Cause Several species of Pythium, including P. ultimum (syn. Globisporangium ultimum), P. violae (syn. G. ultimum), and P. sulcatum. These are fungus-like microorganisms that live in organic matter in soil. Of those, P. sulcatum and P. violae tend to be highly pathogenic; the others vary in pathogenicity.
Symptoms Small, horizontal lesions first appear on carrots 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.
- Avoid fields with a recent history of cavity spot.
- Plant carrots on raised beds in well-drained fields.
- Varietal susceptibility tends to be inconsistent from year to year. However, the following varieties have shown greater than average resistance in some trials: Caropride, Dagger 78, Fannia, Navajo, Orlando Gold, Panther, Six Pak, Six Pak II, and Spartan Premium.
- Ridomil Gold Bravo SC at 1.5 to 2.5 pints/A starting 40 to 50 days after planting and continue on 14-day intervals. Preharvest interval is 7 days. 48-hr reentry.
- Ridomil Gold Copper at 1 to 2 pints/A applied preplant or at-planting. Beginning 40 to 50 days after the first at-planting application can apply 2 lb/A on 14-day intervals. Preharvest 7 days. 48-hr reentry.
- Ridomil Gold SL at 0.5 to 1.3 pints/A applied to the soil. Do not exceed 2 pints/A per season. 12-hr reentry.
- Amplitude at 3.2 to 4.8 fl oz/100 lb seed pieces, 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, or 3 to 4 quarts/A as a chemigation treatment on 14- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
- Double Nickel LC at 0.5 to 4.5 pints/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- SoilGard 12G at 0.5 to 10 lb/A at 4-week intervals will suppress Pythium and Rhizoctonia. See label for specific application types and timings. Can be applied on the day of harvest. 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
References Benard, D., and Punja, Z.K. 1995. Role of Pythium species in cavity spot development on carrots in British Columbia. Canadian Journal of Plant Pathology 17:31-45
du Toit, L.J., Derie, M.L., and Alcala, A.C. 2014. First report of Pythium sulcatum causing cavity spot in processing carrot crops in the Columbia Basin of Washington state. Plant Disease 98:427.
Saude, C., Simon, P.W., and McDonald, M.R. 2014. Incidence and severity of cavity spot of carrot as affected by pigmentation, temperature, and rainfall. Plant Disease 98:929-936.