Factors affecting the development and control of black dot on potato tubers

Field trials were carried out over a 4 year period (2004-2007) to determine the effect of agronomic factors, specifically cultivar resistance, irrigation, crop duration and chemical control (in-furrow application of azoxystrobin), on black dot development on potato tubers grown in fields where soilborne inoculum of Colletotrichum coccodes was present. In 2004, 2005 and 2006, two field trials were performed each year and in 2007, 19 mini-field trials were carried out across Scotland and England. Cultivar resistance was clearly demonstrated to be an effective method of reducing black dot disease severity on tubers (described here as the percentage of unmarketable tubers, i.e. those with symptoms covering a surface area of > 10%). In the four field trials carried out in 2004 and 2005, in irrigated and fungicide-untreated plots, 43.8% of tubers of cv. Maris Piper were unmarketable, compared with 17.0% of tubers of cv. Sante. Assessments of disease development on underground plant parts (stems, stolons and roots) revealed that cultivar resistance acted only at the tuber level, as disease symptoms on other parts were often high irrespective of published disease resistance ratings. Irrigation increased the severity of disease on tubers in two trials (England 2004 and 2006), but its effect was less significant when rainfall was high. Delaying harvest by 2 weeks increased disease severity in all six trials, whilst application of azoxystrobin consistently reduced black dot severity. There were significant interactions between factors. The results clearly show how black dot disease severity can be reduced through an integrated approach to disease management.