Laboratory evaluation methods of apple Valsa canker disease caused by Valsa ceratosperma sensu Kobayashi.

Valsa canker is one of the most destructive bark-rot diseases on trunk of apple tree in China. The aim of the research was to develop a simple but stable, efficient and easy method for disease evaluation in laboratory. Excised leaves, young shoots, fruits and twigs of ‘Fuji’ apple trees were inoculated with a virulent isolate 03-8 after making different wounds. The samples were kept at 25℃ in a saturated moisture box. No disease was found in uninoculated and/or unwounded samples, while lesions were occurred in treatments by wound inoculation. The type of wound affected the disease development greatly. Lesions developed much faster on upper side wound than lower side of the leave. No significant difference was observed between wounds pricking one time and ten times on upper side of the leave. Bigger lesions were formed on young shoots by pricking wound than leaf scar. There were significant differences (P0.05) among wounds of pricking one time, ten times and removing partial peel on fruits. Also, the disease developed much faster on leaf, young shoot and fruit samples (1.5-2 days) than on the twigs (10 days). Furthermore, four different virulent isolates were used to test the reliability and stability of the method. The results showed that expanded leaves or young shoots were better materials than twigs and fruits because different lesions caused by the different isolates could be observed obviously in less than 3 days. Thus, the evaluation method for Valsa canker was suggested as follows: using detached expanded leave or young shoot as plant material in growing season, inoculating one pathogenic fungi disc on the upper side of leaf or surface of shoot after pricking one time with needle, keeping at 25℃ for 2 days in a satisfied moisture box, then measuring the lesion diameter. This me-thod could be used to assess materials in quantity at a short time, obtain good results from replication, screen resistant cultivar, test isolate virulence and chemical efficacy indoor.