Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Field studies were carried out in farmer-based seed
potato production to determine the incidence of potato aphids and potato
aphid-transmitted viruses in two potato-producing areas of Kenya. Parameters
determined included aphid population, virus disease incidence and tuber
yield. Aphid population was monitored on leaves and in water-pan traps.
Virus infection was determined based on symptoms and the viruses were
identified in tubers sprouts by DAS-ELISA. Tuber yield was determined
for plants showing virus symptoms and healthy-looking plants. Five aphid
species were identified, with the most abundant being M. euphorbiae
and A. gossypii on leaves and M. persicae and A. gossypii
in water traps. The average aphid population was between 1.4 and 4.2 aphids
per three leaves and 4.68 and 9.64 aphids per water pan trap. Farms with
higher population of M. persicae had higher virus disease incidence.
The most prevalent viruses were PVS, PLRV and PVM. Healthy looking plants
had a latent infection rate 57.2% compared to 76.6% for symptomatic plants.
Virus infection reduced the number and weight of tubers by 74 and 62.7%,
respectively. However, virus infection increased the number and weight
of the chats grade. The results indicated that aphid infestation and virus
disease incidence were higher than the recommended for seed potato production.
Therefore, there is need to create awareness among the farmers on aphid
and virus symptom recognition and use of clean certified seed potato.
Potato (Solanum tuberosum L.) is the second most important food crop
after maize in Kenya (Ministry of Agriculture, 2006). The
national average yield is about 9.1 t ha-1 compared to 40 t ha-1
under research conditions (Ministry of Agriculture, 2007a;
Kabira et al., 2006). The low productivity has been
attributed to low quality seeds and diseases, especially bacterial wilt and
viruses in smallholder farms. The informal sub sector contributes 99% of the
seed potato in Kenya. This sector involves seed potato production without going
through the certification processes and it includes unregistered growers and
suppliers of seed mainly in their immediate localities (Kenya Agricultural Research
Institute, 2006). It encompasses seed potato production with involvement of
Non-Governmental Organizations (NGOs) and Community Based Organizations (CBOs),
seed private growers and individual farmers through seed plot technologies and
positive selection. However, the seed potato production rules require that the
proportion of plants infected with virus, bacterial wilt and nematodes should
not exceed 10, 3 and 3%, respectively (Ministry of Agriculture,
The low potato productivity is partially attributed to high losses due to pests
and diseases, lack of adequate quantities of healthy planting materials and
inadequate use of farm inputs (Ministry of Agriculture, 2006).
Seed potato of most popular potato varieties has degenerated and need to be
cleaned, multiplied and distributed to the industry. Due to lack of adequate
and affordable certified seed potato, farmers generally recycle seed from previous
harvests. This leads to further spread of tuber borne viruses and other diseases.
About 40 viruses have been reported to affect potato (Bostan
et al., 2006). Most of these viruses, such as potato leaf roll virus,
potato virus X and potato virus Yare spread by aphids and tubers. The potato
spindle tuber viroid is transmitted by aphids only together with potato leafroll
virus. The green peach aphid (Myzus persicae) is the most efficient vector
in transmission of virus diseases and is known to transmit over 100 viruses
in different plants (Braedle, 2006; Ming
et al., 2007; Raman, 1985). However, the capacity
of farmers to recognize and manage aphids and potato virus diseases is very
low (International Potato Centre, 2006; Kabira
et al., 2006; Kibaru, 2003; Machangi,
2003). This lack of awareness about the damage caused by aphids and aphid-transmitted
virus diseases in the informal seed potato production system contributes to
seed degeneration and low yields.
The study was carried out to determine the levels of aphids and viruses
in farmer-based seed potato production system and the associated yield
MATERIALS AND METHODS
Experimental site and design: The study was carried out on smallholder
farmers fields in Njabini, Nyadarua south and Limuru, Kiambu west
districts of Kenya over two seasons between November 2006 and February
2008. It targeted individual farmers or farmer groups involved in seed
potato production. Four seed-potato producing farms were selected in each
district. The experimental design was randomized complete block design.
In each farm, 0.25 ha potato plot was selected and divided into four equal
portions, which acted as replicates while the farm acted as a block.
Assessment of aphid population: Aphid population was monitored
on leaves and by use of yellow water pan traps. Leaves were used to monitor
wingless (apteral) aphids while the yellow water pan traps were used to
monitor winged (alate) aphids. Sampling of leaves was done weekly from
second week after emergence to maturity. Ten potato plants were randomly
selected from each plot and three leaves were picked from top, middle
and bottom of each plant. The leaves from each plant were put in separate
labelled paper bags and kept in cool box until aphids were counted and
identified. The yellow water pan traps consisted of yellow basins 3/4
filled with water and a few drops of liquid detergent added to break surface
tension to make the trapped insects sink to the bottom. Five water pan
traps were placed in each 0.25 ha plot. The traps were replaced weekly
and aphid counts taken. The aphids were counted and identified to species
level under a stereo telescopic dissecting microscope (x40 magnification).
Aphid identification was based on aphids colour in life, lateral abdominal
spiracles, antennal tubercles, shape of siphunculi and dorsal abdominal
Assessment of virus disease and effect on tuber yield: In each 0.25
ha portion of the farm, 100 plants were selected at random and observed for
virus symptoms. Incidence of virus infection was determined as the proportion
of plants showing leaf roll and mosaic symptoms. Virus incidence was determined
weekly from the second week after emergence to crop maturity. At harvest, the
tubers were tested for presence of potato leaf roll virus (PLRV), potato virus
X (PVX), potato virus S (PVS), potato virus M (PVM), potato virus Y (PVY) and
potato virus A (PVA) by double antibody sandwich-enzyme linked immunosorbent
assay (DAS-ELISA) method as described by International Potato
Centre (2007) and Clark and Adams (1977).
At flowering 10 healthy-looking and 10 symptomatic plants were tagged
in each 1/4 portion of the 0.25 ha potato plot such that a total of 40
healthy-looking and 40 symptomatic plants were tagged. At maturity, tubers
from each plant were harvested separately and graded into ware (>55
mm), seed (55-25 mm) and chats ( < 25 mm). The number and weight of
tubers in each grade were determined and percentage yield reduction calculated
as the percentage difference between the number or weight of tubers from
the healthy-looking and the symptomatic plants.
Data analysis: All data were subjected to analysis of variance
(ANOVA) using Genstat software and differences among the treatment means
compared using Fishers Protected LSD test at 5% probability level.
Aphid species identified were Macrosiphum euphorbiae, Aphis
gossypii, Aphis fabae, Myzus persicae and Rhopalosiphum
maidis. The most abundant species on leaves were A. gossypii
and M. euphorbiae while the least abundant was R. maidis (Table
1). The most abundant species in water pan traps were M. euphorbiae
and M. persicae in Njabini but higher populations of A. gossypii
and R. maydis were caught in Limuru during both growing seasons.
However, high levels of A. fabae were traped in water pan traps
in Limuru only during the short rain season. The farms differed in population
and distribution of different aphid species. Aphid population was higher
and more widely distributed in Limuru than in Njabini. However, higher
numbers of Myzus persicae were caught in water traps in Njabini
than in Limuru. But more of this species was found on potato leaves in
Limuru than in Njabini. The aphid population on leaves generally increased
with growth of the crop, then decreased towards maturity, with peak population
being between 6th to 8th weeks after crop emergence (Table
2). There was a higher total aphid population during the short rain
season compared to the long rain season.
||Mean population of different aphid species per 3 leaves
and per water pan trap during short and long rain seasons in two potato-producing
areas of Kenya
|Me = M. euphorbiae, Ag = A. gossypii,
Af = A. fabae, Mp = M. persicae, Rm = R. maidis
|| Mean aphid population trend per 3 leaves and per water
pan trap during short and long rain seasons in two potato-producing
areas of Kenya
|ns: Non significant
||Mean percent virus disease incidence on potato plants
in Njabini and Limuru, Kenya, during the short and long rain seasons
Potato virus incidence was found at higher levels (2-30%) in Njabini
than in Limuru (10-21%) (Fig. 1). Farms with higher
population of M. persicae had higher virus disease incidences.
Potato viruses detected in tubers were potato leaf roll virus (PLRV),
potato virus M (PVM), potato virus X (PVX), potato virus Y (PVY), potato
virus S (PVS) and potato virus A (PVA). The most prevalent virus was PVS
(100%) followed by PLRV (92.5%) and PVM (90.3%) while the least prevalent
was PVY (16.9%) (Fig. 2a, b). Healthy-looking
plants had a lower incidence of latent infection of 57.2% compared to
76.6% for plants showing virus symptoms. A higher virus load was detected
in tubers during the short rains and tubers from Njabini had higher virus
titre in tubers from both the healthy-looking and symptomatic compared
to Limuru (Fig. 2).
||Types and amounts of viruses infecting healthy looking
and symptomatic potato plants in farmer-based seed potato production
units in (a) Limuru and (b) Njabini, Kenya in the 2007 season
The viruses detected in highest amounts were PVS, PVM and PLRV over the
two seasons. Virus infection reduced the number of tubers by between 9.6
and 35.5%. The number of tubers from healthy-looking plants was significantly
(p ≤ 0.05) higher than from symptomatic plants during the two seasons
(Table 3). In Njabini, the reduction in number of potato
tubers was by 9.6 and 29.7% during the short and long rains, respectively,
while in Limuru, reduction was by 17.7 and 35.5% during the long and short
rains, respectively (Table 3). The reduction in the
number of tubers was also significant (p ≤ 0.05) among the potato grades
during the two seasons. The reduction in number of tubers was greatest
(46-74%) for the ware grade but the symptomatic plants produced higher
number of chats compared to the healthy-looking plants. Tuber weight was
significantly (p ≤ 0.05) reduced by between 36.4 and 62.7% (Table
4). The highest weight reduction was in the ware grade at 65 and 47%
in Njabini and 62.7 and 36.8% in Limuru during the short and long rains,
respectively. However, symptomatic plants had higher weight of chats grade
during the two seasons.
||Mean number of tubers per 10 plants for different grades
harvested from healthy-looking and symptomatic potato plants during
short and long rain seasons in two potato-producing areas of Kenya
|Ware >55 mm, Seed 25-55 mm, Chats < 25 mm, H:
Healthy, S: Symptomatic, ns: Not significant
||Mean weight of tubers per 10 plants in kg for different
potato grades harvested from healthy-looking and symptomatic plants
during short and long rain seasons in two potato-producing areas of
|Ware > 55 mm; Seed 25-55 mm; Chats < 25 mm; H:
Healthy; S: Symptomatic, ns: Not significant
Potato aphids were prevalent in small-scale potato farms, with an average of
1.4 and 4.2 aphids per three leaves in Njabini and Limuru, respectively. This
translates to 46.6 and 140 aphids per 100 leaves, respectively, which is higher
than the threshold of between 3 and 10 aphids per 100 leaves recommended for
seed potato production (Capinera, 2001; Thomas,
2002). Among the five species identified, M. persicae is the most
efficient vector of potato viruses (Bostan et al.,
2006; Bunt, 2001; Braedle, 2006;
Ming et al., 2007). The population of this species
was higher during the short rains than during the long rains. This can be explained
by the warm conditions that favour aphids during the short rain season (De
Temmerman et al., 2002). Factors that affect aphid population include
weather conditions, nutrition and presence of predators (Handizi
and Legorburu, 2002). Temperatures of less than 17.8 °C greatly restrict
the number of M. persicae (Radcliffe, 1982) while
vegetation around potato plots play a critical role in aphid population dynamics
(Handizi and Legorburu, 2002). Therefore, the differences
in population of the different aphid species in the two potato-growing regions
could be due to variation in types of other crops grown and the type of vegetation
in the vicinity of potato fields.
The results indicate that aphid control measures, such as chemical spray and
rouging of infected plants, are required to reduce virus spread in the seed
potato (Struik and Wiersema, 1999; Thomas,
2002; Walingo et al., 2004). Peak aphid infestation
was observed at the seventh and eighth week after crop emergence, which agrees
with an earlier study by KARI (Kabira et al., 2006).
Therefore, the control measures should start early in the season, preferably
immediately after crop emergence by rouging infected plants. Chemical control
is applied when aphid population reaches economic threshold of between three
to ten aphids per 100 leaves irrespective of the stage of growth (Capinera,
2001; Thomas, 2002). Apterae aphids have been found
to transmit more viruses into a potato plant than alate aphids (Yvon
et al., 2000) and there is a positive correlation between aphid population
and viral load in tubers suggesting that aphids are directly responsible for
the virus contamination of the tubers (De Temmerman et
al., 2002; Powell et al., 2006; Verheggen
et al., 2007). Therefore, reducing aphid population would have a
direct effect of reducing infection and increase potato yields (Walingo
et al., 2004; Paola et al., 2005).
The study showed that the average virus disease incidence was between 12.4
and 17.7%, which is higher than the maximum threshold of 10% recommended in
seed potato production (Machangi et al., 2004;
Ministry of Agriculture, 2006; Nderitu
and Mueke, 1986). Disease incidence is a measure of seed purity and is used
in seed potato certification under the formal seed potato production system
(Ministry of Agriculture, 2007b). The high incidence can
be attributed to lack of aphid control measures by the farmers who have no capacity
to recognize aphids and they are not aware of the dangers posed by aphids in
seed potato production (Kabira et al., 2006). Recycling
of seed due to unavailability of clean, certified seed is also common. Infected
seed potato tubers transmit viruses to the germinating plants and therefore
the virus load continues to increase in successive seasons until the yield is
diminished (Ming et al., 2007; Paola
et al., 2005; Syller, 2001). Healthy-looking
potato plants were latently infected with viruses but the virus load was lower
than for symptomatic plants. This is consistent with other studies done elsewhere
(Birch et al., 1999; De Temmerman
et al., 2002; Jonathan and Alison, 2008; Bostan
et al., 2006). Therefore, farmers could be advised to rogue symptomatic
plants early after crop emergence to leave healthier plants and thereby increase
production (Kabira et al., 2006). Visual disease
incidence was correlated to viral load of PVY and PLRV suggesting that cumulative
vector intensity and disease incidence could be used to predict virus threat
to seed potato (Basky, 2002).
Reduction in the number of tubers in symptomatic plants ranged between 9.6
and 35.5% while the reduction in tuber weight was between 36.8 and 62.7%. The
results indicate that virus infection has a direct effect on potato yield and
therefore the income the farmer would get from the crop (Khurana,
2000; Saucke and Doring, 2004). Virus infection reduced
the number and weight of ware and seed grades but increased the yield of the
chats grade. The chat grade has no economic benefit to the farmer both in terms
of food or seed. This indicates that even with good agronomic practices, the
farmers would not attain desired yields unless virus diseases management is
incorporated in the production system (Robert and Bourdin,
2000; Hoffmann et al., 2001; De
Temmerman et al., 2002; Kabira et al.,
2006; Kenya Agricultural Research Institute, 2007;
Yvon et al., 2000). Therefore, there is need to
create awareness on importance of aphid management and use of certified seed
in farmer-based seed potato production systems in Kenya.
This research was conducted with financial support from the Commission
for Higher Education (CHE), Kenya (CHE 500-673-002).
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