Management of Paratrichodorus allius Damage to Onion in the Columbia Basin of Oregon.

Production of storage and dehydration (dehy) onions is increasing rapidly in the northcentral Oregon region of the Columbia Basin. Roughly circular patches of stunted onions have been observed in many fields and are often numerous and (or) extensive in area. Onions in these patches often exhibit symptoms typical of stubby-root nematodes, and Paratrichodorus allius has been collected from several damaged areas. This study examined the relationship of P. allius densities to onion yield reduction and benefit from oxamyl application. Transect samples and onions harvested across areas with varying P. allius densities revealed that the total number of onions was not affected by P. allius but that onions were in smaller, less valuable size classes. Yield of the largest size class of storage onions (colossals) declined substantially (57-61%) even at low (5-16/250 g soil) nematode densities. Treatment with oxamyl at 1.1 kg a.i./ha mitigated most of the damage, and the value of both storage and dehy onions was increased by more than $1,200/ha. Early applications were superior to later applications, whereas treatment at both times was no better than the early treatment alone. Damage at low (<10/250 g soil) nematode densities, low treatment cost ($86-172/ha), and high crop values suggest an economic benefit from treatment at population densities of four or more P. allius/250 g soil. However, stunted patches of onions may occur for reasons other than nematodes, and the presence of P. allius should be confirmed before treating fields. This is the first report of stubby-root nematode damage to dehy onions and to storage onions in the Columbia Basin.