Inhibition of mechanical transmission of virus by sap, and infection of Cucurbita ecuadorensis with papaya ringspot virus type W

Abstract

Dilutions of Cucurbita ecuadorensis sap were mixed with dilutions of sap from C. maxima infected with papaya ringspot virus type W (PRV-W), and the mixtures mechanically inoculated to seedlings of C. pepo cv. Small Sugar or Cucumis sativus cv. Supermarket. Plants inoculated with C. maxima sap plus C. ecuadorensis sap (at 1/8 when C. pepo was the test plant, or 1/16 when C. sativus was the test plant) showed reduced infection compared with plants not receiving C. ecuadorensis sap. The reduction caused by C. ecuadorensis sap was greater at a lower virus concentration (1 g infected C. maxima/400 mL sodium-potassium phosphate buffer 0.1 mol/L) than at a higher virus concentration (1 g/25 mL). It was concluded that C. ecuadorensis sap contains an inhibitor of infection, that tissue derived from this species should be diluted at least 1 g/ 10 mL buffer before use in inoculations, and that C. pepo, which was 5 times more sensitive than C. sativus, should be used as the test seedling in reisolations. When C. ecuadorensis, C. maxima and the F1 (C. maxima x C. ecuadorensis) were inoculated on their cotyledons with a severe isolate of PRV-W, symptoms were expressed on at least 1 plant of each species and the F1, and the virus was reisolated in seedlings of C. pepo cv. Small Sugar from the uninoculated leaves of all plants. Severity of symptoms and the proportion of plants infected at reisolation were least for C. ecuadorensis, intermediate for the F1 and most for C. maxima. This is the first report of infection of C. ecuadorensis by PRV-W.