For rice as for other crops, the distribution and behavior of
diseases is expected to change under the influence of climate change. In fact,
it is already doing so. In rice-disease hot spots in Tanzania, 92% of rice
farmers with 15–30 years of experience
have observed increased temperatures and changing rainfall patterns in terms of
timing and amounts — effects they associate with climate change and with
changes in both the incidence and the severity of diseases.
Some 91% of farmers are familiar with the symptoms of rice
diseases, but few, if any, regularly practise any form of disease control. It
has long been known that the most effective way to help farmers overcome
diseases is to provide them with rice cultivars that are resistant to those
diseases. In East Africa, the commonest rice diseases are bacterial blight,
blast (a fungal disease) and Rice yellow mottle virus.
In response to this challenge, Germany’s Federal Ministry for
Economic Cooperation and Development (BMZ) funded a 3-year project, ‘Mitigating
the impact of climate change on rice disease resistance in East Africa’
(MICCORDEA) carried out by the Africa Rice Center (AfricaRice). The project
focused on bacterial blight and blast in Rwanda, Tanzania and Uganda.
“Perhaps the most important outcome of the project is that we have
a number of national scientists qualified at master’s and doctoral levels in
the three countries,” says Drissa Silué, AfricaRice plant pathologist. “This
means that there are now scientists in place in the national programs who can
carry out research on rice diseases in general and on these two diseases in
particular.”
An immediate upshot of this has been the mapping of the
distribution and severity of bacterial blight and blast across the three
countries. “This is the first time that
we have had detailed maps of the distribution of these diseases in East Africa,
which will help target breeding efforts,” says Silué. This work has also
established baseline data for measuring changes in disease patterns as climate
change takes hold over the coming decades.
The
causal organisms of both diseases are highly variable. The variability of blast
pathogens is demonstrated by the gene-for-gene theory of genetic resistance —
specific resistance genes in rice prevent infection by specific virulence genes
of the pathogen. Over 70 major resistance genes have been documented for blast
in rice worldwide. Meanwhile, over 30 resistance genes for bacterial blight are
known, some of them in native African rice species such as Oryza barthii, O. glaberima and O. longistaminata.
In
the MICCORDEA project, rice germplasm known to be carrying resistance genes was
screened at disease hot spots in each of the three countries. This is a quick
and cheap way of identifying material resistant to local strains of the
pathogens and, in the case of blast, the pathotypes prevalent in each hot spot.
Bacterial
blight
From
the work conducted by Rwanda Agricultural Research Institute (ISAR), Rwanda
seems to have the most complex distribution of variation in the two diseases,
with both registering considerable diversity across sites. For bacterial
blight, two of the sites registered three pathotypes/resistance groups each,
but the third site registered uniform moderate resistance across rice lines.
No
candidate resistant varieties for use in a bacterial blight-resistance breeding
program emerged from this work. However, in a separate experiment conducted by
the Rwanda Agriculture Board (RAB), five cultivars proved resistant to all
bacterial blight isolates.
In
Tanzania, researchers at the Agricultural Research Institute in Uyole found two
lines resistant to bacterial blight that show promise for inclusion in the
country’s bacterial blight-resistance breeding program. Research testing rice
genotypes against five strains from bacterial blight hot spots across the
country revealed large variations across seasons and sites, suggesting the
worrying prospect of genetic shifts in pathogen populations.
However,
six genotypes were resistant to four of the site-specific strains of the
disease. In Uganda, the National Agricultural Research Organisation (NARO) and
National Crops Resources Research Institute (NaCRRI) found no lines completely
resistant to bacterial blight, and just two lines showing moderate resistance.
However, in a test of five cultivars against the three most aggressive
isolates, AfricaRice’s WITA 9 and NERICA 4 performed best.
In
parallel with the fieldwork in East Africa, Georg- August University of Göttingen,
Germany, conducted diversity, virulence and toxin production studies on
bacterial blight. A major result from the diversity studies was the diagnosis
of bacterial blight isolate Ug12 from Uganda.
Meanwhile,
the virulence studies identified two genes that conferred broad resistance to
bacterial blight — one providing strong resistance and the other moderate
resistance. The research also confirmed that African strains of the blight
bacterium are distinct from those found in Asia. The toxin production study led
the research team to speculate that a low-molecular-weight toxin may be present
but not playing a major role in bacterial blight virulence.
Blast
The
results of ISAR’s blast screening were more promising than those of its
bacterial blight screening, with at least two and up to seven genotypes (each
with between one and four resistance genes) showing promise for disease control
at each of the three hotspot sites. An inoculation test using the five most
virulent isolates of blast against recently released cultivars showed that
Rumbuka has broad resistance to all five isolates, while Mpembuke is resistant
to two of them. This information should help the extension service target areas
for promotion of these new cultivars.
The
upland site of Kyela in southern Tanzania has a particularly diverse and
aggressive blast population, which destroyed up to 75% of the rice lines
tested, the disease being at its worst early in the rainy season. However, even
here, the screening revealed 10 resistant lines (9 with monogenic resistance, 1
with a four-gene combination). These 10 lines have been recommended for use in
a breeding program to ‘pyramid’ (i.e. combine) the resistance genes in popular
local varieties that are susceptible to the disease.
Five
rice lines showed stable resistance to blast across four hotspot sites in
Uganda (4 monogenic, 1 with a combination of two genes). Moreover, five
accessions (i.e. varieties or landraces originally collected in the field
rather than from breeders) also performed well in these hot spots. These
accessions include the well-known varieties IR24 and AfricaRice’s own NERICA 1.
All these materials were recommended for inclusion in the effort to pyramid
resistance genes.
In
Germany, institutions at three universities — the Institute of Plant Pathology
and Plant Protection, the Karlovsky lab and the Section for Tropical and
Subtropical Agriculture and Forestry (SeTSAF) at the Georg-August University of
Göttingen; the Institute of Plant Diseases and Plant Protection at Leibniz
University, Hannover; and the University of Applied Sciences of Erfurt —
investigated the population structure, pathogenicity and mating type of blast
pathogens in preparation for further studies on the impact of climate change on
disease incidence and severity.
Some
88 blast isolates were used to determine variation in virulence among isolates.
The research into mating type revealed the possibility of recombination via
sexual reproduction of the blast fungus in East Africa, though this has never been
proved to occur in the field.
Resistance
analysis in Germany identified two genes with potential for use in East Africa,
while a study of cultivar reaction to blast strains demonstrated that NERICA 4
has broad-spectrum resistance to East African strains, though the genetic basis
for this is as yet unknown. This makes NERICA 4 potentially doubly interesting,
given the resistance to bacterial blight demonstrated in Rwanda.
How will
climate change affect disease patterns?
A
central aim of the project was to work toward mitigation of the impact of rice
diseases as East Africa’s climate changes. The degree studies and short-course
training provided for national scientists are a major component of this, as
they will give rise to continuing activities over the coming years, enabling
scientists to respond to farmers’ changing needs. However, the project also
included a component of research to find out how the two diseases are likely to
affect the East African rice crop as temperature increases and rainfall becomes
more erratic.
The
crop model RICEPEST, which determines rice losses to diseases under current
climatic conditions, was an obvious place to start. To develop future
scenarios, the climate model EPIRICE was used to generate data on projected
climate to feed into RICEPEST. This was the first time these two models had
been combined. For blast, the news for farmers is good: although it can be
locally virulent, the disease currently has a relatively minor impact on rice
yields in the region as a whole, and this is predicted not to change in the
foreseeable future.
The
combined model predicted a less than 2.5% probability of blast epidemic
outbreak in Tanzania, with low yield losses (no more than 0.017 t/ha) due to
blast up to 2050 (i.e. 35 years hence). However, the news is not so good
regarding bacterial blight: this disease is predicted to reduce yields by
between 0.47 and 0.67 t/ha by 2050. The implication is that, for East Africa,
breeding efforts should focus far more on resistance to bacterial blight than
to blast.
Georg-AugustUniversity tested six blast-resistance genes in two genetic backgrounds at two
temperatures against a Tanzanian strain of blast. The research found that, in
general, both temperature and genetic background tend to affect resistance.
However, the good news is that two of the resistance genes were not affected by
either temperature or genetic background, providing strong resistance in all
cases. Parallel research showed that rice reactions to blast and temperature
are both genetic.
A
major stress under predicted future climate scenarios will be drought. If
rainfall becomes more erratic, rainfed rice in particular is likely to suffer
yield losses as it undergoes increasingly long and severe dry spells during the
growing season. Drought resistance in rice is complex: there is no major
drought-resistance gene, but rather a number of small-effect genes whose impact
is cumulative, so that the more of these genes a plant has the more resistant
it is. These genes are collectively known as ‘quantitative trait loci’
(QTLs).
Project
research demonstrated that rice plants with a selection of drought-resistant
QTLs were more susceptible to bacterial blight than those without them.
Moreover, rice lines with both the drought QTLs and a bacterial blight
resistance gene suffered more severely from the disease under drought
conditions than under ‘normal’ conditions.
This
apparent breakdown in resistance would seem to be a major problem, until one
looks at parallel research which showed that the effects depend very much on
the gene-for-gene alignment of rice resistance with pathogen virulence. With
the right bacterial blight-resistance gene combined with drought-resistance
QTLs, rice plants displayed increased
resistance to the disease under
drought.
The
other major component of climate change is temperature. Over the coming
decades, East African rice systems are likely to experience gradually
increasing temperatures, especially night-time temperatures during the growing
season. Two bacterial blight-resistance genes were studied: higher temperatures
enhanced the resistance effects of one while reducing the effects of the other!
Clearly,
climate-proofing rice against bacterial blight is going to be a complicated
business. Another avenue is to tap the resistance genes of O. glaberrima,
ensuring these enter the widely grown sativa
cultivars. To this end, 18 O. glaberrima accessions
were screened, including 9 that had demonstrated resistance to many Philippine
strains of bacterial blight.
These
accessions were screened against 14 strains of the blight bacterium at two
temperatures. Four of the accessions showed broad-spectrum resistance at high
temperature. One of these carries a known resistance gene, but the others are
still being tested to see whether they carry known resistance genes or novel
ones.
Project
achievements
“I
believe that the project has done what we wanted it to. It has made a
significant contribution to preparing the rice production sector for future
climate change,” says Silué. “We have a gene for bacterial blight resistance
that currently stands up to most of the bacterial blight in East Africa. We
also have a pair of genes for blast that not only show durable resistance
today, but also seem to be effective at higher temperatures. This means we have
the basic tools for climate-proofing existing and new varieties for East
Africa.
“The other element of climate-proofing
is having the skills on the ground to continue to study the diseases as they
evolve over the coming years. This is what the master’s and doctoral training
was all about.” All in all, then, a successful project, though there is no room
for complacency. Much hard work both in the lab and in farmers’ fields remains
to be done in future years to enable East Africa’s rice sector to cope
effectively with the disease challenges associated with climate change.