SURVEY OF FUNGAL DISEASES ASSOCIATED WITH AERIAL STEMS AND LEAVES OF POTATO ( SOLANUM TUBEROSUM L.) IN SOME IRRIGATED FIELDS IN KADUNA AND KATSINA STATES OF

A survey was conducted to determine the incidence of fungal diseases associated with aerial stems and leaves of Irish potato ( Solanum tuberosum L.) in some irrigated fields in Kaduna and Katsina states. The fields were selected based on systematic random sampling method and the samples were collected using stratified random sampling (SRDS) techniques. Disease symptoms were identified using CABI and CIP description of potato diseases. A total of four fungal diseases were found to be associated with aerial stems and leaves that included early blight (17.8%), leaf blight (48.7%) Fusarium wilt (21.1%) and Sclerotinia wilt (12. 29%). The fungi were identified based on standard mycological methods. The colonies observed were distinguished microscopically and macroscopically. Six genera of fungi were isolated from the wilted aerial stems and blighted leaves as Fusarium sp (18.7%), Sclerotium sp (18.7%) and Alternaria sp (18.7%); Helminthosporium sp (18.7%) had the highest frequency of occurrence. Botrytis sp (17.7%) and Stemphylium sp (5.6%) had the least frequency of occurrence. The results of this study could be used for further studies in plant protection and disease management.


INTRODUCTION
The potato (Solanum tuberosum L.) is grown worldwide in continental, temperate, subtropical and tropical climates. It is the world's most widely grown tuber crop (Were et al., 2013) and the fourth major food crop after rice, wheat and maize (Guchi, 2015). Potato originated from South America and was brought to Africa in the 20 th century. Since then, the crop has spread around the world and has been incorporated into the food of nearly every region of the world (CIP, 2011). Potato is now an important crop and invaluable food source all over the world. Potatoes are rich in carbohydrates, which are important for energy (Visser et al., 2009). While potatoes are low in proteins, they contain large amount of vitamins B and C as well as ascorbic acid. Currently in Nigeria, the crop is being cultivated in commercial quantities in Cross River, Plateau, Taraba and Borno states (NRCRI, 2012). Also during the cold months of hamattan, potato can be grown in eleven Northern states of Nigeria, namely Borno, Bauchi, Jigawa, Gombe, Kaduna, Katsina, Kano, Kebbi, Sokoto, Yobe and Zamfara under irrigation. Many countries in sub-Saharan Africa such as Nigeria are in the process of expanding their potato production, and with the effect of climate change leading to floods especially in the rainy seasons, irrigation seems to be a better alternative. Muhammad (2009) noted that over 50,000 hectares of land have been under potato irrigation on the Plateau every dry season by individuals, cooperative groups and government agencies.
The potato business in Nigeria is estimated at 300 billion naira (FAO, 2012;NRCRI, 2012). Potato plays an important role in employment creation, poverty alleviation, food security as well as a source of revenue to government. The bulk of potato production in Nigeria comes from the Jos-Plateau due to favourable climatic conditions.
Most of the work done on etiology of the diseases of Irish potato in the areas of study carried out on potato tuber were either in the store or market sources. Little or no work has been done on both the aerial stem and leaves of potato from irrigated fields. This study reports the identification of different fungal diseases affecting aerial stems and leaves of Irish potato as well as fungal isolates associated with the diseases.

Survey area
During 2015 and 2016 growing seasons, a survey was conducted in some irrigated fields of potato in Kaduna and Katsina States to determine the incidences and prevalence of fungal diseases associated with aerial stems and leaves of potato. The sampling collection techniques were designed and developed based on the size and disease distribution in the fields. A total of eight (8) villages and 32 fields were surveyed in the two states. In both states, two Local Government Areas and four villages were visited. Based on the interview with potato growers and physical observations of all the fields, qualitative data such as seed sources, potato varieties grown, seed cutting process, cultural practices, crop rotation and source of irrigation water were collected in order to track the source of infections.

Sample Collection
Samples were collected from fields based on the stratified random sampling technique (SRSD) (Cochran, 1977). The entire field was divided into sectors (6-8) with the help of quadrant (10 x10). The infected leaf samples were collected and put in polyethylene bags for further analyses.
Hand books (CIP, 2009;CABI, 2013) containing an overview and description of the disease symptoms were used in identifying the symptomatic potato plants. Also, coordinates of sampling areas were determined using hand-held Global Position System (GPS) Magella Trailblazer XL GPS Handheld Receiver. The disease incidence was calculated using the following formula:

Isolation of Fungi from Diseased Samples
The fungi were isolated from infected tissues as described by Jha (1995), Pitt and Hocking (1997) and Nelson and Nabulie (2000) for diseased potato plants. The infected areas of the tissues were washed with sterilised distilled water prior to isolation. The infected area was removed or cut into 5 mm with the help of sterilised blade. The cut tissues containing both the healthy and infected portions were immersed in 1% sodium hypo chloride (NaOCl) for 2-3 minutes and rinsed 3 times with sterilised distilled water, then transferred into Petri dishes containing fresh PDA and incubated for 3-5 days at room temperature. The representative colony types were picked and sub-cultured on the fresh PDA until pure culture was obtained. The isolated fungi were identified based on colonial characteristics on culture plates and microscopic features.

Identification and Characterisation of Isolated Fungi
Fungal isolates were identified based on macroscopic and microscopic characteristics. Wet mounts of fungal colonies were prepared following standard mycological procedures. Lactophenol in cotton blue was used for direct examination of specimens of fungi; 1-2 drops of lactophenol was placed on a slide and small tuff of the fungus was picked up with a sterilised needle and placed into the drop. The fungal material was then teased out, covered with a cover slip and examined under the microscope under x 10 and x40 objectives. Identification was done b a s e d o n t h e B a n n e t and Hunter (1972) method.

Statistical Analysis
Data obtained were subjected to descriptive statistics to determine the incidence of diseases as well as frequency of occurrence of fungal isolates. Values were expressed in percentages after transformation. Also, Student's T test was used to compare the disease incidence among the fields and between the states. All analyses were performed using SAS V.9.2 (2008).

Comparison of incidence of fungal diseases and frequency of occurrence of isolates associated with aerial stems and leaves of irrigated potato field in Kaduna and Katsina States
The comparison of fungal disease incidence and frequency of occurrence of isolates associated with aerial stems and leaves of potato field in Kaduna and Katsina states is shown in

DISCUSSION
The survey showed that early blight disease was found to be high in Gwamusawa and Bakori fields in Katsina State. This agreed with findings of Abbas (2017), who reported that early blight disease tends to be more severe in humid regions because the dew or free moisture on the foliage of potato plants can cause conidia of Alternaria to germinate within 30-40 minutes and the germ tubes arising from conidia penetrate the leaf epidermis directly through micro pores.
The results showed that Hunkuyi had a high incidence of leaf-blight caused by Botrytis sp. This could be associated with the irrigation schedules (time, amount and frequency of water). Irrigation remains a common problem in developing countries due to lack of technology. Layronas et al. (2013) reported that excessive irrigation and high fertilizer input lead to dense canopy making the leaves to remain wet for a long period, which is conducive for the development of botrytis disease.
The results indicated that Hunkuyi had the highest incidence of leaf blight caused by Helminthosporium sp. This may likely be due to the maize cultivation during the rainy season farming before the cultivation of potato in the dry season. Shatty and Franyer (2015) noted that the maize residue harboured Helmiothosporium blight pathogens. With the maize residues in contact with the soil surface the conidia and chalamidiospores were formed and the spread of Helminthosporium pathogens on the soil surface penetrates the leaf tissues.
The study revealed that leaf blight caued by Stemphylium sp was observed in most of the areas surveyed. Suheri and Prince (2000) reported Stemphylium blight in the 1930s in Florida, USA. Since then, the Stemphylium leaf blight disease has spread to Asia, Europe and Africa. Zhang et al. (2010) reported that between 2004-2008 Stemphylium blight also attacked the fields in China, resulting in 30-70% yield loss. The results showed that Dan-Danga had the highest incidence of Stemphylium blight. This might be due to the fact that the Stemphylium sp could have overwintered in the fields. Since the farmers in Dan-Danga normally rotate onion with potato and other crops, it may probably be the carryover of pathogens from onion to potato plants. Mehta (2013) reported that in India the incidence of Stemphylium blight was high in the fields where onion was rotated with potato.
Karfe area had the highest incidence of Fusarium wilt disease, due, perhaps, to acute shortage of irrigation water in the area which makes potato plants to be under water stress. According to Kora et al. (2005), Fusarium wilt is severe at high temperature and dry soil condition.
Sclerotinia wilt disease was observed in nearly all the surveyed areas. The soil could have been infested with Scleretium which has overwintered in the soil due to rotation of carrot with potato in the fields. Akinlabi et al. (2015) reported that Sclerotium is capable of invading the crop from infested soil.

CONCLUSION
Results of this study showed that the Fusarium and Slerotinia wilt diseases were associated with aerial stems. Early blight was observed in the leaves. Alternaria sp, Helminthosporium sp, Sclerotinia sp. and Fusarium sp. Were the other pathogens isolated from the diseased plants. The results showed that rotation of potato with other crops like carrot, onions maize as well as irrigation schedules could have predisposed the crops to infections.

AKNOWLEGEMENT
The authors thank the farmers in the study areas as well as the technical staff of the Department of Microbiology, Ahmadu Bello University, Zaria and Tanimu Tela of the Division of Agricultural Colleges, A.B.U., Zaria for their assistance.