2017 ASHS Annual Conference

Preplant soil fumigation is in integral component of pest management for commercial sweetpotatoes in California. Soil fumigation is done both in the fall and spring to suppress root knot nematodes (RKN), Meloidogyne incognita, and soil insects such as wireworms (Limonius spp) and grubs (Diabrotica spp, Phyllophaga spp). Telone (1,3-D), metam (methyldithiocarbamate), and chloropicrin (pic) are registered for use. Unfortunately, the availability of the preferred fumigant, Telone, is insufficient to meet the needs of the industry because California restricts Telone by implementing “use caps” for the entire state. These caps limit the amount of Telone that can be used on a township basis (one township is 23,040 acres) to 92,500 lbs a.i. These restrictions can limit the amount of Telone used in any year to approximately 50% of demand. In response, growers are using metam sodium (Vapam) and metam potassium (K-Pam). A new nematicide Nimitz (fluensulfone) soon to be registered on sweetpotatoes also offers a potential alternative to fumigants. In 2009-10 and 2014-16, trials were conducted to evaluate the effect of pre-plant fumigants and nematicides on root knot nematodes and yield of sweetpotatoes. Telone was applied at 6, 9, and 12 gallons per acre with and without pic; metam rates were 35 and 50 gallons per acre. Nimitz was applied at 3.5 and 5 pints/A and shank or water incorporated. Treatment design for all trials was a randomized block with four replications. Fumigants and Nimitz applications were applied before transplanting and incorporated. Nematode samples were taken in early June to and again at the end of the season. Regardless of treatment, location, or year, RKN counts in the spring and fall were highly variable. Only in one location in fall 2011 and 2014 were RKN counts significantly different in the Telone treatment as compared to the other treatments. Yield increases from soil fumigation, however, have been very consistent: yields were significantly improved as compared to the untreated control in all treatment years. Metam at 50 gpa was nearly as effective as Telone. Combining Telone with metam improved the performance of both products, and results suggest that reduced rates of Telone can be effective when combined with metam. Results with Nimitz nematicide have shown significantly increased root yield and quality with pre-plant applications in 3 of 4 test sites. These results show that alternatives to Telone can be very effective for sweetpotato production in California. Challenges with application method and treatment timing remain.