Exploring the potential for cost savings through matching late blight fungicide inputs to cultivar resistance in potatoes

The primary objective of this study was to evaluate if integrated control of late blight, using cultivar resistance to replace some of the fungicide input, was feasible under GB conditions.

In 2007, the control of foliar and tuber blight was investigated using combinations of cultivar resistance, fungicide product, spray interval and dose in two trials: one near Aberystwyth and one in Ayrshire.

At each site, three cultivars were treated with three fungicides at full UK label rate, three-quarter rate and half-rate at 7-day intervals, and full and half-rate at 10-day intervals. Cultivars with different foliar and tuber blight resistances were used (National List foliar blight rating/tuber blight rating):

• Shepody (2/3),
• Maris Piper (4/5)
• Lady Balfour (8/7).

The fungicide active ingredients tested were mancozeb (in Dithane NT DG), fluazinam (in Shirlan 500 SC) and fluopicolide + propamocarb HCl (in Infinito SC) and these were evaluated as components of season-long spray programmes. They were applied from the canopy stable growth stage through to haulm desiccation.

When interpreting the results from these trials the following points must be remembered: the extreme blight pressure in 2007, the presence of 13_A2 at both sites and the fact that the 7-day programmes at the Welsh site were adversely affected by 45 mm of rain shortly after the first 7-day treatment was applied.

The results from both trials demonstrate that cultivar resistance could be utilized in Britain to reduce fungicide inputs even where blight pressure is extremely high and the more aggressive 13_A2 genotype is present. The relative efficacies of the integrated control treatments, i.e. the combination of a cultivar with a higher resistance rating with less of the same fungicide, in controlling foliar blight was most often significantly better than the combination of the more susceptible variety treated with the maximum fungicide input. In 26 out of 48 comparisons the integrated control treatments gave significantly better control, in 21 there was no significant difference and in only one was foliar blight control significantly worse.

The relative efficacies of the integrated control treatments compared with the benchmark combination of susceptible cultivar and maximum fungicide input was influenced by the trial, fungicide product and the interaction between these two factors. At both trial sites integrated control treatments using Dithane NT almost always gave significantly better control of foliar blight than the benchmark. However, for the two other fungicide products, Shirlan and Infinito, the results were different at the two sites.

Efficacy against tuber blight was less easy to summarise because of the low incidences of tuber blight in many of the treatments at both sites.

In 2008, twenty-three potato varieties were screened for foliar resistance to late blight. The ratings were reduced for many cultivars and the reduction was clearly greater for the cultivars with higher published ratings.