active against certain groups of
pathogens. With monogenic resistance, the plant host will have a
(R-gene) product that specifically
recognises a (avr-gene) product
produced by the pathogen. The
pathogen, however, can overcome
single-gene plant resistance by
modifying their gene products to
prevent recognition by plant gene
products. An example of this type
of resistance is that the wheat
gene, Lr34, which is associated with
resistance to leaf rust (caused by
Puccinia triticina) and stripe rust (P.
striiformis f. sp. tritici), as well as
powdery mildew (Blumeria graminis
f. sp. tritici) (Figure 1).
The other type of resistance, polygenic resistance, is often described
as “quantitative” or “partial resistance”, and does not result in
immunity. It is more difficult to
study polygenic resistance in plants,
as several genes jointly provide the
resistance response, and because
such resistance is often influenced
by the environment. Polygenic resistance is rather non-specific, and
often protect plants against differ-
ent pathogens and pests. An example of polygenic resistance is that of
several sugar beet varieties that are
moderately resistant to downy mildew (Peronospora farinosa), powdery mildew (Erysiphe betae), Ramularia leaf spot (Ramularia beticola),
and rust (Uromyces betae) (Figure
2).
Plant resistance has contributed
significantly to the control of plant
diseases. The genetics of disease
resistance in host plants will continue to be a practically imperative
research topic.
Figure 2. Examples of polygenic resistance: Downy mildew of sugar beet caused by Peronospora farinosa
(picture taken from http://www.kws-uk.com/aw/KWS/united-kingdom/Products-TopMenu/Sugar-beet/
Technical-Reference-Folder/Diseases/Artikel-Krankh-GB/~drg/Downy-mildew-GB/), powdery mildew of
sugar beet caused by Erysiphe betae (picture taken from https://agriculture.basf.com/en/CropProtection/Powdery-mildew-beet.html), Ramularia leaf spot of sugar beet caused by Ramularia beticola
(picture taken from http://www.kws-uk.com/aw/KWS/united-kingdom/Products-TopMenu/Sugar-beet/
Technical-Reference-Folder/Diseases/Artikel-Krankh-GB/~drj/Ramularia-GB/), and sugar beet rust caused
by Uromyces betae (picture taken from http://www.kws-uk.com/aw/KWS/united-kingdom/ProductsTopMenu/Sugar-beet/Technical-Reference-Folder/Diseases/Artikel-Krankh-GB/~biwd/Image-gallery-ofDiseases/).