The Ethiopian highlands are densely populated and home to millions of smallholder farmers. The area is blessed with cool temperatures and high rainfall, which should make it fertile ground for the production of disease-free seed potatoes. However, Abdulwahab Abdurahman reports that this may not be as easy as previously thought.
Background
Farms in the south of Ethiopia are very small and highly fragmented: 81% of rural households have less than 1ha and most of these farms are fragmented into smaller plots. The average farm size becomes even smaller in the populous highlands, such as the Chencha district, where 95% of households have less than half a hectare of land. The prospect of increasing crop production by expansion of agricultural land is very unlikely, demanding highly productive and efficient crops such as potato instead.
In Chencha, potato is an important staple crop, with almost every household allocating a small plot of land for the crop each year. Moreover, for poor households it is a hunger breaker because of its short crop cycle compared with cereals. Potato is produced in two growing seasons: the Belg season (a short rain season: March to June), during which the bulk of production takes place, and the Meher season (a long rain season: July to November). Despite its high potential, however, the actual productivity of potatoes in Ethiopia is very low (8.2t/ha), mainly as a result of the lack of access to quality seed potatoes of disease-resistant varieties.
Seed potato degeneration in Chencha
Seed potato degeneration is defined as the accumulation of pathogens and pests in planting material as a result of successive cycles of vegetative propagation causing reduction in progeny yield. We studied the rate of seed degeneration due to virus disease accumulation at three locations in Chencha, ranging in altitude from 2,500m to 2,800m above sea level, starting with laboratory-tested disease free mini-tubers. The results of two successive cycles of propagation in farmers’ fields in both the Meher and Belg seasons indicated the absence of the most important aphid-transmitted viral diseases, i.e., Potato Virus Y (PVY) and Potato Leaf Roll Virus (PLRV), and the presence of mechanically transmitted viruses (PVS and PVX), with no visual symptoms. Indeed, no winged aphids were trapped by yellow water traps in the field. Therefore, in the first instance, we concluded that Chencha is a low-degeneration area for virus diseases and suitable for seed potato production without a need for frequent seed tuber renewal, provided production sites are carefully selected to avoid other soil-borne pathogenic bacteria and that clean material is available.
Reversing degeneration
There was also empirical evidence available that there are simple practices available that can successfully prevent seed degeneration or even reverse it. The best-studied practice is the so-called positive selection: marking vigorous and healthy looking plants before crop senescence and harvesting these plants as seed for the next crop. Positive selection not only slows down degeneration, but can actually improve (‘regenerate’) degenerated seed over generations. Hence, we started to investigate the mechanisms behind positive selection in improving (regenerating) an otherwise degenerated farm-saved seed.
Bacterial wilt disease of potato in Chencha
However, a wilt disease with the characteristic symptoms of brown rot caused by the bacterium Ralstonia solanacearum subsequently affected our trial plots beyond recovery. Bacterial wilt is a soil and seed-borne quarantine disease that causes strong yield declines in ware crops, prevents the use of the tubers as seed, and contaminates the farm for many years. As a result, we adapted and changed the focus of our investigations, to address the emerging and pressing issue of the wilt disease.
Our field observations during the last three years showed that the wilt disease incidence and severity was increasing at an alarming rate from year to year, with complete crop loss in some instances. In the 2015 Belg season, the incidence of the disease had reached 97% per potato farms, based on visual assessment. However, the severity of disease on these farms varied from low infection levels to almost complete infection.
It was not obvious whether the disease is endemic to the district and manifested itself on recently introduced potato varieties, or is a recent introduction brought into the area with infected seed. Understanding the source of the pathogen is crucial to help stop the spread of the disease. For this, the pathogen was isolated from regions of the country known to be both the source of seed potatoes and also endemic for the disease. We subsequently used DNA fingerprinting techniques for the detection, identification and genetic profiling of the pathogen for possible source tracing.
Results and outlook
Our research has shown that Ralstonia solanacearum is widely spread in Ethiopia, as well as in Chencha. The bacterium is both seed and soil-borne, and difficult to control by widening the crop rotation; certainly under Chencha conditions it can be latent and, therefore, dangerous if seemingly healthy seed potatoes are transported for planting in warmer, more conducive environments. Moreover, the disease can easily spread through contaminated water and host plants other than potato.
Our research has also confirmed that Ralstonia has been endemic in Chencha for many years and is not a recent introduction. Coupled with a low frequency of potato in the rotation, large quantities of bacterial, wilt-free seed will be required to flush the disease from the farming system.
We have identified land in the Chencha region that is free from bacterial wilt and has not previously grown potatoes. This raises the possibility of importing small quantities of tested, disease-free seed, and multiplying these locally to replace local, existing infected stocks. If this local community model proves successful in Chencha, it could be adopted as a model for bacterial wilt mitigation across communities in sub-Saharan Africa.
Bacterial wilt has rapidly developed into a nationwide problem in Ethiopia, threatening the cultivation of potato. In addition to the production model described above, awareness programmes and community-based strategies to increase farm hygiene, as well as designs of an agronomic tool box to control and suppress disease are urgently needed. As a first step in this process, several Ethiopian agronomists have received training on laboratory diagnosis of bacterial wilt disease.
Acknowledgements
- This work has been co-financed and supervised by the Teagasc Walsh Fellowships Programme, Wageningen University and Research Centre, and Vita (www.vita.ie).
- Abdulwahab Abdurahman,Teagasc Walsh Fellow
- Denis Griffin, Senior Research Officer, Potato Breeding, Teagasc, Crops Research Department, Oak Park, Carlow
- Steffen Schulz, Ethiopia Country Representative, International Potato Center,
- Paul C. Struik, Centre for Crop Systems Analysis, Wageningen University and Research, The Netherlands
- Correspondence: abdulwahab.abdurahman@wur.nl
- Reprinted with permission from TResearch – Teagasc’s research and innovation magazine.
