Crop Profile for Onions in California - Regional IPM Centers

(12)
Root knot nematode (Meliodogyne hapla, M. incognita, M.. javanica and M. chitwoodi). The root
knot nematode enters the onion as a second stage juvenile and remains there to feed until it reaches
maturity. Mature female nematodes produce an egg mass just outside the root or just below the surface
of the root. Damage can result in severe stunting of the plant and stand reduction. Infestations cause
galls on roots. M. hapla produces small galls that are difficult to see while other nematodes produce
larger, more visible galls. (12)
Stubby root nematode (Paratrucgidirys spp.). The stubby root nematode is a migrating ectoparasite
that feeds on but does not penetrate the root. This nematode lays individual eggs directly in the soil.
Populations increase during cool, damp weather. Symptoms of feeding are extremely short roots with
yellow to brownish cast. Damage can result in stunted plants and stand reduction. (12)
CULTURAL CONTROL- Sanitizing equipment and preventing the movement of infested soil to
uninfested areas can decrease nematode migration. In addition, well-drained soil will prevent nematode
problems. The root knot nematodes (M. incognita, M. javanica and M. arenaria) are inactive at
temperatures below 64° F. Thus planting onions when the soil temperatures are in this range can
minimize nematode damage. Growing nonhost crops such as carrots and lettuce for several years helps
decrease stem and bulb nematode populations. However, this is not feasible with root knot nematode and
stubby root nematode because of their wide host range. (12)
BIOLOGICAL CONTROL- Myrothecium verrucaria, botanical extracts, pathogenic fungus, and
diatomaceous earth are biologically derived nematacides that may be used in onion crops. (5)
CHEMICAL CONTROL
● Oxamyl – see onion thrips and western flower thrips.
● Pyrethrins – see onion thrips and western flower thrips.
● Metam sodium – see bulb mites.
Weeds
Weeds can be problematic to onion producers. Onions are slow growing plants with shallow roots
and narrow, upright foliage and are among the least competitive of all crops. Several flushes of