Relationship between <i>Alternaria radicina</i> soil population density and carrot black rot

Abstract

Alternaria radicina is a seed and soilborne pathogen that causes black rot of carrot tap roots and black rings on the root crown The A radicina population density in spring was determined in carrot field soils to investigate the relationship between soil conidial populations and black rot levels in summer In each of 15 carrot fields four randomly selected 30 m² plots were used for soil sampling to a depth of 5 cm in September Alternaria radicina population densities (cfu/g soil) were determined using a soil dilution method and selective agar Black rot disease incidence was expressed as the number of infected plants/m of row and severity was assessed using a 0 to 4 rating scale Alternaria radicina soil population density (33233 cfu/g soil) correlated positively (P<001) with black rot severity (R² 084) and incidence (R² 082) In Canterbury disease incidence (1090) and severity (078218) of black rot varied with the soil population density and their average score was 587 and 161 respectively Soilborne inoculum therefore has a major role in the development of black rot infection