Principles of Plant Genetics and Breeding

90 CHAPTER 6
What is genetic vulnerability?
Genetic vulnerability is a complex problem that involves
issues such as crop evolution, trends in breeding, trends
in biological technology, decisions by crop producers,
demands and preferences of consumers, and other factors.
As a result of a combination of the above factors, a
certain kind of crop cultivar (genotype) is developed for
the agricultural production system. Genetic vulnerability
is a term used to indicate the genetic homogeneity
and uniformity of a group of plants that predisposes it
to susceptibility to a pest, pathogen, or environmental
hazard of large-scale proportions. A case in point is
the 1970 epidemic of southern leaf blight (Helminthosporium
maydis) in the USA that devastated the corn
industry. This genetic vulnerability in corn was attributed
to uniformity in the genetic background in corn stemming
from the widespread use of T-cytoplasm in corn
hybrid seed production.
Genetic uniformity per se is not necessarily the culprit
in vulnerability of crops. In fact, both producers and
consumers sometimes desire and seek uniformity in
some agronomic traits. The key issue is commonality of
genetic factors. Genetically dissimilar crops can share a
trait that is simply inherited and that predisposes them
to susceptibility to an adverse biotic or abiotic factor.
A case in point is the chestnut blight (Cryphonectria
parasitica) epidemic that occurred in the USA in which
different species of the plant were affected.
Introduction
K. Hammer, F. Heuser, K. Khoshbakht, and Y. Teklu
Key factors in the susceptibility of crops
The key factors that are responsible for the disastrous
epidemics attributable to genetic vulnerability of crops
are:
1 The desire by breeders or consumers for uniformity
in the trait that controls susceptibility to the biotic or
abiotic environmental stress.
2 The acreage devoted to the crop cultivar and the
method of production.
Where uniformity of the susceptible trait is high and
cultivars are widely distributed in production (i.e., most
farmers use the same cultivars), the risk of disaster will
equally be high. Further, where the threat is biotic, the
mode of dispersal of the causal agent and the presence of
a favorable environment will increase the risk of disasters
(e.g., wind mode of dispersal of spores or propagules
will cause a rapid spread of the disease). In biotic disasters,
the use of a single source of resistance to the
pathogen is perhaps the single most important factor in
vulnerability. However, the effect can be exacerbated by
practices such as intensive and continuous monoculture
using one cultivar. Under such production practices, the
pathogen only has to overcome one genotype, resulting
in rapid disease advance and greater damage to crop
production.
Industry highlights
Plant genetic resources for breeding
Institute of Crop Science, Agrobiodiversity Department, University Kassel, D-37213 Witzenhausen, Germany
In recent years the maintenance of plant genetic resources (PGRs) has attracted growing public and scientific interest as well as
political support since it is accepted that there is a close relationship between biological diversity and the health of the biosphere
(Callow et al. 1997).
PGRs can be divided into two groups (Evenson et al. 1998). The first one consists of the cultivated plants themselves, their wild
forms, and related species. The value of this group for improving crop plants is well recognized and expressed in the investment
made by collections, evaluations, and conservation of these PGRs in past and present times. The second group is composed of
PGRs from other, non-cultivated plants like weeds and even in species outside the plant kingdom. This group was neglected until
the modern methods of biotechnology appeared, which allowed the integration of “alien” genetic material into valuable plant species.
While a great amount of the PGRs of the first group are conserved in gene banks (ex situ), there is an increasing requirement for
conserving or better preserving the non-cultivated PGRs of the second group in situ, meaning to save biodiversity as a whole in
the natural habitat. All kinds of plants might have useful properties (in the future) that would be worth maintaining. Frankel (1974)
has already pointed out that there is “no doubt [that] primitive and wild gene pools will continue to serve as important sources of