A LEAF DISK ASSAY FOR DETECTION OF RESISTANCE OF MELONS TO SPHAEROTHECA-FULIGINEA RACE-1

The resistance of 10 entries of melons (Cucumis melo) to powdery mildew, caused by Sphaerotheca fuliginea race 1, was evaluated by means of a leaf disk assay. Disks (9 mm in diameter), removed from fully expanded leaves, were placed in petri dishes containing water agar amended with 25 mug/ml of benzimidazole and maintained at 25 C. Disks were inoculated by applying a 10-mul drop of a water suspension of conidia or by blowing air over an infected plant toward the test disk. Differences in inoculation efficacy between the two methods were insignificant. Infection efficacy and sporulation were greater for the adaxial position of the disk. Powdery mildew development and sporulation were reduced following exposure to high light intensity, compared with lower light intensities. The percentage of sporulating disks originating from the cotyledons of resistant plants was low (0-5%), indicating that preliminary selection can be done as early as the cotyledonary stage. The response of the host to powdery mildew, based on the inoculation of disks originating from the third leaf, correlated well with results obtained with the whole plant, indicating that a disk assay using this leaf may accurately predict the response of melon plants to S. fuliginea.