Experiments were conducted in an irrigated, sandy loam soil to evaluate mulches and hill sizes as barriers to prevent the development of potato tuber blight caused by Phytophthora infestans. In mulching experiments, five treatments were applied to field plots of cv. Red LaSoda: 1, no mulch; 2, polyurethane spray foam in an 8-cm-diameter area immediately surrounding the plant stem; 3, black polyethylene film over the entire hill except near the stem; 4, a combination of treatments 2 and 3; and 5, a water-permeable, agricultural textile treated with copper hydroxide applied over the same hill area as in treatment 3. In 1998, the incidence of tuber blight in plots mulched with black film (treatments 3 and 4) averaged 32% compared with 56% in plots without this mulch (treatments 1 and 2). In 1999, incidence of tuber blight in plots with and without black film averaged 9 and 20%, respectively. Mulching the stem area with spray foam (treatments 2 and 3) did not reduce the incidence of blighted tubers when compared with the appropriate control. The copper-treated textile mulch (treatment 5) provided reductions in the incidence of tuber blight similar to those observed with the use of black polyethylene film. In a hill size experiment conducted once in 1998 and twice in 1999, three hill size treatments were established on cvs. Red LaSoda, Shepody, and Russet Burbank. Red LaSoda was the most susceptible and Russet Burbank the least susceptible to tuber blight. Comparison of blight incidence in tubers classified by depth in the hill revealed few differences among the hill size treatments, although over all treatments, tubers covered with more than 15 cm of soil had a lower incidence of blight (1 to 14%) than tubers with less soil cover (13 to 59%). Most tuber infections were apparently initiated in eyes and were not concentrated on a portion of the tuber such as the stolon (proximal) or distal end. The fact that black film and textile mulches reduced tuber infection indicates that inoculum of P. infestans can move from foliage to tubers through soil and that inoculum movement is not limited to large channels in the hill such as those created by the potato stems. The mulch treatments, however, provided only partial protection of tubers, limiting the practicality of such treatments to commercial producers. Hill size treatments had little effect on tuber blight incidence, indicating that adequate suppression of tuber infection in an environment conducive to late blight may be inseparably linked to adequate suppression of the foliar phase.