Acfa Agric. Scatid, Serf. B, Soil and Plant Sci. 1991: 44. 226-229 Primed in De)t))wrk . All rights reserred. ). Effect of plant age and humidity on development of bacterial wilt of forage grasses. Accepted January 11, 1991. Acta Agric. Scand., Sect. B, Soil and Plant Sci. 44: 226-229, 1994. 0 Acta Agric.Scand. 1994.The effect of plant age on the development of bacterial wilt of forage grasses wvas studied by dipping cut roots of Loliirrii pereririe L. cv. 'Svea' in a suspension of Xnrithonionns cnriipesfris pv. grmniiiis. The effect of humidity on disease development wvas studied by cutting leaves with scissors dipped in inoculum, and by spraying inoculum onto unwounded leaves. Young plants were more susceptible to the pathogen than old ones. X.c.grniiiinis was easily isolated from young leaves and tillers, especially from the leaf bases. Disease symptoms were most severe at low humidity. However, high humidity is necessary for infection to take place through stomata and hydathodes.