EFFECTS OF SINGLE AND DOUBLE INFECTIONS WITH Potato virus X AND Tobacco mosaic virus ON DISEASE DEVELOPMENT, PLANT GROWTH, AND VIRUS ACCUMULATION IN TOMATO

The tomato cv. Fukuju n. 2 was used for studying the effect of single and double infections with Potato virus X (PVX) and Tobacco mosaic virus (TMV). Mixed infection resulted in a synergistic increase of disease severity, where more growth reduction was seen with simultaneous inoculations than with sequential inoculations at four-day intervals. At five and 12 days postinoculation, the increased severity of the disease coincided with enhancement of virus accumulation in the rapidly expanding upper leaves. The PVX concentration in leaves n 5 to 7 of doubly infected plants was three to six fold that of singly infected ones, as determined by DAS-ELISA. Mixed infection with the L strain led to higher enhancement of PVX than with the TMV-L 11 A strain. The concentration of TMV-L was lower in double infection and significantly higher than TMV-L 11 A in both singly and doubly infected plants. Analyses of the PVX ORF2 by Western blot and Northern hybridization revealed the pattern of accumulation of the 25 kDa protein and the RNAs, respectively, following those of the virion and coat protein. The strain TMV-L 11 A overcame the resistance gene in cv. GCR 237 (Tm-1). In the upper leaf n. 8, the concentration of PVX was three times higher in plants with mixed infection than with L 11 A. The concentrations of the L and OM (TMV strains) in both singly and doubly infected plants were at very low levels, and the synergistic effect on PVX concentration and disease severity was not observed. Additonal keywords: synergism, Western blot, Northern hybridization, DAS-ELISA, virus enhancement.