Abstract Physical shocks to barley coleoptiles prior to inoculation, such as electric and heat shocks and UV irradiation, allowed the nonpathogen Erysiphe pisi to produce haustoria. UV irradiation was highly effective in that a 30 min irradiation period allowed 85% haustorium formation. The effect of UV irradiation varied with the location of cells in the coleoptile. A 7 min irradiation resulted in 90% haustorium formation in proximal cells but only 27% in distal cells, suggesting that metabolic activity varied among those cells. Although the average size of papillae was not affected by up to 7 min UV irradiation for subpopulations where haustoria either were or were not formed, the sizes of papillae formed when haustoria were formed were significantly smaller than when haustoria were not formed.