THE RELATIONSHIP BETWEEN WALNUT BLIGHT EXPRESSION AND TREE WATER STATUS

Walnut Blight (WB) is currently the disease which leads to the highest yield reduction in most of the European walnut production countries. In 2006 and 2007, the percentage of fruits falling due to WB measured on 15 plots in the South-East of France ranged from 9 to 56% and from 11 to 66%, respectively. The symptoms of WB include necroses that are clearly visible on fruits and these may be used to identify and quantify WB expression in orchards. However, necroses on fruits can also result from water and/or nutrient stresses occurring during the growing season. This phenomenon has been observed on several species of fruits or annual shoots including tomato or peach. These results indicate that the existence of necroses is not restricted to the expression of Xanthomonas arboricola pv. juglandis. Interaction between the physio¬logical status of the tree (water, nutrient) and the presence of the bacteria may explain orchards sensitivity to WB. It means that cropping practices such as irrigation or fertilizing may play an important role on WB expression. Across three years, the monitoring of micrometrical variation of shoots has been used to assess the water status of trees. Water status was also directly recorded at the same time. In 2007, we observed that a higher expression of WB was associated with more frequent stress observed on trees. These results show interesting trends that we will discuss in the context of the improvement of water supply and water resource restriction.