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John, 1988). According to Foti et al. (1995) and Haverkort (1990),
water stress and moderate rise in temperature
generally leads to yield and tuber quality losses
. Potato responds to any change of precipitation regime; excess or
deficient soil moisture is said to cause significant yield losses (Saue and Kadaja, 2009). According to Mackerron
and Jefferies (1986), water stress before tuber initiation reduces tuber set per stem.
5 Disease Incidences
Potato production in Kenya is affected by a number of diseases, chief among them being late blight (Kaguongo et
al., 2008). It is highly destructive during the rainy season especially in the cool highlands (Kaguongo et al., 2008).
With unpredictable weather, unprecedented heavy rains in potato growing areas are likely to create conditions
favourable for serious late blight outbreaks. For example, in 2002, 2003 and 2006 unexpected heavy rains
pounded Tigoni area (Kiambu county) leaving a trail of destruction on potato fields due to late blight. Although
development of late blight resistant varieties has been a focus of the Kenya national potato programme in
collaboration with the International Potato Centre (CIP) (KARI, 2000), durable resistance has been elusive.
In addition to late blight, bacterial wilt (caused by
Ralstonia solanacearum)
is a common disease contributing to
potato yield reduction (Kaguongo et al., 2008). Although the cool-climate adapted race 3 of the pathogen is
the principal cause of bacterial wilt of potatoes in the highlands (Smith et al., 1995) it also occurs in potato plants
grown in warmer locations from seed tubers harvested from cool areas (French, 1994). With the expansion of
potato production into warmer lowlands and the warming up of the cool highlands, cases of lowland bacterial wilt
caused by race 1 (biovars 1, 3 and 4) have occurred (French, 1994; EPPO, 2004). In Kenya, both pathogen races
are present causing an estimated yield loss of between 50% and 75% (Rotich et al., 2010).
Furthermore, the hot and dry conditions are likely to increase the incidences of insect vectors and viral diseases.
The most common viruses in Kenya are Potato Virus X (PVX), Potato Virus Y (PVY), Potato Virus S (PVS) and
Potato Leaf Roll Virus (PLRV) (KARI, 2000). Most potatoes in Kenya are grown from seed tubers retained by
farmers from previous harvests, acquired from markets or from neighbours (Khurana and Garg, 2003). Viruses
are spread by insects especially aphids; the incidences of which increase as conditions become warm and dry.
Seed tubers from infected plant most likely pass the infection to the resultant plant upon planting; the rapid
buildup of viruses in warm and dry conditions leads to rapid degeneration of seeds.
6
Possible Mitigation Measures
To mitigate against the effects of climate change, it is necessary to grow potatoes that are tolerant to high
temperatures, common diseases and pests, and drought (IPCC, 2007). In addition, other measures such as
minimum tillage and irrigation may have to be adopted. Coping strategies may be broadly grouped into breeding,
seed, postharvest and agronomic interventions.
6.1
Breeding
The identification or the development of potato cultivars with increased heat and drought tolerance appears to be
important in coping with climate change (Hijmans et al.,
2003).
Breeding for increased heat tolerance will need to
focus on the effect of temperature on tuberization (Haverkort, 2008) since potato tuber initiation and development
are much more sensitive to high temperature stress than photosynthesis.
Development of early-maturing varieties
with shorter growth cycles will allow the crop to 'escape' stresses such as drought (Robert, 2003; David, 2007)
.
When breeding for heat tolerance, quality aspects such as susceptibility to secondary tuber growth, accumulation
of reducing sugars and reduced glycol-alkaloids are other aspects that must also be factored in during
development of new varieties (Haevekort, 2010). Since diseases are likely to intensify under high temperatures, it
is necessary to breed for resistance to the most critical ones such as bacterial wilt,
Erwinia
and viruses
.
It is
instructive that resistance to bacterial wilt (
R. Solanacerum
) has remained elusive (
Wolfgang, 2009
).
6.2
Seed
Hot and dry conditions favour rapid multiplication of insects especially aphids which are vectors of viruses in
potatoes. Accumulation of viral load in seed tubers leads to rapid degenerating from one season to the next. The
