Control of Phoma Leaf Spot and Root Decay of Table Beet in New York

Abstract

Disease caused by Neocamarosporium betae (syn. Phoma betae, Pleospora betae) results in reductions in plant populations, foliar disease (Phoma leaf spot [PLS]), and root disease and decay in table beet. Disease caused by N. betae has reemerged as prevalent in organic table beet production in New York. The disease can also cause substantial issues in conventional table beet production. To evaluate in-field control options for conventional and organic table beet production, small-plot, replicated trials were conducted in each of two years (2019 and 2021). The fungicides, propiconazole and difenoconazole, and premixtures, pydiflumetofen + fludioxonil or pydiflumetofen + difenoconazole, provided excellent PLS and root decay control. Azoxystrobin provided excellent (69.9%) control of PLS in 2019 and lesser (40%) control in 2021. Field trial results complemented in vitro sensitivity testing of 30 New York N. betae isolates that were all highly sensitive to azoxystrobin (mean effective concentration to reduce mycelial growth by 50%, EC50 = 0.0205 µg/ml) and propiconazole (mean EC50 = 0.0638 µg/ml). Copper octanoate and microbial biopesticides containing either Bacillus amyloliquefaciens D747 or B. mycoides strain J provided moderate (68.5 to 74.6%) PLS control as reflected in epidemic progress. The Gompertz model provided the best fit to PLS epidemics reflecting a polycyclic epidemic. Reductions in PLS severity were associated with significant decreases in Phoma root decay and increases in canopy health and the time-to-death of leaves compared with nontreated control plots. Prolonging leaf survival is critical for mechanical harvest of roots. These findings underpin the design of programs for foliar disease control in conventional and organic table beet production. Assessment of PLS severity in the field will better inform postharvest management decisions.