PEROXIDASE-ACTIVITY AS A BIOCHEMICAL MARKER FOR RESISTANCE OF MUSKMELON (CUCUMIS-MELO) TO PSEUDOPERONOSPORA-CUBENSIS

Peroxidase activity in uninfected muskmelon plants was used to predict the resistance and susceptibility of 257 plants, including cultivars or breeding lines and crosses of susceptible and resistant plants. When values of peroxidase activity greater than or equal to 30 (changes in absorbance at 470 nm min-1 g-1 fresh weight of leaf tissue) were used, 100% of the resistant plants were detected, whereas at values of less than 30, 89% of the susceptible plants were detected. When values of greater than or equal to 35 were used, 88% of the resistant plants were detected. After infection, peroxidase activity increased with time in both susceptible and resistant plants. The ratio of activity in infected to uninfected leaves increased over time in the susceptible plants. This ratio, however, was lower and remained unchanged in the resistant plants. Using peroxidase activity as a preliminary marker for resistance of muskmelon to certain races of Pseudoperonospora cubensis is suggested in this study.