Pathogenicity and control ofAlternaria radicinaandA. dauciin carrots

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Abstract
Alternaria radicina and A. dauci were shown to be virulent leaf pathogens of carrots in New Zealand. A. dauci was predominant in winter and A. radicina predominant in summer. Seed infection was controlled most effectively by a hot thiram soak Effective sprays were organo-tin compounds, especially triphenyltin hydroxide, and propineb showed promise. In field trials 50–78% of plants showed leaf blight, caused predominantly by A. dauci. The outer mature leaves were more susceptible to blight than younger inner leaves. Several alternative hosts were found: wild carrot and parsnip and a common weed, Fumaria muralis for A. dauci; celery and F. muralis for A. radicina. These are new host records for New Zealand and Fumaria is possibly a new world record. Infected debris can provide a further source of infection. Young carrot roots were not susceptible in this study despite severe foliar infection. Neither of these diseases had any effect on carrot roots in the field or in a carrot root storage trial.