Competition Between Cucumber Mosaic Virus Subgroup I and II Isolates in Tobacco

Historical reports indicate that Cucumber mosaic virus (CMV) strain subgroup I is more prevalent than subgroup II in most parts of the world, but recent reports suggest that subgroup II isolates may be far more abundant than previously found in China. In order to evaluate the dominance of CMV subgroup I and subgroup II strains in co‐infected tobacco plants, four isolates, NX and YQ in subgroup I, and ZL and AG in subgroup II, were tested in competition experiments. In these comparisons, the frequency of infection was assessed, and ratios between singly and doubly infected plants were calculated based on ELISA tests of tobacco leaves. In contrast to previous reports suggesting that subgroup I strains are usually more competitive than subgroup II strains in the field, the results from the present study indicate that the subgroup II ZL isolate was more competitive than the subgroup I YQ isolate, even though the ZL isolate caused milder symptoms than YQ in singly infected tobacco. In contrast, the subgroup I strains NX and YQ were more competitive than subgroup II AG. This information provides evidence for variation in the competitive abilities of subgroup II strains in tests with subgroup I strains, and suggests that direct competition during mixed infections may account in part for the recent spread of some subgroup II strains in China and elsewhere.