Anaerobic Soil Disinfestation for Suppressing Verticillium dahliae in Strawberry Production in California

Anaerobic soil disinfestation (ASD), a non-chemical alternative to methyl bromide (MB) fumigation developed in Japan and the Netherlands, can control soilborne pathogens and nematodes in strawberries and vegetables. To optimize ASD for California strawberries, field experiments were conducted in Watsonville and Salinas in 2009-10, and are being repeated in Ventura, Watsonville and Castroville in 2010-11. Soil types in trials ranged from sandy loam to silty clay loam. Randomized block experiments with four replicates were established to compare ASD treatments with different carbon sources including rice bran 10 Mg/ha (RB10), rice bran 20 Mg/ha (RB20), and rice bran 16 Mg/ha plus mustard seed cake 4 Mg/ha (RB+MC) with untreated checks (UTC) and MB or Pichlor 60 fumigation controls (except at the Ventura site). For ASD treatments carbon sources were rotor tilled into the soil before or after bed formation (Ventura site) and drip tape and plastic tarp installed, a standard practice. Then 7.5 to 12.5 cm of drip-irrigation was applied intermittently and left for three weeks at which time holes were punched in the plastic and transplants planted ~7 days later. Native Verticillium dahliae populations in soil (0-15 cm) were quantified before and after treatments and during the harvest period. During the ASD treatments, anaerobicity (Eh), temperature, and moisture content were continuously monitored at 15cm depth. Marketable fruit yield was evaluated. Soil Eh reduction near or exceeding 50,000 cumulative Eh mVhr below 200mV (a threshold for suppressing V. dahliae at 25 °C suggested by earlier pot experiments) was observed for most ASD plots across trials. ASD led to a 73 to 100% reduction in V. dahliae in soils compared to the UTC in Salinas, Ventura and Castroville under moderately high V. dahliae pressure (11 to 25 microsclerotia/gram soil). Strawberry fruit yield in ASD plots in Salinas (2009-10) with moderate V. dahliae pressure was similar to MB and UTC. In Watsonville (2009-10) with 0 microsclerotia/gram soil, marketable yield was the highest in MB (100%), followed by RB+MC (85%), RB20 (82%), RB10 (76%), and UTC (75%). In the summer-planted Ventura trial (2010-11) with moderately high disease pressure, early marketable yield in ASD plots was 80% greater than UTC (P=0.0015). Overall, ASD was very effective in suppressing V. dahliae in soils and resulted in 82 to 100% of the marketable fruit yield observed with fumigated controls in coastal CA strawberries. The combination of RB and MC in ASD appears particularly promising.