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Nematodes as Virus Vectors
Perhaps the most serious nematode-related problem with fruit production in the Cumberland-Shenandoah region is the transmission of plant viruses. The tomato ringspot virus (TmRSV) and tobacco ringspot virus (TbRSV) are both vectored by common species of dagger nematodes (Xiphinema sp.). The nematode acquires the virus when it feeds on an infected plant, then transmits the virus when it feeds on a healthy plant. Figure 2.1 illustrates this exchange. Feeding by dagger nematodes is the only natural means of infection for these viruses. In the absence of dagger nematodes, the virus does not spread naturally from plant to plant. Because dagger nematodes transmit TmRSV, their damage threshold is much lower than that of a parasite that causes damage only by feeding. TmRSV causes serious disease problems in a number of different fruit crops, including peach, apple, plum, cherry, grape, raspberry, and blueberry. TmRSV infects many different plants, including most of the common broadleaf weeds. Good broadleaf weed control may be the most effective way of avoiding these virus problems. TmRSV can be spread in the seed of some weeds such as wind-blown dandelion seed.