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Fungicide application interval and hybrid tolerance for management of downy mildew (Pseudoperonospora cubensis) in pickling cucumber

Cheryl Trueman: University of Guelph

<div>Cucurbit downy mildew (CDM) (<em>Pseudoperonospora cubensis</em>) is an important cucumber disease in Ontario, Canada. CDM tolerant hybrids ‘Citadel’ (CT) and ‘Peacemaker’ (PM) may allow for longer fungicide application intervals. Early (E) and late (L) seeded trials were completed at Ridgetown (RT) and Simcoe (SI) to evaluate CT, PM, and susceptible ‘Vlaspik’ (VP) with no fungicide, and seven, 10, and 14-day (d) intervals. At E-RT, CDM severity was low, seven-d VP had 100% lower area under the disease progress curve (AUDPC) than the untreated, and there was no interval effect on total yield. E-SI had moderate CDM pressure, and AUDPC among hybrids was 78 to 100% higher with no fungicide than any interval. Total weight was 20% higher for the 10-d interval than the untreated. High CDM pressure occurred in L-seeded trials. At L-RT AUDPC decreased with decreasing interval, regardless of hybrid tolerance. Results were similar at L-SI except 10 and 14-d intervals were equivalent. Total fruit weight was 80% higher for all hybrids using a seven or 10-d interval at L-RT, but at L-SI, any fungicide increased yield in VP, while for CT only seven and 10-d intervals increased yield versus no fungicide. There was no interval effect on yield in PM at L-SI and yield was like fungicide treated VP and CT, suggesting higher CDM tolerance. Tolerant hybrids offer potential to reduce fungicide use, but this may depend on CDM pressure. More research is required to develop grower recommendations.</div>