Optimizing methods to sample and quantify stem and bulb nematode, Ditylenchus dipsaci, in garlic, Allium sativum, field soil

Abstract

Stem and bulb nematode (Ditylenchus dipsaci) is a plant parasite that can cause severe damage to garlic crops in Ontario, Canada, and other garlic growing regions. Accurate soil sampling is important to determine the risk of nematode damage before planting garlic in a field. However, it is not clear where the nematode is most concentrated in the soil profile or how the nematode is best extracted. A field survey and laboratory experiments were conducted in the autumn of 2015 and 2018 to determine the distribution of stem and bulb nematode in the soil profile and to determine the most effective extraction method. The soil in 20 garlic fields throughout southern Ontario was sampled and the top 5 cm and the bottom 5–20 cm of soil were collected in a single core and then separated. Nematodes were extracted from all soil samples using both the Baermann pan and sugar centrifugal flotation methods. Significantly more stem and bulb nematodes were extracted from the top 5 cm of soil and using the sugar centrifugal flotation method. An additional extraction efficacy experiment was conducted using a known quantity of stem and bulb nematodes in soil to compare various extraction methods, and the sugar centrifugal flotation method continued to be the more effective method. These results demonstrate that only the top 5 cm of soil should be collected and assessed for populations of stem and bulb nematode in fields intended for garlic production, and the sugar centrifugal flotation method should be the extraction method of choice.