Scientists in four African countries have identified two coffee varieties that appear to be resistant to both diseases.
In a research programme headed by international agricultural science organisation CABI, in partnership with national research organisations in India, Zimbabwe, Uganda and Rwanda, scientists selected and tested a range of coffee varieties from India for their resistance to coffee leaf rust. India was selected as the country that harbours the most strains of leaf rust in the world.
"We knew that if a coffee variety survived the onslaught of so many different strains of coffee rust in India, it would almost definitely prove resistant to the strains of the disease in Africa," said Noah Phiri, senior plant pathologist at CABI.
Coffee leaf rust, caused by the fungus Hemileia vastatrix, attaches to the underside of the leaves of coffee bushes and causes them to fall. Many trees retain only two or three pairs of leaves on branches where they might ordinarily have 15 or 20. This leads to stunted bushes, which are low in yields and usually die after a few years.
Resistance to replace fungicide
Control of the disease has always been problematic due to the ease with which the fungus spreads and develops new strains. Coffee leaf rust is spread by wind and rain from lesions on the underside of the leaves of the coffee plant, and the only way to prevent this is to spray frequently with a fungicide; this has both economic and environmental costs for the smallholder farmers; the predominant coffee growers in Africa.
However, the two resistant varieties of coffee found in Karnataka, South West India, which the researchers refer to as Selection 5A and Selection 6A, have not developed the disease, despite numerous tests. They were tested first in a nursery, then in research fields, and finally in farmers' fields. Noah Phiri explained how they were deliberately planted next to varieties known to be susceptible to coffee leaf rust, and the disease did not develop.
"Entire crops have been lost to coffee leaf rust in the past," he said. "We are very encouraged by the results of this research which will make a big difference to coffee farmers in Africa. Replacing diseased or dead coffee bushes with the new varieties will mean they lose less of their crops and have a higher quality crop overall."
Resistance to CBD
There are also encouraging signs that Selections 5A and 6A will be resistant to CBD, another fungus-based disease that damages yields. CBD, which was discovered in Kenya, affects the cherries, greatly reducing production of berries. It is caused by a fungus which lives in the bark of the coffee bush and produces spores which attack the coffee cherries, causing annual losses of between 30 and 60 per cent. The researchers are now waiting for the new varieties to flower and bear berries so that resistance to CBD can be tested under field conditions.
"What we have seen so far in the nursery has been very encouraging," said Noah Phiri. "What is more, we have noted some interesting data that shows that one of the two Indian lines has the capability to tolerate coffee bacterial blight (CBB), another disease that has been a headache among Kenyan coffee farmers. We are continuing to test this and hope to be able to confirm resistance."
The researchers hope soon to see real practical results in the fields of the farmers who have been involved with the project. Coffee is a major source of income for many in Zimbabwe, Uganda, Kenya, and Rwanda; planting coffee bushes that will not succumb either to coffee leaf rust or CBD will be a major step forward in protecting and advancing the industry in these countries.
In the next stage of the research programme, scientists will evaluate the two Indian varieties for the taste and aroma of the final coffee product, since the price of coffee is determined by its taste. "Our first priority was to make sure we were actually able to harvest a coffee crop," said Noah Phiri. "Now we need to make sure that farmers will be able to get the best possible price for it."
by Kimani Chege, CABI