NATURE OF RESISTANCE IN CUCUMIS-MELO CVS TO PAPAYA RINGSPOT VIRUS TYPE W

Comparative studies were made of the reactions of a 'resistant' (Cinco ), a susceptible (Planters Jumbo) and a Cinco x Planters Jumbo (Cinbo) cultivar of muskmelon to papaya ringspot virus-watermelon strain (PRS V-W). Neither Cinbo nor Cinco supported levels of virus that were dete ctable by biotin-ELISA, but PRSV-W was detected in individual cells of the three cultivars. In inoculated leaves of Planters Jumbo most cell s were infected, but in both Cinbo and Cinco only 3-7% of cells were i nfected. When cotyledons of Cinbo were inoculated, mild systemic sympt oms were observed, but plants remained symptomless when true leaves we re inoculated. Virus was recovered from both cotyledon - inoculated Ci nbo grown at 25 or 40-degrees-C and true leaf-inoculated Cinbo, but on ly when the latter were grown at 25-degrees-C. There was no difference in the percentage of infected cells in these leaves despite the diffe rences in symptom expression. Cinco was highly resistant and only occa sionally showed mild systemic symptoms on cotyledon-inoculated plants grown at 25-degrees-C. Unlike those of Cinbo, leaves with symptoms wer e not infective, but there was no difference in the percentage of infe cted cells in these leaves compared to those in Cinbo. For both Cinbo and Cinco, several lines of evidence are presented that suggest PRSV-W is restricted to a few cells throughout the plant.