf white clover entries, as 37% of the plants selected as resistant were rated as susceptible upon retesting. Res istance to M. phaseolina was observed in adapted white clover germplasm, an d development of new cultivars with this resistance should improve white cl over summer survival.