Can crop rotation reduce inoculum of the carrot pathogen Alternaria radicina?

Abstract

The fungal pathogen Alternaria radicina causes pre- and post-emergence damping off, leaf and umbel blight and black rot of carrot. It is seed and soil-borne, and in mid-Canterbury, soil populations ranging from 33-223 colony forming units (CFU)/g soil have been recorded. How long A. radicina remains viable in New Zealand soils was not known. Fields which had last produced a carrot seed crop from twelve down to one year previously were identified and their A. radicina population determined. The pathogen was still present six years after a carrot seed crop had been harvested, but was absent from soils where a carrot seed crop had been harvested seven or more years previously, suggesting that a seven to eight year gap between carrot seed crops in the same field would be required to avoid disease problems from this source. In a pot trial, A. radicina inoculum was reduced by 29% (from 240 CFU/g) after four and a half months in the presence of wheat plants, and was also significantly reduced in the presence of barley and faba bean, but not perennial ryegrass or pea. It is possible that root exudates contain antifungal compounds, but this was not determined. Whether growing the non-host crops wheat, barley or faba bean after carrot can reduce A. radicina soil-borne inoculum requires further investigation.