Dr Antoinette Swart of the National Collection of Nematodes found two female nematodes and several juveniles of the genus Ditylenchus in a garlic sample submitted for analysis by the Department of Agriculture, Land Reform and Rural Development. Both females were con-specific with the quarantine nematode, Ditylenchus dipsaci.
No males were present in the sample, so only female characters could be used to diagnose the species utlilising Nemapix, which employs morphology and morphometrics.
According to the Diagnostic Protocols for Regulated Pests (2015), molecular identification of the species D. dipsaci is required when specimens cannot be distinguished conclusively. Specimens were subsequently sent to Dr Ebrahim Shokoohi at the University of Limpopo for molecular identification, which reached him on 24 August 2020.
Nematodes as endoparasites
Molecular identification was deemed the best way to confirm the identity of the specimens. The molecular results were received on 21 September 2020 and confirmed that the specimens were D. dipsaci with high similarity of two genes (ITS and 18SrDNA).
D. dipsaci lives mostly as an endoparasite in aerial parts of plants (stems, leaves, and flowers). However, it also attacks bulbs, tubers, and rhizomes. This nematode is seed-borne in Medicago sativa (lucerne/alfalfa), Allium cepa (onion), Trifolium spp. (clovers), Dipsacus spp. (teasel) and Cucumis melo (melon). Of great importance is the fact that the fourth-stage juvenile can withstand desiccation for a long time, sometimes 20 years or more.
‘Wool’ on seeds in heavily infested pods
Nematodes clump together in a cryptobiotic state to form ‘nematode wool’ when the plant tissue begins to dry. The wool can often be observed on the seeds in heavily infested pods and in dry plant debris. The presence of the infective fourth-stage juveniles in seed and dry plant material is important in the passive dissemination of the nematode over long distances. The nematode in its desiccated state can survive passage through pigs and cattle on infected seed.
Although D. dipsaci is regarded as a parasite of higher plants, it was reported that a Californian population of D. dipsaci from Allium sativum (garlic) could reproduce on soil fungi (Verticillium and Cladosporium) under laboratory conditions. It is also known that D. dipsaci is of potential economic importance on Agaricus bisporus (mushroom). Ditylenchus dipsaci is known to vector bacterial plant pathogens externally causing alfalfa wilt. – Plant Protection Newsletter, ARC