When tall fescue and perennial ryegrass (Lolium perenne L.) infected with Neotyphodium endophytes were found to cause disorders in grazing animals, it seemed sensible to eliminate these fungi from the grasses. However, the performance of these grasses when free from their endophytes made it apparent that endophytes played a role in the host fitness and survival. The presence of the endophyte enabled tall fescue in the southeastern United States to withstand better drought conditions and water deficit stress (see Chapter 4).
In New Zealand the endophyte in perennial ryegrass (N. lolii Latch, Christensen & Samuels) was found to be essential to the grass to provide protection from attack by the Argentine stem weevil [Listronotus bonariensis (Kuschel)] that was previously thought to be of little economic importance (Prestidge et al., 1982). These observations inspired research into determining what other benefits these endophytes conferred on their host grasses. The reader is referred to previous reviews on diseases of tall fescue independent of endophyte effects by Chapman (1979) and Leath et al. (1996).
Research into the effects of endophyte infection of grasses on pests and diseases was undertaken in laboratories around the world; many of the results obtained are outlined in this chapter. A great deal of this research has explored the effects that endophyte infected grasses have on insects because they appeared to be most affected. Research into the effects that endophytes have on plant diseases caused by fungi, bacteria, and nematodes (see Chapter 10) has not been as extensive as that of investigations on insects (see Chapter 9). This may be explained partly by the fact that so few diseases have been shown to be affected by endophytes, in contrast to the numerous insects affected. There is still vast opportunity for further research on the effects of endophytes on pests and diseases and the chemical compounds produced by endophytes, which are responsible for these effects.
The results of some investigations have been confusing because particular pests and diseases were affected by the presence of endophyte in grasses in some trials and not in others. This is understandable because of the many variables among trials. In most trials referred to in this chapter, there were three biotic components: the grass, the endophyte, and the pest or disease agent. Although the species of all three components in the trial may be stated, the genetic composition of each of the three can vary substantially. The cultivar of grass may differ among trials. The many strains of endophyte that exist within a species vary in their growth, and the chemicals they produce in plants can differ. There may be many strains of the pest or disease-causing organism. Finally, environmental conditions can differ greatly among trials as they interact with the function of one or more of the organisms. Given these variables, it is not surprising then that researchers may arrive at different conclusions as to the effect of an endophyte on a particular organism. Nevertheless, it is obvious that many insects and some diseases are affected by the presence of endophytes in grasses and that these grasses benefit greatly from endophyte infection. The fact that endophytes are common in grasses growing wild in their centers of origin testifies to their benefit.