| Summary: | Phytophthora infestans (Mont) de Bary is a devastating pathogen in potato
producing regions around the world. Populations of the organism in Canada shifted
during the mid-1990’s as the US-1 strain (A1, metalaxyl-sensitive) was displaced by the
highly aggressive, US-8 strain (A2, metalaxyl-insensitive). An increase in the incidence
and severity of late blight has followed. Late blight is controlled by cultural practices
aimed at eliminating disease sources and by the application of foliar fungicides. Tubers
can become infected at harvest from contact with blighted vines leading to severe losses
in storage. In many production areas, growers desiccate vines two to three weeks prior to
harvest to reduce late blight tuber rot. However, in Manitoba, because of the loss of
potential yield that results from vine killing prior to harvest in a late maturing cultivar
such as Russet Burbank, growers are reluctant to adopt this practice. The objective of this
study was to develop recommendations for preharvest management practices that reduce
storage losses due to late blight. Field trials were conducted in 1997 and 1998 to
investigate the effect of vine desiccation with diquat and/or a late season application of
chlorothalonil and copper hydroxide on yield, processing quality, and disease
development in storage. Desiccating vines with diquat two weeks prior to harvest reduced yield and tuber size. Compared to the untreated control, the largest reductions in
marketable yield were observed for the early September harvest. By the late September
harvest, however, the effect of vine killing in reducing marketable yield was less
apparent. Specific gravity was lower in the vine killed treatment for all harvest dates in
1997 and in the early and mid September harvests in 1998. Vine killing did not contribute
to elevated levels of reducing sugars or consistently darker fry colour at harvest or during storage. Skin-set was improved when vines were desiccated for all harvest dates in 1997
and at the early September harvest date in 1998. Vine killing reduced tuber rot in storage
caused by Fusarium dry rot and Pythium leak for the early and mid September harvest
dates in 1997. The incidence of late blight tuber rot was reduced in storage for the early
September harvest in 1998 when vines were desiccated. The late-season application of
chlorothalonil and copper hydroxide did not reduce tuber rot in storage in storage either
year. Results from this study indicate that vine killing two weeks before an early to mid-
September harvest is not recommended in Manitoba because of reductions in yield and
specific gravity. Alternative management practices to reduce late blight tuber rot in
storage should be investigated.
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