Cause Species of Pythium violae, P. sulcatum, and P. ultimum (syn. Globisporangium ultimum). These are fungus-like microorganisms that survive in organic matter in soil.
Symptoms Small, horizontal lesions first appear on parsnip storage roots when they are about ready to harvest. Later, lesions may darken and enlarge to about 0.25 inch in length. There usually is no secondary rotting, but the lesions, if numerous, make the carrots unmarketable. Pythium and Globisporangium can also infect and kill the fine roots, which will take on a brown color.
Cultural control
- Avoid fields with a recent history of cavity spot.
- Plant parsnip in well-drained fields.
- Harvest in western Oregon before the fall rains commence.
Chemical control
- Ridomil Gold SL (Group 4) at 1.0 to 2.0 pints/A applied to the soil is labeled for Pythium and Globisporangium control. 48-hr reentry.
Biological control
- SoilGard at 2 to 10 lb/A as an in-furrow treatment will suppress Globisporangium, Pythium and Rhizoctonia. See label for specific application types and timings. 4-hr reentry. O
- Stargus at 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, 3 to 4 quarts/A as a soil drench on 10- to 14-day intervals, or 3 to 4 quarts/A as a chemigation drip treatment on 14- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Guba, E.F., Young, R.E., and Ui, T. 1961. Cavity spot disease of carrot and parsnip roots. Plant Disease Reporter 45:102-105.