Cause Paratrichodorus allius and P. minor, ectoparasitic nematodes that overwinter in soil. The largest nematode populations and conspicuous plant symptoms occur in cool, damp springs. P. minor affects a wide range of plant species.
Symptoms Roots are extremely short with yellow to brownish cast; plants are stunted. Root tips become darker, stubby, and more branched.
Sampling These nematodes are found only in soil. Before planting susceptible onions, take soil samples with sufficient lead time to implement management procedures if necessary. For example, if control is by soil fumigation, it may be several weeks before onions can be planted. Fall sampling for crops to be planted the following spring is an excellent strategy. For diagnosis, submit both soil and onions containing roots.
- Avoid planting onions after a mint crop.
- Preplant soil fumigation.
- Telone II at 9 to 25 gal/A in fall. Seal soil for 7 to 14 days, then aerate 1 week for each 10 gal/A applied (longer in case of heavy rains or temperatures below 60°F) until fumigant odor is gone. Do not treat extremely heavy soils or apply near living plants. Restricted-use pesticide. 5-day reentry.
- Telone C-17 at 10.8 to 30 gal/A. Caution: Fumigation must be done before planting, so it is important that the planting row be directly over the fumigated soil if only the planting row is treated. Variations of a few inches can cause complete loss of control. Use highest rates on peat soils. Restricted-use pesticide. 5-day reentry.
- Apply at planting.
- Vydate L at 1.5 to 2 gal/A (broadcast basis) in 20 to 50 gal water as an in-furrow spray, or at 0.75 to 1 gal/A (broadcast basis) in 100 to 150 gal water as an in-furrow drench. Restricted-use pesticide. If nematode populations are extremely high, make two (2) or three (3) post-emergence applications. Do not use more than 2.25 gal/A/season of Vydate L. Not for use on green onions. Do not harvest tops of treated onions. Do not apply within 14 days of harvest. 48-hr reentry.
- MeloCon WG at 2 to 4 lb/A at 4- to 6-week intervals for nematode suppression. See label for specific application types and timings. 4-hr reentry. O