Control of stem nematode, Ditylenchus dipsaci, in onions, by granular nematicides applied to the soil

Aldicarb at 1.4–5.6 kg a.i. ha^-l or oxamyl at 1.3–5.2 kg a.i. ha^-1 applied over the seed furrows during sowing prevented much of the damage resulting from stem nematode attack in spring-sown onions. Rotting of bulbs in store due to stem nematode infestation was sometimes lessened by these treatments. Neither sowing onion seeds on soil ridges, nor applying aldicarb in two equal doses, one over the seed furrows during sowing, the other over the plants mid-season, controlled the nematode better or increased onion yields more than a single equivalent dose over the seed furrows. Applied as two doses in this way, aldicarb was sometimes more effective against the nematode in summer-sown onions than was an equivalent amount applied over the seed furrows during sowing. Only 2.5 kg or less aldicarb ha^-1 can be applied to onions without leaving unacceptable residues (>0.15 μg g^-1) in the harvested bulbs.