Biological suppression of the root-knot nematode Meloidogyne incognita following winter cover crops in conservation tillage cotton

Abstract Winter cover crops are often planted in temperate and subtropical environments. The residue from these cover crops may enhance suppression of plant-parasitic nematodes by providing alternative food sources for predators and parasites. The primary objective of this study was to determine whether winter cover crops enhance early season suppression of Meloidogyne incognita compared to a weedy fallow. Two secondary objectives were to determine the effect of winter cover crops on free-living nematodes, with an emphasis on carnivorous (Ca; obligate predators) and omnivorous (Om, facultative predators) nematodes, and determine the role of these predators in suppression of M. incognita. Two experiments were conducted over 3 years at different field sites. The experiments consisted of three cover crop treatments replicated four times in a randomized complete block design. The cover crop treatments for the Jones Farm were rye, a rye-crimson clover combination, and fallow. The cover crop treatments for the Belflower Farm were rye, hairy vetch, and fallow. The cover crops were killed and the fields planted with cotton. At the time of planting cotton, soil samples were collected to conduct bioassays to evaluate suppression of M. incognita and abundance of predatory nematodes. One bioassay measured nematode survival after 5 days in natural compared to defaunted soil, and the other bioassay measured differences in nematode reproduction among the cover-crop treatments. The native soil community reduced survival of M. incognita J2 by 55% in 2016 at the Belflower Farm and in all years at the Jones Farm by 60% compared to defaunated soil; however, the cover crop treatments had no effect on the level of suppression. Predatory nematodes may have contributed to reduced survival. Like the survival bioassay, predators were also generally not influenced by cover crop treatment. At the Jones Farm, there was a significant correlation between relative abundance of Om, Ca, and Om + Ca and suppression of J2. Reproduction of M. incognita was suppressed in soil that had been planted to cover crops by 54–66% compared to the weedy fallow at the Belflower but not the Jones Farm. Planting winter cover crops enhanced early season suppression of M. incognita reproduction at the Belflower Farm; however, cover crops had no effect on suppression of J2 survival at the Jones Farm. Enhancement of nematode suppression by cover crops is likely dependent on the organisms involved in suppression.