Impact in Walnut Production
Root-lesion nematode, Pratylenchusvulnus is pathogenic to most perennial crops and infests 85% of the CA walnut acreage. It is considered the “Hidden Enemy” and its direct impact on walnut roots can be debilitating. Symptoms include: root lesions, missing roots, and halted tree growth.
Although it seldom kills tree crops it can rapidly reproduce on walnut and severely stunt the trees. Within a single season, starting from small initial populations, Pratylenchus vulnus can reach damage thresholds and within 2 years at the population threshold P. vulnus can result in a 20% reduction of tree growth. Root damage induced by P. vulnus can result in fewer and smaller nuts and limb dieback in trees of bearing age, and root stunting in replant sites can destroy young trees.
Fig. 1. Major nematode species of concern to perennial crop growers: root lesion nematode (P. vulnus), root knot nematode (Meloidogyne spp.) and ring nematode (Mesocriconema xenoplax).
Fig. 2. After a soil sample indicated hundreds of P. vulnus per sample in this 6 year old orchard, excavation confirmed missing and damaged roots resulting from nematode infection.
Clonal plants and seedlings have been planted in the field and inoculated with P. vulnus and Meloidogyne sp.
Screening of Juglans hybrids for nematode resistance and tolerance occurs in field settings.
Root sampling, nematode extraction, tree growth, and root damage are used to detect levels of resistance or tolerance.
J. cathayensis, is the first source to exhibit resistance to root-lesion and root-knot nematodes.
Resistance ratings will be used to map the genetic loci mediating nematode resistance.