Evaluation of spinach cultivars for downy mildew resistance in Yuma, AZ 2022

Authors: ZIMA, HANNA - University Of Arkansas; Clark, Kelley; POUDEL-WARD, BINDU - University Of Arizona; SLINKSI, STEPHANIE - University Of Arizona; Klosterman, Steven; CORRELL, JIM - University Of Arkansas

Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2022
Publication Date: 8/9/2022
Citation: Zima, H.V., Clark, K.J., Poudel-Ward, B., Slinksi, S.L., Klosterman, S.J., Correll, J.C. 2022. Evaluation of spinach cultivars for downy mildew resistance in Yuma, AZ 2022. Plant Disease Management Reports. 16. https://doi.org/10.1094/PDMR16.
DOI: https://doi.org/10.1094/PDMR16

Interpretive Summary: Downy mildew disease on spinach is caused by a microscopic pathogen known as Peronospora effusa. The pathogen can produce wind-dispersed spores, and requires cool, wet conditions for infection. Due to the increased canopy density that spinach is planted, low levels of disease incidence can lead to unmarketable spinach because of the rapid establishment and spread of the disease. Downy mildew is successfully managed with fungicides and resistant cultivars in conventional fields, but only the latter is implemented for organic production, which represents about half of U.S. production. In this study, different commercial spinach cultivars were examined for downy mildew disease incidence under field conditions near Yuma, Arizona. The amount of disease appearing on susceptible plants in the experiment was adequate to evaluate resistance as in four cultivars disease incidence was greater than 60%. The overall range of disease incidence was from 0.0 to 96.7%. In five cultivars, disease incidence was less than 3.3%, and thus some of these cultivars may be informative in programs that aim to breed for plant resistance.

Technical Abstract: Downy mildew on spinach (Peronospora effusa) is a major constraint to spinach production. Downy mildew is successfully managed with fungicides and resistant cultivars in conventional fields, but only the latter are implemented for organic production, which represents about half of U.S. production. At present, there are 19 described races of P. effusa, as well as isolates with novel virulence patterns on spinach differentials. The purpose of this study was to assess resistance to P. effusa among commercial spinach cultivars, with a total of 70 cultivars that were rated at the University of Arizona, Yuma Agricultural Center between 11 Jan-16 Mar 2022. Seed was sown on 10-11 Jan, sprayed with Dual Magnum herbicide on 12 Jan, and with a wet date of 13 Jan. The plots were 15 ft by 6 ft and seed was planted at a density of 4 million seed/A, and beds were about 600 ft in length with three separated replications. Each replication even rows with 10 cultivars per row. The plot was set up as a completely randomized block design. The trial was flanked by single beds of the cultivar Woodpecker and a mixture of cultivars. Maximum air temperature ranged from 71-86°F and minimum from 46°-55°F, cumulative rainfall was 0.02 in., and the maximum wind peak was 35 mph. Plants were watered with overhead sprinklers 2-3 times per week for the duration of the experiment. Downy mildew disease pressure for this trial originated from naturally occurring inoculum and was first observed on 8 Feb. Disease incidence (DI) was evaluated on 16 Mar when plants were 64 days old. Disease pressure was high as indicated by the DI detected on several cultivars and the overall range of DI was from 0.0 to 96.7%.