Potatoes were introduced to the Highlands of Kenya in 1900 and, being free of blight, they were a very productive and popular crop for the next forty years. However, blight was accidentally introduced during World War II, and it proved devastating. Potato production all but ceased, particularly among subsistence farmers, who had neither the cash nor the expertise for fungicidal spraying. One old Dutch cultivar, known locally as Dutch Robijn (Robijn rhymes with 'no pain') had sufficient horizontal resistance to produce a reasonable yield without spraying, and it is still being cultivated. The temperate viruses of potato lack epidemiological competence in the highlands of equatorial Kenya, and the only important tuber-borne disease is bacterial blight, caused by Pseudomonas solanacearum. A Scottish cultivar, bred for vertical resistance by William Black, also proved to have a reasonably high level of horizontal resistance to blight. This was named Roslyn Eburu in Kenya, where it is still being cultivated.
My own potato breeding was aimed specifically at horizontal resistance to both blight and bacterial wilt, with resistance to potato tuber moth as a secondary objective. Because potatoes can be cultivated throughout the year, if irrigation is available, it was possible to breed on a 'production line' basis. One thousand true seedlings of potato were introduced into the system each working day, with about 150,000 seedlings in each breeding cycle, and with two breeding cycles per year. Month-old seedlings were flood-inoculated with bacterial wilt, and sprayed with a spore suspension of blight. Less than 1% of the seedlings survived these diseases, and the survivors were transplanted to the field where the majority succumbed. Survivors with acceptable tubers became the parents of the next generation of recurrent mass selection.
As a result of this breeding, two cultivars called Kenya Akiba and Kenya Baraka, are largely responsible for a greatly expanded potato acreage in Kenya and neighbouring high altitude, equatorial areas. These cultivars have good yields of commercially acceptable tubers. Furthermore, these crops are grown by subsistence farmers without any cash expenditure. Neither certified seed, nor crop spraying are required. At the time of writing, they had been cultivated for some seventy vegetative generations without decline from tuber-borne diseases, and the total potato production has increased by thirty-five times. However, it is feared that the inevitable arrival of the A2 mating type will increase the epidemiological competence of the blight fungus, and that these levels of horizontal resistance will then become inadequate. It is important that this unique situation must not be claimed as a 'breakdown' of horizontal resistance, making it comparable to vertical resistance.
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