Sampling techniques and detection methods for developing risk assesments for the root-knot nematode (Meloidogyne incognita) on lima bean (Phaseolus lunatus) in the mid-Atlantic region

Lima bean (Phaseolus lunatus) is a cornerstone crop for the vegetable processing industry in the Mid-Atlantic region and Southern root-knot nematode (Meloidogyne incognita), considered to be one of the most widespread and destructive of all plant-pathogenic nematodes, can cause yield losses of up to thirty percent. Effective chemical controls are limited due to availability and associated cost. Risk evaluation research is needed to provide the lima bean industry with the predictive tools they currently lack. Such tools could allow for more targeted sampling and nematicide use in fields where edaphic factors and root-knot nematodes (RKN) are heterogeneous. Data was collected via grid sampling from nine lima bean fields from 2013 to 2014; before planting, during mid lima bean growth, after harvest, and after overwintering. The effects of edaphic factors (pH, organic matter, electrical conductivity, texture, Mehlich 3 extracted phosphorus, moisture, temperature, and depth to hardpan) on RKN distribution, populations, and lima bean root galling were studied. Assay techniques; root ratings, J2 extraction, and pre-planting bioassay were evaluated to determine the most cost effective and reliable root-knot nematode detection solutions for growers. The results suggest that RKN density is greater in areas of high sand content and low electrical conductivity and RKN galling was significantly associated with higher elevation. RKN population sizes generally increased with presence of phosphorus as evidenced by RKN galling. Depth to hardpan; pH and organic content had little to no correlation with RKN activity. RKN populations in the spring were expected to be increased by vegetation; however, when measured by comparing normalized vegetation index (NDVI), there was not a significant correlation. Rhizobia nodulation increased as galling by RKN decreased. For assays, the sugar flotation method showed a greater range of J2 content and detection per sample than did the Baermann funnel technique. Of all the assays performed here to assess RKN presence and activity, those based on gall-ratings appear to be more sensitive and robust, than J2 counts. The data from the survey suggests that a bioassay would be a good predictor of spring J2s if fall galling data were unavailable. In all fields there was a reduction in the overall amount of nematodes in each individual grid over the winter. A dummy variable model showed that J2 Summer, RKN galling on pigweed, Soil EC-H and depth to hardpan were the most predictive of lima bean root galling. Correlations of edaphic factors and lima bean root galling will provide information to start building a working predictive model.