Forecasting and control of Alternaria on carrots

Contact: Dr Jane Thomas
   Dr David Kenyon

Introduction
Alternaria blight of carrots, caused by Alternaria dauci, is a common disease of carrot occurring in all major growing areas of the world. Incidence and severity on UK carrot crops increased during the early 1990s. Fungicide application to control disease development has become more common, but there is little information on the effects of A. dauci infection on the yield and quality of crops. Foliage infection can increase rapidly, starting on the older leaves first, before spreading to cause a widespread blight. 'Green top' lifting systems may be adversely affected, as well as root yield. A forecasting system for the disease is commercially available to growers in the UK, though the benefits that this may confer in terms of improving spray timing and reducing unnecessary sprays have not been quantified. There is evidence that carrot breeding material and cultivars vary in resistance to A. dauci, but no comparative information exists for cultivars commonly grown in the UK; thus, the extent to which cultivar resistance could contribute to reducing disease severity is unknown.

Results
The effect of Alternaria on yield was examined in a total of three fungicide trials, which were inoculated with Alternaria. Controlling the disease with fungicides produced yield benefits, but these varied greatly from year to year.

Experiments with a commercially available Alternaria forecasting system showed that in two out of three years, the forecasting system reduced the number of sprays applied, compared with normal grower practice, without affecting the degree of disease control. There were consistent differences between varieties in the level of Alternaria developing in controlled inoculations. Differences were maintained over the growing season as varieties came to maturity. However, no variety had complete resistance and the best partial resistance was seen in less widely grown varieties.

The financial support of Horticultural Development Council (FV234) is gratefully acknowledged.