Genetic Resources in Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 199

received the most attention, excluding a very large number
of much of the world's food crops. An added problem is
that these ex situ genetic conservation efforts remove crops
from their original cultural-ecological context, severing
the adaptive tie between genome and environment (Hamil-
ton, 1994; Nevo, 1998).
To achieve sustainability, conservation of genetic
resources must also occur in situ or in the setting of the crop
community (Brush, 2004). In situ conservation involves
ongoing selection and genetic change, rather than static pres-
ervation. It allows genetic screening to occur, maintaining
and strengthening local landraces. It attempts to mimic all
the conditions — location, timing, and cultivation
techniques — under which future cultivation of the crop
will occur. As a result, cultivars remain well adapted to (1)
the conditions of the local environment, (2) the cultural
conditions of the local environment (such as irrigation, culti-
vation, and fertilization), and (3) all the locally important
biotic crop problems (such as pests, diseases, and weeds).

In situ conservation requires that farms be the reposi-
tories of genetic information and farmers the repositories
of the cultural knowledge of how crops are cared for
and managed. At one extreme, therefore, the principle of
in situ conservation argues for each farm having its own
breeding and preservation program. Indeed, farmers ought
to be able to select and preserve their own locally adapted
landraces, where feasible. But a more practical approach
focuses at the regional level (Figure 14.1). Because
regional characteristics of a farming region establish
important selection criteria, screening programs can be
centralized to a certain extent for a particular geographi-
cally- and ecologically-defined region, as long as constant
exchange of crop genetic material takes place among
farmers of that region (Brush, 1995; Cunningham, 2001).
Ultimately, in situ and ex situ genetic resource con-
servation efforts must be integrated. Already, partnerships
between nonprofit groups and farmers show that the two
kinds of programs can complement each other and

BREEDING FOR HORIZONTAL RESISTANCE IN BEANS

In the Mixteca bean-growing region of Mexico, farmers confront a variety of potent disease organisms. Bean
common mosaic virus (BCMV), the common blight bacterium ( Xanthomonas sp.), and the fungus Macrophomina
sp., a soil-borne root pathogen, are three of the worst among a large cast of viruses, bacteria, and fungi. Using
traditional breeding techniques, farmers have stayed a half-step ahead of these diseases — generally avoiding
disastrous epidemics but always losing some potential production because of disease-weakened plants.
More than 10 years ago, Dr. Roberto García-Espinosa at the Colegio de Postgraduados (College of Postgradu-
ates), in Montecillos, Mexico, began a bean-breeding program designed to help Mixtecan farmers increase bean
yields without using fungicides. His approach was to develop bean varieties with horizontal resistance — the ability
to resist the pathogen community as a whole, or what he calls the “pathosystem.”
García-Espinosa's trials show that he has succeeded remarkably well. More than 5 years of data have consistently
demonstrated that his varieties are more disease resistant than their conventional counterparts, and produce yields
up to four times larger without the use of fungicides (García-Espinosa et al., 2003; Lotter, 2004).
García-Espinosa's breeding process is a radical departure from that of mainstream plant breeders, who focus
on developing complete resistance to specific races of pathogens, or vertical resistance. His first step is to eliminate
vertical resistance from the parental lines, because plants with vertical resistance tend to lose their horizontal
resistance. He collects as wide a spectrum of germplasm as possible, plants these different genetic lines, and
inoculates the plants with pathogens at a level intended to be sublethal. Then among the surviving plants, he collects
seeds from those that were most susceptible to the diseases. If he were a mainstream breeder, he would be doing
the exact opposite.
After this susceptibility screening, García-Espinosa has a handful of genetic lines with no vertical resistance,
but as-yet-unexpressed horizontal resistance. He hand-cross-pollinates these lines, plants out the progeny, and
subjects them to the same diseases. This time, however, he selects seeds based on resistance to the diseases. This
process, called recurrent mass selection, is repeated at least three times, with only 1 to 10% of plants selected at
each round. This allows the selected cultivars to accumulate polygenes for broad resistance. Then, the selected
cultivars are planted out in the bean-growing region, where dozens of diseases and pest insects exist. In these
additional on-site breeding cycles, resistance to all the locally active pests and diseases is looked out for.
This breeding process produces varieties that yield 2000 to 2400 kg/ha, compared to 1500 kg/ha for commercial
and green revolution varieties. Despite this success, García-Espinosa has trouble attracting funding for his work, both
in breeding the resistant varieties and in disseminating them to farmers (Figure 14.10). García-Espinosa attributes
this to several factors. A big problem is that his breeding process does not fit into conventional notions of plant
breeding. But perhaps more importantly, his horizontally-resistant cultivars cannot help agribusiness corporations
turn a bigger profit, because they are difficult to patent and they don't require agrochemical inputs (Lotter, 2004).