Identifying resistance to, and interactions of, root-knot nematodes and Cylindrocladium black rot in peanut
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Root-knot nematodes (RKN, Meloidogyne spp.) and Cylindrocladium black rot (CBR, Cylindrocladium parasiticum) are important soilborne diseases on peanut. Utilization of peanut cultivars with resistance to both diseases is a desirable management approach. Greenhouse, microplot, and field studies were conducted to improve screening techniques and identify new resistance sources and to enhance RKN and CBR resistance breeding in peanut. Greenhouse and microplot studies were also conducted to evaluate the interactions between M. arenaria and C. parasiticum in peanut. A gall index based on percentage of galled root was a reliable indicator of resistance to M. arenaria at early harvest dates after inoculation with 8000 or more eggs/plant from 10 to 30 days after planting. The selected genotypes could then be assessed for eggs/g root at 8 weeks after inoculation to verify the resistance level based on egg production. Twenty-six peanut genotypes with some resistance to M. arenaria, M. javanica, or M. hapla were identified from 60 accessions based on eggs/g root and gall index in the greenhouse. Results with molecular markers indicate that different resistance genes exist in the selected genotypes. The resistant selections should be valuable sources for pyramiding resistance genes to develop new cultivars with broad and durable resistance to Meloidogyne spp. Differentials of resistance to CBR exist in runner type peanuts. Peanut genotypes are most reliably screened in inoculated or uniformly infested natural fields, but greenhouse evaluations may be useful to identify and characterize components of resistance. In the greenhouse experiments, root rot ratings were increased by addition of 500-3000 eggs/plant of M. arenaria with low inoculum level of C. parasiticum, but not at high levels. Gall indices were not affected by C. parasiticum inoculations in the greenhouse or microplots. In microplot experiments, synergistic interactions between M. arenaria and C. parasiticum were observed on plant mortality in both 2006 and 2007. The root rot ratings from nematode-susceptible genotypes were higher in plots infested with M. arenaria and C. parasiticum than those with C. parasiticum alone. Simultaneous inoculation with M. arenaria decreased yield incrementally on RNK-susceptible C724-19-25 and Georgia-02C as C. parasiticum inoculum levels increased, but not on RNK-resistant C724-19-15.