- Late blight, caused by Phytophthora infestans, is a devastating problem to potato production in many parts of the world. While the foliar phases of this disease are well characterized, tuber infection, leading to quality losses and disease perpetuation, is less well understood. Experiments were conducted during 1998 and 1999 in irrigated sandy loam soils to evaluate the relative importance of cultural treatments, i.e. mulches and hill sizes, as barriers to the movement of P. infestans inoculum from potato foliage to developing tubers. In the mulching experiment, five treatments were applied to field plots of cultivar 'Red LaSoda' in a randomized block design: 1) no mulch, 2) expandable polyurethane spray foam in the 8 cm-diameter immediately surrounding the potato stem, 3) black polyethylene film over the entire hill area except the 8 em-diameter immediately surrounding the potato stem, 4) a combination of treatments 2 and 3, and 5) a copper hydroxide-treated textile applied over the same area as in treatment 3. When compared to the appropriate control, black polyethylene film reduced tuber blight incidence by 10 to 24% while polyurethane
spray foam did not reduce (P>0.1) tuber blight. Copper hydroxide-treated textile
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also reduced (P≤0.05) tuber blight incidence by 10 to 33%. In the hill size
experiment, conducted once in 1998 and twice in 1999, potato cultivars 'Russet Burbank', 'Red LaSoda' and 'Shepody', were planted as whole plots in a split-plot design with three hill size treatments. Hill size treatments were established by hilling with the planter only (small hill) and by supplementary hilling with tractor mounted disks once or twice after potato emergence (medium and large hills, respectively). In all three trials, consistent and significant differences (P≤0.05) in tuber blight incidence were observed among the cultivars with 'Red LaSoda' the most susceptible and 'Russet Burbank' the least susceptible to tuber blight. Hill size had a significant effect (P≤0.05) on tuber blight incidence only in 1998 when 40% of tubers in small hills and 30% tubers in medium and large sized hills were blighted. Comparison of tuber blight incidence examined by tuber depth in the hill, however, revealed few differences among hill size treatments, although, deep tubers had a lower incidence of tuber blight (5.5%) than tubers at shallow (34.1%) or intermediate (19.7%) depths. Because black polyethylene film and copper hydroxide-treated textile reduced the incidence of tuber blight, movement of P. infestans inoculum from potato foliage to tubers does not appear to be restricted to large channels in the soil such as those created by the stems. The difficulty of direct tuber blight suppression after establishment of a foliar epidemic suggests that prevention of tuber blight in a conducive environment may be inseparably linked to suppression of the foliar phase.
Management of tuber blight remains one of the least understood areas in the late blight disease cycle, in part because tuber blight is difficult to assess and quantify. To help overcome problems associated with tuber blight assessment, a P. infestans
(US-8) isolate was transformed with the reporter gene β-glucuronidase (GUS) for use in future epidemiological studies of the tuber stage of the late blight disease cycle.