Methods for quantifying low populations of root-knot nematode (Meloidogyne spp.) in vegetable soils and their potential for use in monitoring programs to improve nematode management decisions

Abstract

Most Australian vegetable growers apply fumigants or nematicides as a precautionary nematode control measure when crops susceptible to root-knot nematode (RKN, Meloidogyne spp.) are grown in soils and environmental conditions suitable for the nematode. The only way growers can make rational decisions on whether these expensive and environmentally disruptive chemicals are required is to regularly monitor RKN populations and decide whether numbers prior to planting are high enough to cause economic damage. However, such monitoring programs are difficult to implement because nematode quantification methods vary in efficiency and the damage threshold for RKN on highly susceptible vegetable crops is often < 10 root-knot nematodes /200 mL soil. Consequently, five nematode quantification methods were tested to see whether they could reliably detect these very low population densities of RKN. Two novel methods produced consistent results: 1) extracting nematodes from 2 L soil samples using Whitehead trays, quantifying the RKN DNA in the nematode suspension using molecular methods, and generating a standard curve so that the molecular results provided an estimate of the total number of RKN individuals in the sample, and 2) a bioassay in which two tomato seedlings were planted in pots containing 2 L soil and the number of galls produced on roots were counted after 21–25 days. Both methods could be used to quantify low populations of RKN, but bioassays are more practical because expensive equipment and facilities are not required and they can be done at a local level by people lacking nematological or molecular skills.