The incidence of some viruses in grasses can be affected indirectly by the presence of endophytes in the plant. Endophyte infected grasses produce toxins such as lolines, which deter insect vectors of virus and mycoplasma diseases from feeding on them. The main vector of barley yellow dwarf virus (BYDV) is Rhopalosiphum padi, but this aphid will not feed on E+ tall fescue (Latch et al., 1985). The deterrent chemical is the alkaloid loline (Siegel et al., 1990) (see Chapter 13). Mahmood et al. (1993) sowed plots of E+ and E- tall fescue and noted 3 yr later that the incidence of BYDV was twice as great in the E- plots than in the E+ plots. It is likely that the incidence of other insect-transmitted plant diseases may be reduced in E+ grasses if the endophyte produces a compound that deters the insect vector from feeding.
Zabalgogeazcoa et al. (2000) reported that families of fungal viruses have been found in natural populations of the endophyte Epichloë festucae Leuchtm., Schardl, and Siegel, suggesting that other endophytes also may be infected with viruses. Whether these viruses affect the ability of the endophytes to alter the resistance of their grass hosts to pests and diseases has yet to be determined.
It is likely that further research will add to the list of diseases affected by endophytes in grasses but, at the moment, it appears that endophytes affect a far greater number of species of insects than plant diseases.