The recent dramatic increase in world rice prices drastically affected
African countries, which are heavily dependent on rice imports. Consequently,
many African countries are trying hard to develop their largely unexploited
rice potential to boost domestic production. However, both expansion (the
development of new areas) and intensification of rice cultivation will bring
new problems, including diseases.
Three main diseases are hampering the effort to intensify rice
cultivation in Africa: rice blast (due to pathogenic fungus Magnaporthe oryzae), bacterial blight (caused by Xanthomonas oryzae pv. oryzae) and rice yellow mottle disease (initiated
by a Sobemovirus).
Rice yellow mottle
virus (RYMV) was first observed in Kenya in the late
1960s. It is now a major disease of rice in the irrigated and lowland ecologies
in almost all rice-producing countries in Africa, causing yield losses of
25–100% (depending on the rice variety grown).
For a long
time, breeders had access to only one gene for RYMV resistance (rymv1), albeit in four versions — one (rymv1-2) available in an indica variety (Gigante) originating
from Mozambique and the others (rymv1-3, rymv1-4 and rymv1-5) in Oryza glaberrima
landraces (TOG5681, TOG 5672 and TOG5674,
respectively).
These O. glaberrima genes are in several of
the Lowland NERICA varieties. Subsequently, a second gene for RYMV resistance
was found in O. glaberrima (RYMV2). Unfortunately, there are natural
resistance-breaking isolates of the virus. This means that if we release a
variety with one of the resistance genes into a hotspot area, the
resistance-breaking viruses will survive and multiply, so that the final state
is no better than the first!
As an
insurance policy against this happening, the AfricaRice breeding strategy is to
combine two resistance genes in varieties for release in hotspot areas. This
work is being done in collaboration with Institut de recherche pour le
développement (IRD).
In
addition, since it has been shown that the occurrence of resistance-breaking
strains is correlated with high disease inoculum, AfricaRice is seeking ways to
reduce the disease pressure as a component of an integrated management system
to reinforce the durability of the resistance.
Screening
for RYMV resistance is conducted in a screenhouse in Cotonou, Benin. The
screenhouse is designed to prevent the disease from escaping into the ‘wild’.
Segregating populations (the offspring of crosses that are not yet
true-breeding) are mechanically inoculated with the virus, and the disease is
allowed to run its course.
In
addition to visible symptoms, we now have access to three molecular markers for
RYMV resistance, which can be detected in the laboratory to show the presence
or absence of the resistance gene (or genes) in any particular plant.
Blast disease affects (particularly upland) rice
in most parts of the world, inflicting yield losses of 70–80% under
disease-favorable conditions. Bacterial blight
(BB) has been known in Africa since the late 1970s in the irrigated ecology of the
savannah and Sahel regions — yield losses attributable to BB are in the range
of 20 to 78% in Southeast Asia and India.
There are
several known major resistance genes for each of these diseases (blast resistance:
Pi1, Pi2, Pia; BB resistance: Xa1, Xa2,
… Xa26), but these are not durable, so
new pathogens can emerge and ‘break’ the resistance conferred by these genes. The
aim of the AfricaRice breeding program is to combine (pyramid) two or more of
these non-durable resistance genes into varieties for farmers.
In addition, some major
genes that induce partial durable resistance to blast in Asia (e.g. Pi40) will be tested under African
conditions to see if they can be used to protect African rice varieties against
blast disease.
The plant
pathology laboratory is working in collaboration with the Japan International
Research Center for Agricultural Science (JIRCAS) and the Green Super Rice
project to determine which resistance genes are best for which populations of
the disease — this will indicate the best candidates for pyramiding.
For both
diseases, artificial screening of breeding and introduced material will soon be
conducted in a climate-controlled greenhouse (expected to be operational in mid-2012).
