Spatial Efficacy and Economic Return Evaluations of Soil Fumigation for Agroathelia rolfsii Management in Florida Plasticulture Tomato Production

Agroathelia rolfsii (A. rolfsii), the causal agent of southern blight (SB) of tomato, is a soilborne fungal pathogen that produces sclerotia to survive during fallow periods and is commonly managed with pre-plant soil fumigation in commercial tomato production. However, fumigant efficacy against A. rolfsii sclerotia distributed throughout raised bed soil profiles and the economic return of fumigant inputs have not been assessed for currently available fumigant systems. Across four trials, this study evaluated the efficacy of three fumigant treatments against A. rolfsii sclerotia distributed at different soil depths and placements, on SB disease incidence, and marginal economic returns in a plasticulture fresh-market tomato system. Treatments included the grower-standard chloropicrin and 1,3,-dichloropropene (Pic-Clor) applied at the bed base, metam potassium (Kpam) applied in the top half of the bed, and a combination treatment (Pic-Clor+Kpam). Beds treated with Pic-Clor+Kpam saw 2-3% sclerotia survival across all burial depths and placements, compared to the grower’s standard, Pic-Clor, which had 11% recovery at 1.5 cm and 5% at 10 cm depths. Across all seasons, Pic-Clor+Kpam and Pic-Clor limited disease incidence below 2% and 4% respectively, compared to 9% for Kpam and 43% for the nonfumigated control. All fumigant treatments returned at least $20,000/hectare more than the nonfumigated control during high disease pressure seasons. Moreover, both Pic-Clor alone and Pic-Clor+Kpam returned $25,0000/hectare, suggesting that the additional cost of Kpam was not justified based on marginal economic return when only considering disease management. While additional bed-top Kpam application does lower surviving inoculum levels and improves SB disease management compared to Pic-Clor alone, the prospect of limiting other pesticidal applications may be needed to justify the additional cost as marginal returns are comparable across systems.