Article by Karlien van Zyl,
Department Plant Pathology, Stellenbosch University
T
he genetics of plants and
pathogens explain why
some plants are resistant
to diseases, whereas others
are susceptible. It also explains why
plants, after being resistant for
years, suddenly become susceptible.
To ensure sustainable agriculture,
we need to understand how plant
resistance mechanisms function to
successfully employ resistant crops
in an integrated disease management program. Two mechanisms
govern disease resistance and are
referred to as monogenic and polygenic resistance. Monogenic resistance is controlled by a single
gene, and polygenic resistance is
controlled by a number of genes,
also called quantitative resistance.
Although response of the host
plant is determined by the genetics
of plants and pathogens, the environment also plays a major role in
disease development. For example,
the single gene conferring re-
sistance in wheat to black stem rust
is expressed at 20°C, but not at 25°
C.
Resistance breeding to control plant
pathogens is most often exploited
by breeders to obtain monogenic
resistance, since such resistance
confers immunity (no disease) to
pathogens. Breeders can also easily
transfer single resistance genes
from one host genotype to another.
This resistance, however, is only
Figure 1. Examples of monogenic resistance: Leaf rust of wheat caused by Puccinia triticina (picture taken
from https://www.agric.wa.gov.au/mycrop/diagnosing-leaf-rust-wheat), stripe rust/ yellow rust of wheat
caused by Puccinia striiformis f. sp. tritici (picture taken from http://rusttracker.cimmyt.org/?page_id=9),
and powdery mildew of wheat caused by Blumeria graminis f. sp. tritici (Photo credit: Thomas Lumpkin,
CIMMYT; https://www.flickr.com/photos/cimmyt/5123746065).