Epidemiology and control of potato virus Y in Kenya

John Onditi

Research output: Thesisinternal PhD, WU

Abstract

Potato (Solanum tuberosum L.) is the second most important food crop after maize in Kenya. Majority of farmers plant their own saved tubers from the harvest of previous seasons or ware tubers from neighbours and local markets, a practice that is known to contribute to the spread of seed borne diseases and most importantly viruses. Previous virus prevalence surveys indicated high incidences of viruses and virus vectors in the major potato growing regions. Potato viruses are particularly a problem because once a plant is infected, the disease is spread through seed tubers from the mother plant to daughter tubers. The viruses can also be spread by aphids (vectors) which have been reported to occur with high incidence in the major potato growing regions in Kenya. This results in successive reductions in yield and crop quality over seasons of replanting virus infected tubers. Controlling potato viruses are among the things that can be done to improve potato yields in the farmer’s fields.

The main aim for conducting this study was to give recommendations which can easily be adopted and immediately be available for potato virus control based on the current potato production situation in Kenya. Chapter 1 therefore gave the background of the potato growing situation in Kenya, the existing intuitional infrastructure in the potato sector and the past efforts in potato viruses control in Kenya. This included a description of the potato virus problem in Kenya including previous potato virus prevalence surveys and efforts by local scientist to propose various methods of managing the viruses and major challenges hindering virus control. Potato virus Y (PVY) was identified as one of the most important viruses in Kenya and also all over the world in all regions where potatoes are grown and was highlighted for a more detailed study among the viruses. There were unanswered questions about the general potato virus epidemiology and control from a farmer’s perspective.

In Chapter 2, a farmer household survey of 147 respondents was conducted in two major potato growing regions in Kenya. Lack of sufficient technical information among the farmers on potato virus symptoms (81.8%), vectors (63.9%), modes of virus transmission (94.5%) were found to be among the hindrances to the efforts towards virus control. Most farmers engaged in: maintaining volunteer potatoes as an early season crop in their farms (79.8%), planting own saved seeds tubers (77.4%), maintaining plant species of the same botanical family as potato  that can act as sources of virus transmission to their farms (19.7%), growing potatoes all months of the year in one region which allows for virus transmission all through the year and either not applying pesticides at all to control (aphids) vectors (55.1%) or incorrect application of pesticides which could be promoting virus spread. To set up a strategy for virus control, six different aspects of potato virus control were compared: (a) aphid (vector) virus transmission, (b) virus transmission through seed tubers, (c) mechanical or contact virus transmission to the potato crop, (d) sources of virus inoculum around the potato crop, (e) lack of use of host resistance by the farmers and (f) lack of effective use of pesticides by farmers. These were analysed in a spider diagram in terms of the scores assigned to them as per the present levels of farmer knowledge and also based on experts opinion on their potential impact on overall potato virus epidemiology and control. Use of virus resistant cultivars was found with the lowest score meaning it required the most urgent attention compared to other aspects of virus control and was expected to have greater impact in lowering virus prevalence in farmer’s fields.

This was followed by an investigation on the actual prevalence (Chapter 3) of the six viruses (PVY, PLRV, PVX, PVM, PVA, and PVS). Among 354 potato leaf samples tested in DAS-ELISA, PVX was the most prevalent virus (54.8%) followed by PVS (47.5%), PVM (13.0%), PVY (8.2%), PLRV (6.8%) and PVA (2.5%) but multiple infections occurred in 55.9% of the samples. Virus distribution maps were used to locate viruses occurring only in particular geographic regions indicating that any effective approach in virus control should be region specific. Four cultivars, Sherekea, Shangi, Kenya Karibu and Asante were identified with the lowest average ELISA absorbance (OD) values of the six viruses which was used as an indication of field resistance to the viruses.  Chapter 4 focused on PVY and was intended to identify cultivars resistant to the most predominant PVY strains in Kenya. Among the 13 different cultivars in the five major potato growing counties PVYN-Wi was the most prevalent PVY strain (7.3%) followed by PVYO (2.3%) and PVYNTN (1.4%) while PVYN was not detected. The PVYN-Wi strain occurred across the three seasons and across the counties but only on specific cultivars. Three cultivars were identified as PVY resistant and the most widely grown cultivar Shangi (60.2%), had only a low PVY prevalence of 0.6%. In Chapter 5, the cultivars identified as (partially) resistant to PVY from prevalence surveys were further screened under controlled sap inoculation experiments both in the screen-house and in the field. Three cultivars; Kenya Karibu, Sherekea and Unica were confirmed to be resistant  and susceptible cultivars Dutch Robyjn, Nyayo and in Kenya Mpya, PVYN-Wi infection had yield losses of 21.6%, 39.0% 53.1% respectively after three seasons.

This study discusses and recommends (Chapter 6) deployment of potato virus control methods which are innovatively based on the existing level of farmer knowledge and practices. The use of virus resistant cultivars was identified as one of the methods with potential for lowering the virus prevalence in farmer’s fields and is applicable to the Kenyan potato growing situation. This study demonstrates that virus prevalence surveys can provide useful information about resistance levels to the viruses in potato cultivars which can immediately be used to advice farmers. Virus resistant cultivars can therefore be recommended especially in regions identified with high virus prevalence.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • van der Vlugt, Rene, Promotor
  • van Oers, Monique, Promotor
  • Nyongesa, M.W., Co-promotor, External person
Award date23 Apr 2020
Place of PublicationWageningen
Publisher
Print ISBNs9789463953221
DOIs
Publication statusPublished - 23 Apr 2020

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