Optimisation of cultural practices for organic potato: effect of irrigation regimes on the development of late blight, crop yield and quality

Summary

A trial was established in 2003, in Southern France, to assess under organic farming conditions the effect of top irrigation with sprinklers on potato yield, but also to evaluate side effects on epidemics, with a focus on late blight (Phytophthora infestans L.). Two varieties, one susceptible (Charlotte) and one with a good level of resistance to late blight (Eden), have been planted. Three irrigation regimes have been applied : no irrigation [NO], reduced or optimized irrigation [OPT] and irrigation according to usual grower practice [NORM]. Irrigation had no effect on foliar late blight development. For both varieties, commercial yield in the non-irrigated plots was significantly lower (16,36 t.ha-1) than in the OPT and NORM plots (resp. 26,12 t.ha-1 and 24,15 t.ha-1), with a higher proportion of small tubers for NO (11,96%) than for OPT and NORM (resp. 4,25% and 3,32%). Irrigation regime had no effect on proportion of tubers with affected by either late blight or Rhizoctonia (black scurf). As expected, Charlotte confirmed its susceptibility to foliar late blight, while Eden had a significant higher total yield than Charlotte (resp. 47,19 t.ha-1 and 32,19 t.ha-1). However, there was no significant difference between the two varieties for commercial yield (resp. 20,51 t.ha-1 and 23,91 t.ha-1), due to a large proportion of tubers with black scurf and of mis-shaped tubers in Eden. For conclusion, irrigation in this region is necessary to obtain reasonable yields and tuber sizes compatible with market requirements. It is possible to reduce irrigation doses without reducing significantly yields, in order to preserve water resources. Under the conditions of this trial, top irrigation on foliage did not increase late blight or silver scurf severity.