Agriculture Reference
In-Depth Information
5.2 HORIZONTAL RESISTANCE
Other terms have been used in some cases more or less synonymously, to describe
horizontal resistance including race non-specific resistance, general resistance,
durable resistance, polygenic resistance and partial resistance. However, Van der
Plank (1963,1968) provides compelling reasons why the term horizontal is the most
appropriate. Horizontal resistance slows down the rate at which disease increases
within a single plant or population of identical plants. It can, therefore, usefully be
described as rate-reducing resistance. The mechanisms by which horizontal resis-
tance slows the rate of epidemic development may be active or passive and may be
expressed through reduced infectivity, lower levels of sporulation (in both cases
resulting in a reduction in the basic infection rate), lengthened latent periods, or
increased rate of removal of infectious tissue (reducing the infectious period).
Frequently, experimenters have examined the nature of individual components of
horizontal resistance to specified pathogens within selected varieties and have
correlated these with the established reactions of the same varieties when infected by
that same pathogen under field conditions. Under such circumstances, it is relatively
easy to establish a relationship between, for example, lesion development and field
response. Guzman-N (1964) showed that the numbers of late blight lesions
established per unit of inoculum by
Phytophthora infestans
on a known susceptible
potato variety were 50% higher than on a known resistant variety (Table 5.1).
Similarly, the numbers of sporangia per unit area were seven times greater on a
susceptible compared with a resistant variety (Guzman-N, 1964) (Table 5.1).
In the same way, Hakiza (1997) reported that the latent period for robusta coffee
leaves infected by rust (
Hemileia vastatrix
) was over three times longer in an estab-
lished resistant variety compared with a known susceptible variety (Table 5.1). In a
study of epidemics of
Diaporthe adunca
on experimental and natural populations of
Plantago lanceolata,
Linders
et al.
(1996) used non-linear models to examine the
rate of disease increase and the final disease levels in susceptible and partially
resistant cloned genotypes. For both of the models used, the estimate of final disease
level was significantly lower in the partially resistant genotype (69% and 66% for
logistic and Gompertz models) compared with the susceptible genotype (100% for
both models, Table 5.1). Similarly the rate of disease growth was lower in the
partially resistant genotype (0.048 and 0.083 for the logistic and Gompertz models)
than in the susceptible type (0.078 and 0.111 for the logistic and Gompertz models,
Table 5.1).
When novel plant selections are generated from a breeding programme, initial
resistance screening is often conducted under controlled conditions. Although indi-
vidual components of horizontal resistance are relatively easy to measure under
controlled conditions, they are difficult to assess in the field. Furthermore, advanced
knowledge about which component of resistance may relate to field response is lack-
ing. There is no guarantee that values obtained for each of the individual
components measured under controlled conditions will be mirrored in the field
response. Some of the reasons for this lack of correlation arise from the differences
in environmental conditions and their individual effects on the expression of
resistance.
