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Surveys of eight carrot growing regions in South Australia conducted from December 1998 to May 1999, showed that poor seedling establishment was associated with high levels of infection by Alternaria radicina. Seedling losses were widespread throughout the state and were most frequent between February and April 1999. Up to 47% of seedlings and 88% of mature plants on some properties were infected by A. radicina. Isolations from diseased plants showed that A. radicina attacked carrots at all stages, causing damping-off, and rotting of roots, crowns, seedlings, petioles, leaves and crowns of maturing carrots. Up to 70% of mature carrots with crown and shoulder infections were unmarketable. Of the 19 lines of commercial seed tested, 18 were infected with A. radicina with the highest infection level of 35% occurring on imported seed. A. radicina at levels between 0.2 and 14% was also found in 11 of 16 seed batches treated with thiram and/or iprodione. Isolates of A. radicina from seeds or infected seedlings were pathogenic to carrot seedlings and carrot discs. Alternaria dauci was also isolated from two seed samples, with levels of infection ranging from 0.1–0.3%, and on foliage in three of the eight carrot growing regions. |
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