Post-harvest applications of zoxamide and phosphite for control of potato fiber rots caused by oomycetes at harvest

Potato storage tuber rots caused by the late blight and pink rot pathogens at harvest can cause severe economic losses, warranting the need for effective post-harvest fungicide applications. The purpose of this study was to evaluate the efficacy of select post-harvest fungicides in reducing tuber infections by the late blight and pink rot pathogens when applied at various post-inoculation time intervals. 'Russet Burbank' potatoes were inoculated by submersion in an aqueous suspension of Phytophthora infestans or Phytophthora erythroseptica zoospores at 0, 1, 2, 4, and 6 h prior to receiving a post-harvest treatment. Products evaluated were zoxamide (various rates and formulation), phosphite (335 g a.i/MT), and a hydrogen peroxide/peroxyacetic acid mixture (HPPA, 9 g a.i./MT), all applied at 2.08 UNIT of tubers as a low pressure spray prior to storage. Zoxamide and phosphite significantly reduced late blight and pink rot incidence and severity when applied immediately after inoculation. HPPA was less effective at controlling disease development. Phosphite was effective at reducing late blight development at all time intervals up to 6 h post-inoculation (7% vs 80% in untreated). Zoxamide appeared to have good post-harvest disease control if applied soon after inoculation. The maximum time intervals between inoculation and treatment where significant reductions in pink rot incidence were observed was 0 h for HPPA (28%), 2 h for zoxamide (55%; 64 g a.i/MT) and 6 h for phosphite (13%) compared to the untreated (73%). Phosphite provided consistent disease control even when applied several h after inoculation and has potential to be a reliable post-harvest fungicide for the potato industry.