First Report of Stem and Bulb Nematode Ditylenchus dipsaci on Garlic in New Mexico

Ditylenchus dipsaci is a serious temperate-climate nematode pest of over 500 plant species in over 40 plant families (Subbotin et al. 2005), including onions, garlic, and alfalfa. Among plantparasitic nematodes, it ranks fifth in economic importance and is of nearly universal international quarantine concern (Jones et al. 2013; OEPP/EPPO 2008). NewMexico is the eighth largest producer of onions in the United States, cultivating 5,200 acres with a crop value of $91million, making onions the third-highest grossing crop in New Mexico in 2015 (USDA-NASS). New Mexico also has a vibrant small-farm garlic industry that supplies local markets with fresh garlic and seed for producers. New Mexico has never reported an infestation of D. dipsaci and its presence could have serious ramifications for New Mexico’s onion, garlic, and alfalfa producers. In May of 2015, garlic plants (Allium sativum) from a home garden in Dona Ana Co. were submitted to the Plant Diagnostic Clinic at New Mexico State University. The grower reported poor growth in ~30% of 1200 plants originating from seed purchased in southern New Mexico. Early symptoms included chlorosis, wilting, and poor root and bulb development. As the disease progressed, roots turned brown, were easily separated from the bulb, and older foliage had collapsed and turned straw colored (Fig. 1). Microscopic examination showed large numbers of nematodes present in symptomatic garlic plants. Two regulatory samples with similar symptoms were submitted to the USDA-APHIS Nematode Containment Facility #76. Nematodes from both the homeowner and regulatory samples were morphologically consistent with descriptions of D. dipsaci, possessing small stylets, small but distinct metacorpus, and conical, pointed, slightly asymmetric tails (Sturhan and Brzeski 1991) (Fig. 2). Males exhibited prominent retracted spicules and long, slender bursae. Measurements taken from 10 mature male and female nematodes produced the following mean lengths: 1.01 mm (_) and 1.06 mm (\), which are consistent with values reported for D. dispsaci. The DNA sequence of the ITS-1 region, which is useful for identification of Ditylenchus species (Subbotin et al. 2005), was determined for 22 individual nematodes picked from infested garlic samples. The ITS-1 region sequences were identical between all 22 individuals, indicating the population of nematodes infesting the garlic was clonal. BLAST analysis and pairwise comparisons showed that the ITS region sequences were 100% identical to numerous D. dipsaci sequences in GenBank, with the next closest related species being D. gigas (GenBank Accession No. HQ219239) and D. weischeri (AF396322), both being 95% identical. The New Mexico garlic nematode ITS-1 sequence was only 46 to 57% identical to reported D. destructor sequences (DQ471334 and HQ235677). Taken together, the symptoms present, microscopic characterization, and ITS-1 sequence data all indicate that the garlic samples were infested Ditylenchus dipsacimaking this the first known report of this pest in New Mexico. This discovery is highly significant as D. dipsaci can be a persistent pest (Seinhorst 1956) that has the potential to cause significant economic losses on agronomically important hosts grown in the state and region including onion, garlic, and alfalfa. The longevity of this pest in the soil and international trade issues will also be concerns for producers going forward. Based on information from the grower, it appears that this is an isolated introduction from infested plant material which can potentially be contained as the presence of D. dipsaci was discovered before any material was harvested or distributed. Monitoring of production areas in the region will be performed to determine if this