Walnut blight: development of fruit infection in two orchards

Walnut is one of the major regional crops produced in Dauphiné (SE France). Unfortunately, walnut blight has been recorded as the main disease for years. Crop losses due to this disease may reach high levels in spite of repeated copper sprays. In order to monitor the bacterial contamination of the fruits, two orchards differing in soil, irrigation and treatment were selected. Fruit sampling was performed once a week for 11 weeks. Results showed that the percentage of contaminated fruits was more important at the beginning of a maturation, that bacterial populations rapidly reached a maximum level and that there was no obvious relation between the number of bacteria infecting a fruit and the occurrence of necrosis. These results did not indicate any significant difference between the two orchards, in these respects.