Risk Assessment for Root-knot Nematodes in Lima Beans

Southern Root-Knot Nematode (RKN), Meloidogyne incognita, is a major yield limiting pest in lima beans (Phaseolus lunatus). RKN are not evenly distributed through fields and population dynamics are fluid making whole field management challenging. The objectives of this research were to characterize RKN distribution in the field in relation to soil factors and to quantify RKN populations and impacts on root infestation of lima beans in order to develop field level risk assessments. Surveys were conducted on eight lima bean fields with a history of RKN in 2013 and 2014 and six fields in 2015. Fields were divided into one hectare grids and evaluated for edaphic parameters [field water capacity (FC), electrical conductivity (EC), depth to hardpan, and pH]. Twenty soil cores were taken at a 30 cm depth within each grid for nematode assays. Fields were sampled in fall, spring, summer, fall, and the following spring. Plant growth was evaluated indirectly using NDVI. RKN second stage juveniles were extracted from soil samples using elutriation and sucrose centrifugation. Greenhouse cucumber seedling bioassays were conducted on grid soil samples and rated for galling on a 1 to 10 scale. Test crop roots were evaluated for gall ratings at mid-season and after-harvest. Spring J2 counts were significantly correlated with after-harvested root ratings (p=.009, R²= .431); whereas bioassays were less predictive (p<.001, R²=.335). Of the edaphic factors, FC was also correlated with after-harvest root ratings (p<.001, R²= .111). A model combining spring J2 counts and FC or EC gave best prediction of lima bean galling (P<.001, R²⁼.64). This research will be used to develop site specific RKN management practices for lima beans.