University Distinguished Professor Michigan State University East Lansing, MI, USA
Abstract Text: Pseudoperonospora cubensis causes cucurbit downy mildew (CDM) requiring fungicides to protect cucumber plants and yield. Our objective was to test the disease forecaster BLITECAST (BC) to determine the fungicide application intervals for CDM susceptible ‘Vlaspik’ and intermediately resistant ‘Peacemaker’ cucumbers. The fungicides cyazofamid + chlorothalonil alternated with chlorothalonil + oxathiapiprolin (premix) alternated with ametoctradin + dimethomorph (premix) + chlorothalonil were applied to field plots based on BLITECAST at 15 or 18 disease severity value (DSV), every 7-days or not treated (control). The 7-day treatment received seven (2023) or eight (2024) sprays. BC 15 DSV received seven (2023) or six (2024) sprays. BC 18 DSV received six sprays each year. CDM was visually assessed (0% = healthy, 100% = symptomatic foliage). Each year, control plots of ‘Vlaspik’ and ‘Peacemaker’ exhibited 71% to 87.5% and 26.3% to 38.0% CDM, respectively. In 2023, ‘Vlaspik’ and ‘Peacemaker’ treated with fungicides using BC (18 DSV) had <9.8% CDM and the 7-day treatment had <2.0% CDM; BC 15 DSV (<4.5% CDM) and the 7-day treatments were similar. In 2024, CDM ratings were similar for ‘Vlaspik’ (<24.5%) and ‘Peacemaker’ (<18.3%) treatments. Area under disease progress curve (AUDPC) data indicated that all treatments limited CDM compared to the control each year. According to AUDPC data, both cultivars treated based on BC 18 DSV had more CDM than the 7-day treatment each year. In 2024, sprays were reduced by 25% using BC 15 DSV compared to the 7-day treatment with similar CDM control.
