Cavity spot is caused by persistent, soil-borne Pythium water moulds, and wet soil is a major part of the problem. OMAFRA notes lesions commonly appear later in the season; BC's current carrot guide specifically identifies cool, wet conditions, excessive soil moisture, poor drainage and heavy soils as favouring disease.

Cavity Spot is caused by several soil-borne Pythium species. These water moulds persist in the soil and infect developing carrot roots, producing small lesions that gradually enlarge into the characteristic sunken, dark cavities. Lesions may become more obvious later in the season and can continue to enlarge during storage.

Cavity spot is strongly associated with wet soil. Cool, wet conditions, poor drainage, heavy soils and periods of waterlogged soil all increase the risk. Low areas of the garden where water collects can be particularly troublesome.

How to Fix It
Improve drainage: Avoid planting carrots where water sits after rain. Raised beds can help in poorly drained soil.
Don't overwater: Keep carrots adequately watered without keeping the soil saturated. Avoid heavy irrigation that temporarily waterlogs the root zone.
Avoid cold, wet soil: Don't rush early carrot sowings into soil that is persistently cold and soggy.
Rotate carrots: Avoid repeatedly growing carrots in an area with a history of cavity spot. Because Pythium persists in soil, rotation reduces risk but won't necessarily eliminate it.
Choose less-susceptible varieties: Where cavity spot is a recurring problem, cultivar choice can reduce losses.
Harvest promptly: Don't leave mature carrots sitting unnecessarily in wet soil late in the season.