Dutse Journal of Pure and Applied Sciences (DUJOPAS) Vol. 2 No. 2 December 2016
INCIDENCE OF PHYTOPHTHORA INFESTANT THE
IRISH POTATO FAMINE FUNGI AT YANKABA
MARKET
Yahaya, S. M.
Department of Biology,
Kano University of Science and
Technology, Wudil, Kano State
Saratu, B.
Department of Biology,
Kano University of Science and
Technology, Wudil, Kano State
Amina, L. A,
Department of Biochemistry,
Kano University of Science and
Technology, Wudil Kano State
Sadiya, A. B.
Department of Biochemistry,
Kano University of Science and
Technology, Wudil Kano State
Maimuna, M. D.
Department of Biochemistry,
Kano University of Science and
Technology, Wudil Kano State
Tasiu, A. M.
Department of Biochemistry,
Kano University of Science and
Technology, Wudil Kano State
Ibrahim, A. M.
Department of Biochemistry,
Kano University of Science and
Technology, Wudil Kano State
Musa, M. J.
Department of Biochemistry,
Kano University of Science and
Technology, Wudil Kano State
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Dutse Journal of Pure and Applied Sciences (DUJOPAS) Vol. 2 No. 2 December 2016
S
Abstract
tudy was carried out to determine the fungal pathogens responsible for post-harvest losses of potato
(Solanum tuberosum) on sale at Yankaba market. The investigation period lasted for five month
between September, 2015 to January, 2016. Potato samples were collected and analyzed for the
presence of fungal species using standard methods. Fungal infection at Yankaba markets were found mostly
to be due to Phytophthora infestant 20 (33.3%}, Alternaria solani 15(25%), Aspergillus niger 6(10%),
Rhizopus stolonifer 9(1.58%), and Mucor spp 10 (16.7%). Although more colonies were recorded on
Monday 28(46.7%) than Thursday exposure with 32 (53.3%) but the differences was not statistically
significant (P>0.05) between the two sampling dates. This showed that date of sampling has no effect on the
isolation of fungal colonies. However, the results of this study showed that fungal pathogens play a
significant role in post-harvest loss of the potato at Yankaba market.
Keywords: Fungi, Irish, Potato, Yankaba
Introduction
Potato (Solanum tuberosum L.) belongs to the tuber crops and there are two main types’ Irish potato and sweet
potato (Ipoema batata) which is raised through vines, whereas the former are raised through tubers. Irish
potato was first introduced into Nigeria in the late 19th Century, through missionary activities (Obigbesan,
1976). It is estimated that over 200,000 tonnes of potato tubers are marketed in the major cities of Nigeria and
about 100,000 tonnes of table potato are informally sold across the Nigerian boarders in the West African Subregion (Ugonna et al., 2009). Irish potato is a staple crop in about 130 countries worldwide. A brief analysis of
potato production in various countries of Africa reveals that Egypt is Africa’s number one potato producer,
followed by Malawi (Ugonna et al., 2013). Although Nigeria is known to be the fourth biggest producer of
potato in Sub- Saharan Africa, it is the seventh biggest producer of potato in Africa. The production was
encouraged by the British Colonial Government during the Second World War, as the tubers were needed to
feed their armed forces in West Africa (Ugonna et al., 2013). Since then, the importance of potato has been
widely realized such that it is now an important commodity in both local and international trade. Although
production of potato has increased by over 120% in the last 10 years in Nigeria (FAO, 1990; Ugonna et al.,
2013), it is still grossly below demand. Apart from low quality seed and poor storage facilities, diseases are
also a limiting factor to potato production in Nigeria (Kuku et al., 1980; Ifenkwe and Suchomel, 1983; Eckert
1999; Jay, 2003). The main potato growing area in Nigeria is the Jos Plateau, where altitudes ranging from
1200 to 1400 m and summer temperatures that rarely exceed 35°C make for a temperate climate, well suited
for potato production. However, productivity is constrained by lack of suitable varieties and high land and
labour costs. In fact, Nigeria records one of the world's lowest average potato yield/hectare, little more than
3.1 t/ha. Potato consumption is also very low, barely 3.2 kg per capita per year. However, Nigeria's taste for
potatoes, especially in rapidly growing urban areas, is increasing. Since the year 2000, imports of raw and
processed potatoes have risen from less than 9,000 tonnes to 40,000 tonnes a year (Ugonna et al., 2013).
Potato is known to be prone to more than 100 diseases caused by bacteria, fungi, and viruses (Williams et al.,
1991). Epidemics in different parts of the world were devastating, sometimes causing total crop loss and
severe economic hardship for many potato growers. The resurgence of previously control infectious diseases
and the emergence of new ones are of increasing concern to scientists and the general public (Berkelman et al.,
1994; Cassell, 1994; Garrett, 1994). Optimism fostered by the discovery and successful application of potent
antibiotics has been dampened by the appearance of drug-resistant pathogens. The diseases are strongly
influenced by environmental conditions, being favored by high moisture and moderate temperatures (15-25
oC).
Opadokun (1987) and Arene (2009) reported that a lot of fungal pathogens of different species are associated
with post harvest losses of potato in Nigeria, these pathogens includes Fusarium spp, Aspergillus spp, Mucor
spp, Alternaria spp, Penicillium spp. Phaeoisariopsis batatticola, Pseudocercospora timorensis, Phyllosticta batatas,
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Septoria bataticola, Corticium spp., Elsinoe batatas, Mcliola clavulata, Coleosporium ipomoeae, Puccinia spp., Uromyces
spp., Plenodomus destruens, Sclerotium rolfsii, Rhizopus stolonifer, Monilochaetes infuscans. However, FAO (1989)
reported that the most important fungal pathogen causing disease in potato known as potato late blight
disease (Figure 1) is the filamentous fungal-pathogen oomycete, Phytophthora infestant. Many years after
Phytophthora infestant causes devastating effect on potato crops in the world which causes famine popularly
known as the Irish potato famine, in Ireland, 1.5 million people died and a similar number emigrated (Bourke
1993).
In Nigeria the pathogen is resurfacing again creating plant health problem (Ugonna et al., 2013). Phytophthora
species are sometimes called "water molds" because their growth (and disease development) is favored by wet
conditions. Each individual is bisexual, capable of producing both female (oogonia) and male (antheridia)
structures. Sexual reproduction requires the interaction of two mating types (Al and Al). Fertilized oogonia
develop into oospores that can survive adverse conditions, such as freezing or drying, for as long as months
or even years. The potato late blight disease can be a remarkably rapid and destructive disease: Fields, stores
and warehouses that appear healthy, but contain low incidence of disease, can be devastated within few days
(Legard et al., 1995). It is difficult to detect low levels of incidence of the disease, and the pathogen can
reproduce rapidly. The disease cycle (penetration, colonization, sporulation, and dispersal) may occur within
or less than five days; each individual late blight lesion can produce as many as 300,000 sporangia per day
(Legard et al., 1995).
Figure 1: Potato late blight caused by
the oomycete Phytophthora infestant,
(a) Infected leaves (b) Infected
stems (c and d) Infected tubers and
fruits
Therefore, in order to underscore the importance of potato in addressing the world food crisis, the United
Nations officially declared 2008 as the “international year of the potato”, in order to raise its profile in
developing nations, calling the crop “Hidden treasure” (FAO, 2008). Although potato has been identified to
be the fourth most important root crop in Nigeria, after Cassava, Yam, and Cocoyam (Okonkwo et al, 2009)
with an efficient tuber crop in the country in terms of tuber yield and days of maturity however, no detailed
studies on the nature of the disease under Nigerian condition have been carried out on most of the pathogen
attacking potato. Therefore, the present study tested two hypotheses; first there is the likely hood of
resurgence of previously control infectious diseases of potato Phytophthora infestant and the emergence of new
ones are on the increase. Second, previously control infectious and other new emerging pathogens are
responsible for pathogenic losses of Irish potato grown in Nigeria and marketed at Yankaba market.
Material and methods
Sampling site
Yankaba market is located at Nassarawa Local Government Area of Kano state, a distance of about 50 Klm
away from Kano University of Science and Technology Wudil. It is one of the largest vegetable markets in
Kano state. Despite being one of the largest vegetable markets in Kano state, there are no good storage
facilities in the market, some marketer’s store their vegetables on the bare floor of their stores, while others
store their vegetables packed in basket or sacs. Marketers hardly use chemicals on vegetables they however,
washed them either with water or with detergent.
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Experimental procedure
In this study an investigation was carried out to provide information on fungal colonies responsible for postharvest losses of Irish potatoes. The investigation period was between September 2015 to December, 2015.
Determination of fungal diseases of Irish potato
This involved the isolation and identification of fungi associated with losses of quality and quantity of
potatoes respectively. The methodology used in this research follows the one used by Yahaya et al., (2016) and
is described below.
Collection and handling of samples
Five sample of Irish potato were obtained twice a week at Yankaba market and rinsed in three changes of
running tap water and allowed to dry. Portion (2mm) were cut with sterilized scalpels. Cut pieces were
placed on PDA and incubated at 25.7+ 2oC for 3 days. The number of colonies that appeared was counted and
recorded. Each colony was sub-cultured into fresh PDA.
Pathogenicity Test
Pathogenicity test was conducted to prove Koch postulate. All fresh samples were separately washed in 10%
(v/v) sodium hypochlorite solution and rinsed in 3 changes of running tap water and allowed to dry. A ruler
was used to marked 2mm diameter on each potato sample. Each sample was streaked with fungal hyphae on
the marked portions. Controls were inoculated with sterile distill water. Materials were placed on the
laboratory bench. Sterilized forceps was used to remove portions from the disease area on the 4 th day and
placed on freshly prepared PDA plates and incubated at 25.7 ± 2OC for 3 days. Fungal growth that appeared
was recorded.
Microscopic Examination
A streak of fungal mycelium was placed on a clean glass slide, drop of cotton blue lactophenol was added
and the cover slip was placed. The slide was mounted on the microscope and observed at magnification of
x10, x40 and x100. A morphological characteristic of fungi isolated was determined and identified using
method described by Dorothea et al., (1976). Lengths of the hyphae were determined with eyepiece graticule
and identified by using colonial and morphological characteristics. Photographs of fungi mycelia were taken
from mounted slide using camera Lucida at biology laboratory Kano University of Science and Technology,
Wudil.
Statistical Analysis
The data was analyzed statistically using one way analysis of variance (ANOVA) and difference among the
means were determined for significance at p<0.05. This was achieved using computer program (spss, 16.0).
Result
Infection of fungal species was recorded on all the potato samples. A total of 60 colonies were counted and
recorded which belongs to the Alternaria solani, Aspergillus niger, Rhizopus stolonifer, Mucor, and Phytophthora
infestant (Figure 2). Thus the compositions were Alternaria solani 15(25%), Aspergillus niger 6(10%),
Phytophthora infestant 20 (33.3%}, Rhizopus stolonifer 9(1.58%), Mucor sp 10 (16.7%). Control plates showed no
growth (Table 1).
Variation in the colony counted on Monday and Thursday
A total of 28(46.7%) were recorded on Mondays, while 32 (53.3%) were recorded during Thursdays exposure
(Table 2).
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Dutse Journal of Pure and Applied Sciences (DUJOPAS) Vol. 2 No. 2 December 2016
Pathogenicity test
The results of the pathogenicity test confirmed all the four criteria outline in Koch postulates for identification
of the causative agent of a particular disease. The pathogens were present in all cases of the disease. Similar
pathogens were isolated from the diseased host and grown in pure culture when inoculated into a healthy
sample (Table 3 and 4).
TABLE 1: Number of colonies identified from Potato on sale at Yankaba Market
__________________________________________________________________________
Day of Exposure
Isolated mould
Monday
Thursday Total Mean
%
A. solani
4
11
15
7.5
25.0
A. niger
3
3
6
3
10.0
P. infestant
13
7
20
10
33.3
Mucor sp
3
7
10
5
16.7
R. stolonifer
5
4
9
4.5
1.50
__________________________________________________________________________
TOTAL
28
32
60
30
100
Table 2: Variation in the colony count of all the species for the two selected sites on samples
collected on Mondays and Thursdays
Location
Yankaba
% abundance
Monday
28
46.6
Thursday
32
53.3
Total
60
100
Table 3: Pathogenicity test on fresh/apparently healthy potatoes
Effect of fungal isolates on Potato
Inoculated
Control
Test organisms
Diameter on 4th
Texture on 4th day
Diameter on 4th
Texture on 4th day
day
day
A. niger
2.8
Rot
1
Turgid not soft
Mucor spp
2.2
Soft
1
Turgid not soft
R. stolonifer
1.6
Soft
1
Turgid not soft
P. infestant
2.5
Rot
1
Turgid not soft
Table 4: Re- isolation of inoculated isolate from fresh Potato
_________________________________________________________
A. niger
Mucor sp R. stolonifer P. infestant A. solani
__________________________________________________________
+
+
+
+
+
+
+
+
+
+
____________________________________________________________
Key: + = Isolates grow with a similar growth characteristic features to the original diseased samples
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Figure 2: Phytophthora infestant. sexual reproduction is an oospore (a), which is the only)' form of the
fungus that can survive for months or years in the absence of a living host. Oospores are typically 20- 30 pm
in diameter. Asexual reproduction occurs via multinucleate spores called sporangia. sporangia (b), which are
typically 20-25 x 25-35 pm. These sporangia are produced under moist conditions (relative humidity at or
near saturation or with free moisture) and can be dispersed by wind currents or in splashing water droplets.
Sporangia germinate directly via germ tubes or indirectly (c) by the release of uninucleate, biflagellate
swimming zoospores. Zoospores can remain motile for nearly one hour before encysting. The Oospores
subsequently form germ tubes that can penetrate host tissue.
Discussion
A total of 60 fungal colonies were isolated during the study. The isolated pathogens include Phytophthora
infestant, Alternaria solani, Aspergillus niger, Mucor, and Rhizopus stolonifer. P. infestant was the highest
occurring colony, while most, Pathogenicity test confirmed the pathogens as originally isolated pathogen of
potato sample from Yankaba market. More colonies were recorded on Thursday than on Monday’s at
Yankaba market.
In a study by Alao (2000) shows that post-harvest losses of grains, legumes, fruits and vegetable are the result
of infections mostly by pathogens in the field which continue to develop after harvest. In a similar study
Yahaya (2005) reported that fungi are the most common cause of spoilage on fruits and vegetables and several
fungi like Penicillium spp, Cladosporium spp, and Altternaria phomopsis are known to cause large scale storage
loss of fruits and vegetables from harvest to storage.
The finding of this study is in agreement with the report of Haris and Lapwood, 1976; Arene 2009), that
Alternaria solani, Aspergillus niger, Rhizopus stolonifer, Phytophthora infestant and Erwinia carotovora were the
most virulent microorganisms associated with storage rot of potato. It was clear that marketers at Yankaba
market store their potatoes either on the bare floor of their stores or in sacs. However, good storage is an
important component of potato production. Potato tubers are living organs. They consume oxygen and give
off carbon dioxide and heat. Their living characteristics in storage are influenced not only by the storage
environment but also by genetic variety, agronomic practices during growth, pest and disease attacks and
particularly by the physical condition of the tuber. Poor storage practice where healthy potatoes are mix with
infected ones made Okonkwo et al., (1988) to conclude that in Nigeria, as high as 40 % of the stored Potato
tubers are lost within three months of storage as a result of poor storage conditions.
Observations during the study at Yankaba market showed that Thursday’s recorded more influx of customers
than Monday’s. In addition high consignments of potatoes from different locations in Nigeria were delivered
to Yankaba market on Thursday’s than on Monday’s. Such influx of customers and new supply of
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Dutse Journal of Pure and Applied Sciences (DUJOPAS) Vol. 2 No. 2 December 2016
consignment on Thursday’s may have create condition that might favour the growth of the fungi as against
the lower number of colonies isolated on Monday
The result of the study shows that previously controlled fungal pathogens Phytophthora infestant the causative
agent of late blight disease was the highest occurring fungal colony followed by Alternaria solani the causative
agent of early blight disease with percentage occurrence of 33.3% and 25.0% respectively. However,
Berkelman et al. (1994), Cassell (1994) and Garrett (1994) observed that resurgence of previously control
infectious diseases and the emergence of new ones are of increasing concern to scientists and the general
public. Phytophthora infestant is known to be moisture loving and tends to occur in fruits with relatively high
moisture content, such as potatoes. Contrary to this work, Uzuegbu and Emifoniye (1984) in their study of
post-harvest fungal spoilage of some Nigerian fruits and vegetables isolated Alternaria in 40% of the total
samples used.
In the present study Mucor sp and Aspergillus niger were the third and fourth occurring colonies with
percentage occurrence of 16.7% and 10.0% respectively. The occurrence of these pathogens might be
attributed to their ability to produce resistant spores, as reported by Jay (2003) that spores of Aspergillus are
more resistant to high temperature. And in conjunction to this, Aspergillus species have been implicated in the
spoilage of fruits and vegetables in Nigeria (Uzuegbu and Emifomiye (1984; Hayatu, 2000; Yahaya, 2005). It
was clear from the colonies isolated some potatoes were observed to have been contaminated with one
pathogen while others have multiple infections. FAO (1998) reported that temperature and relative humidity
of the fungi have a significant effect on the growth of the pathogens and their subsequent relevance to
spoilage.
It was clear that the presence of these fungi or their resistant spores is most likely to have originated from the
farms where the potato were harvested and some from the stores due to horizontal contamination by the
infected tubers. This observation agrees with the finding of Bukar et al., (2009) that most spoilage organisms
may be present on tuber, fruits and vegetables from the farm, during harvest operations, and this may result
in post-harvest contamination and spoilage of tuber, fruits and vegetables during storage and marketing. The
presence and subsequent spoilage due to these fungi, if not checked could lead to serious economic loss and
possible health hazards when these tubers are consumed.
In conclusion, the results of the present study has revealed that post harvest losses of Irish potato at Yankaba
was attributed to the activities of Phytophthora infestant, Alternaria solani, Mucor sp and Aspergillus niger.
Although, control of these pathogens is possible with significantly increased application of dosage and
frequency of available fungicides however, this strategy is not only expensive but also inconsistent with
worldwide efforts to decrease pesticide dependency in agriculture. Therefore, the resurgence of potato late
blight on potatoes at Yankaba market illustrates the need for governments at state and national levels to
intensify research on the biology of the pathogen, and on the events that lead to the host susceptibility or
resistance, and on additional technologies that can mitigate the disease. In addition to new and intensified
control efforts, thorough investigations into the disease epidemiology and pathogen genetics are needed. In
addition there is also the need to encourage more research work on the seed variety that is currently used by
famers. Also the farmers and processors of potatoes need to be assisted with improved technologies and
increased financial assistance to be able to acquire modern equipment for production, storage, processing and
marketing of potatoes in Nigeria.
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