Black dot  - Colletotrichum coccodes

Symptoms   Tubers develop brownish-grey lesions, similar to silver scurf, during storage.  Minute black dot (microsclerotia) give an obvious sooty appearance. High levels of Black dot can cause stems, roots and stolons to be affected leading to wilting and premature withering similar to the symptoms of Rhizoctonia and Verticillium wilt. Visual quality deteriorates making the crop unsuitable for some high-value pre-pack markets.

Sources and Factors Favouring Development

Seed and soil-borne infections favoured by wet field conditions.  Development can be rapid in stores that are warm and moist for extended periods of time. It is most severe in soils that are coarse textured or low in nitrogen, at high temperatures or where soil drainage is poor.  Close rotations increase disease risk and severity.

Control          Avoid planting heavily infected seed tubers and increase rotation length where possible; Trials in 2004 showed that there was little or no recognisable benefit from sourcing seed that was free from Black dot if it was to be planted into land which is known to be infected.   Plant stress increases susceptibility to Black dot, therefore adequate soil moisture and sufficient fertility throughout the growing season can help to provide some control. Injury from the herbicide metribuzin has been associated with increased Black dot severity. Adverse effects on yield due to Black dot occur late in the growing season therefore, early cultivars are generally recommended where the disease is severe.  If possible avoid long term storage (particularly in warm humid ambient stores) of severely affected stocks; assess suitability prior to harvesting. Azoxystrobin has an off label approval for controlling black dot and black scurf at 3l/ha when applying in-furrow through specialist equipment at planting, or 6l/ha applied as an overall spray to the soil using conventional tractor mounted spray equipment.