Influence of nematicide on the damage caused by Meloidogyne incognita (Tylenchida: Meloidogynidae) to sweet potato in sandy fields in northeastern Tokushima Prefecture

Nematicides, especially 1,3-dichloropropene (D–D), are empirically used to control damage caused by the root-knot nematode Meloidogyne incognita (Kofoid and White) Chitwood in fields of sandy soils in northeast Tokushima, a major sweet potato (Ipomoea batatas (L.) Lam.) producing area. The objective of this study was to elucidate an economic threshold against the nematode damage. Relationship between the nematode density before nematicide use and the degree of nematode damage at harvest was evaluated in nematicide-treated and non-treated plots. There was a significant correlation between the pre-plant nematode densities and nematode damage in the nematicide non-treated plots. Marketable quality of sweet potato decreased in four out of 16 in the non-treated plots, while it did not decrease in all 11 nematicide non-treated plots in which the pre-plant nematode densities were less than 100 second-stage juveniles (J2) equivalent 20 g−1 of soil, estimated using a real-time PCR method, suggesting that this density is an economic threshold for damage by root-knot nematodes to sweet potato. The present study demonstrates that nematicides can be avoided in fields containing root-knot nematodes at densities less than the economic threshold.