Finally, if parasites provide a rare advantage, then recently common host clones should be more infectible than rare clones, which we tested in a laboratory infection experiment at the end of the field study.
In addition, experimental exposures to parasites showed that recently common clones were more infectible than rare clones, indicating that rare genotypes have an advantage under parasite attack [ILLUSTRATION FOR FIGURE 5 OMITTED].
Indeed, in both soybean and Macroptilium, root hairs older than 5 hours after initiation from the root meristem are no longer infectible. This is in contrast with root hairs of clover, which not only developed tip infections in the emergent root hair zone but also formed lateral or basal infections (initiated at a branch near the base of a hair) in the mature, fully elongated root hair zone (Callaham & Torrey, 1981).
Instead, Frankia, present in a septate hyphal form in the substrate, makes direct contact with an infectible, deformed root hair (Callaham et al., 1979; Berry & Torrey, 1983; Berry et al., 1986; Mansour & Torrey, 1991) to initiate the infection process.