Agriculture Reference
In-Depth Information
Van der Plank (1968) predicted a small effect of host diversity on epidemics in
larger plants. This is because inoculum on small plants may be more effectively
mixed throughout the host population rather than landing principally on the source
host individual. Nonetheless, Garrett and Mundt (2000) showed a significant effect
of host diversity on the severity of foliar symptoms of late blight in potato cultivar
mixtures consisting of cv. Red LaSoda (susceptible) and a resistant breeding
selection. The AUDPC was reduced by an average of 36 and 37% in consecutive
years although the yield and the level of tuber infection increased and decreased
respectively only in the first year. Similar trends were observed in experiments using
inoculum sites throughout the field and sites restricted to one corner of the field. The
mechanisms postulated for the observed effects included a reduction in the proportion
of susceptible tissue, physical barriers to inoculum spread, or compensation or
competition between host genotypes. It is probable that the resistant plants produced
little inoculum as the epidemic proceeded on the susceptible plants resulting, overall,
in a greatly reduced inoculum level in the mixtures (Garrett and Mundt, 2000). The
size of potato plants apparently failed to produce significant autoinfection effects as
inoculum was probably being spread far enough from source individuals. The plant
size effect may have been further reduced by the intertwined nature of foliage
growth (Garrett and Mundt, 2000).
Predictions of the likely extent of host diversity effects in specific host-pathogen
systems can be made on the basis of the five major characteristics described by
Garrett and Mundt (1999) (Table 5.8). Of course, the extent of the host diversity
effect on disease progress will also be affected by additional factors including
growth compensation, plant competition and competitive interactions between
pathogen genotypes. Furthermore, environmental conditions and crop management
activities will also influence the host diversity effect by imposing changes on the
host density, epidemic length and epidemic intensity (Garrett and Mundt, 1999).
5.5. INDUCED RESISTANCE
At present, reference to induced resistance is fraught with the problems of
semantics. Clarifications have been attempted (see for example, Kloepper
et al
.,
1992), but confusion remains. Induced systemic resistance (ISR) can best be
described as: activation, by biotic or abiotic agents of a resistance dependent on the
host plant's physical or chemical barriers. Systemic acquired resistance (SAR) is
regulated through a distinct signal transduction pathway. Salicylic acid is implicated
in SAR regulation, whereas jasmonic acid is implicated in ISR regulation (Graham
and Leite, 2004).
The effectiveness of induced resistance under controlled environmental condi-
tions now appears not to be under any doubt. Although successful induced resistance
under field conditions has been demonstrated (see for example Reglinski
et al
.,
1994), as Lyon and Newton (1997) point out, the question is one of how well the
treatment works under field conditions and what are the epidemiological conse-
quences when the plants come under attack from a variety of pathogens and other
stresses.
