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Poster: Biology & Disease Mgmt: Chemical Control


Baseline sensitivities of Rhizopus stolonifer and first effective fungicides for management of almond hull rot in California
S. HAACK (1), H. Förster (1), J. Adaskaveg (1) (1) University of California, Riverside, U.S.A.

Almond hull rot, caused primarily by Rhizopus stolonifer (Rs) and sometimes Monilinia fructicola (Mf), is an increasingly important disease in California almond production. These fungi infect the hull either during late fruit development (Mf) or at hull split (Rs). Fruit colonization leads to production of toxins that translocate into the fruiting wood, causing twig and branch dieback. Hull rot has been managed culturally by deficit irrigation and by limiting nitrogen application, but this opposes high-density, high-input farming that increases disease pressure. Fungicide treatments add to an integrated management approach. Baseline sensitivities to six fungicides representing DMI (difenoconazole, metconazole), SDHI (fluopyram, fluxapyroxad), and QoI (pyraclostrobin, trifloxystrobin) FRAC groups for inhibition of Rs mycelial growth or spore germination were determined using the spiral gradient dilution method. Mean EC50 values for these fungicides were 0.102, 0.082, 0.292, 0.850, 0.002, and 0.002 mg/L, respectively. Sensitivities to each fungicide were within a ten-fold range, and thus, all isolates are considered sensitive. In field efficacy and timing studies, a single application of these fungicides and selected pre-mixtures at the beginning of hull split timed with a standard insecticide application was sufficient to reduce the disease by 60 to 80% when Rs was the primary pathogen. This represents the first fungicide-based field management strategy for hull rot.