Articles
VIRUS INFECTION OF WALNUTS IN FRANCE
Presently in France, there are 2 millions trees, half growing as separate trees.
The percentage of trees grafted in orchards on J. nigra varies with the region from 5 % to 95 % with an average of 15–20 % : most trees grafted on J. nigra are less than 20 years old.
Black line (BL) symptom at the union was observed on J. regia grafted on J. nigra as it was described in the USA on trees grafted on J. hindsii or Paradox.
The black line symptom was observed on 15–20 years old trees and seems to appear earlier when the trees grow in poor soils.
Trees die within 2–4 years after the first BL symptom appears.
BL symptom was never observed on J. regia grafted on J. regia.
Cherry leaf roll virus was isolated from trees grafted on J.nigra with BL symptoms but was also present in some symptomless trees without any symptoms.
Such symptomless infections were also detected on trees grafted on J. regia.
The virus was identified by bioassay on herbaceous plants and serology (ELISA). Presence of CLRV was demonstrated on 6 varieties.
The French Walnut CLRV isolates have been successfully transmitted onto the peach seedlings GF 305 which show a stunt reaction very similar to that observed when infected with CLRV strains from other origins.
The virus is seed borne and the percentage of infected seedlings in J. regia is 5–20 %.
From the serological tests, the French isolates appear to be closely related to the US Walnut isolates and the Elderberry strain of Cherry leaf roll and are also related to the Walnut ring spot and Walnut yellow vein viruses described in Italy.