Identification, Pathogenicity, and Management of Fungi Isolated From Castor (Ricinus Communis L.) in Samaru, Nigeria
CHAPTER ONE
Objectives of Study
The objectives of this study are to:
Isolate and identify fungi associated with castor and ascertain their pathogenicity. Determine minimum infectious inoculum density of fungi on castor.
Evaluate in vitro and in vivo effects of some fungicides on the pathogen. Determine the effect of sowing dates on disease incidence and severity.
CHAPTER TWO
LITERATURE REVIEW
Origin and Distribution of Castor
Ricinus communis originated from Ethiopia and grows wild in East and North Africa (Adefris and Nigussie, 1993). It was first cultivated in ancient Egypt as far back as 4000 BC (Purseglove, 1968; Gobin et al., 2001). In Nigeria, castor is an indigenous source of vegetable oil used in industries due to its high oil content. However, its potentials under Nigerian conditions are yet to be fully exploited (Yayock, 1985).
Ecology of Castor
Castor requires warm climate and at least 140 – 180 days growing period and can be grown over a wide altitude range in the tropics and sub tropics with both low and medium rainfall (Purseglove, 1968; Weiss, 1971; Gobin et al., 2001). It grows best in deep well drained soils which encourages deep and drought resistant root system (Duke, 1983; Gobin et al., 2001). Castor is grown on sandy and clayey soils, and thrives well on poor soils (Kulkarni, 1959; Duke, 1983).
The castor plant which takes 5 – 8 months to mature is usually sown around June – July or September – October under rainfed and irrigation respectively in India (Duke, 1983; Gobin et al., 2001). The seeds are sown in rows 1 – 2 meters apart for tall varieties, and 40 – 50 cm intra row spacing for the dwarf types (Kulkarni, 1959). Soaking the seed for about 12 hours prior to sowing has been found to improve germination (Yegna, 1965).
Botany of Castor
The wild cultivars are short-lived perennials growing up to 9 to 12 meters tall, with well developed tap and lateral roots. The cultivated varieties are annuals with height of between 1 and 7 meters. The leaves are large laminate carried on leaf stalks coming off the main stem and branches. Castor is monoecious and is mostly wind pollinated although cross pollination is common due to activities of bees and the flower nectar is used by bees to produce excellent honey (Purseglove, 1968; Gobin et al., 2001). The fruit is green when unripe, brown when matured and each capsule contains 3 seeds, which are 0.5 – 1.5 cm long (Purseglove, 1968; Duke, 1983). Each seed has a shape of a well-fed tick with colour varying from pale grey, brown or almost black with darker mottlings (Purseglove, 1968). Castor seeds contain between 50 – 55 % oil and a toxin ricin, which is poisonous, (causes blood coagulation) (C.S.I.R., 1948-1976; Duke, 1983). Upon extraction of oil, the toxin is eliminated (Duke and Wain, 1981).
Production and Yield of Castor
The demand for castor seeds and oil in world market has increased greatly over the years (Adefris and Nigussie, 1993). In 1960s, World production had reached over 500,000 tons with India and Brazil producing very large quantities (C.S.I.R 1948-1979). The average seed yields in pure castor stands vary from 500 kg/ha in India to 1000 kg/ha in Thailand (Gobin et al., 2001). Under improved cultural management with irrigation, yields of 2,500 – 3000 kg/ha were achieved in USA (Duke, 1983; Gobin et al., 2001). World production is about 25296 MT per annum with Africa producing 1863MT (FAO, 2003). The three largest importers of castor seeds and oil are the United States, France and United Kingdom with India (1087 kg/ha), China (993 kg/ha) and Brazil (823 kg/ha) being the largest growers and exporters (FAO, 2005).
CHAPTER THREE
MATERIALS AND METHODS
Isolation and Identification of Pathogen
Infected castor leaves were collected from the Institute for Agricultural Research (IAR) research fields and brought to the laboratory. The infected leaves were washed, cut into pieces, sterilized for five minutes using 0.5 % sodium hypochloride and rinsed thrice with sterile distilled water (SDW). The pieces were placed in petri dishes containing Potato Dextrose Agar with Streptomycin (PDAs) and labelled. The plates were incubated at 28 oC. Fungal mycelia of the isolated organisms were sub-cultured on fresh PDAs to get pure culture. Pure cultures of the organisms were examined using a compound microscope and the fungi identified. Pure cultures obtained were placed in slant bottles and kept for future use. Specimen of the isolated pathogen was sent to International Mycological Institute (IMI), Egham London, for identification.
Pathogenicity tests in the screenhouse
Raising of seedlings
Castor Seeds obtained from infected fields were sown in plastic pots (12 cm diameter) containing sterile soil at three sowing dates of seven days interval in order to have plants of varying ages,11, 18 and 25 days at inoculation. Three inoculation methods, smearing, spraying and soil inoculation were used. One stand per pot was planted and three pots were used for each inoculation method for the different plant age.
CHAPTER FOUR
RESULTS
Isolation, Identification and Symptoms of the Pathogen
The fungi isolated from the infected leaves were Fusarium sp, Helminthosporium sp and Curvularia sp. The pathogenicity tests showed that the Fusarium sp was pathogenic on castor. The Fusarium sp was later identified at International Mycological Institute (IMI) Egham, London as Fusarium pallidoroseum (Appendix A).
Symptoms
The symptoms on naturally infected leaves were large yellowish brown irregular lesions extending towards the leaf margins and the leaf mid – ribs (Plate I). While on inoculated plants, the symptoms were initially light or pale green irregular lesions of less than 3 mm in diameter around the leaf margin. Later these lesions enlarged advancing towards the leaf mid rib. After 6 – 7 days the leaves became yellowish brown, wilted and finally fell off while the stem showed rot and eventual collapse (Plates II and III).
CHAPTER FIVE
DISCUSSION
This study has shown that, Fusarium pallidoroseum isolated from infected leaves is the causal organism of leaf blight and wilt of castor seedlings in Samaru. This is in line with the reports of Anjani et al. (2004), Booth (1971), Nelson et al. (1981) and Anon. (2006), who independently reported, that wilting in castor is induced by Fusarium pallidoroseum formally semitectum. Even though the pathogen has been confirmed by International Mycological Institute (IMI), Egham London, the cultural and morphological characteristics observed in 14 – day-old culture on PDAs at 28 oC was similar to those described by Booth (1971) and Ellis (1971). The differences observed in conidia sizes; for the isolated 10 – 20 x 3 – 4.2 µm was obtained while in the literature 17 – 35 x 3.5 – 4 µm was reported could be as a result of media used which were different from what the previous workers used. Booth (1971) used Potato Sucrose Agar (PSA) at 25 oC and recorded the conidia size of F. pallidoroseum to be 17 – 35 x 3.5 – 4 µm. In this study the size of conidia were 10 – 20 x 3 – 4.2 µm at 28 oC on Potato Dextrose Agar (PDA). The observed variation in size could be attributed to varying culture media and temperature.
CHAPTER SIX
SUMMARY AND CONCLUSION
The fungus inducing leaf blight and wilt on castor leaf was isolated, identified and its pathogenicity confirmed in the glasshouse. The fungus was identified at Crop Protection Department IAR, ABU Zaria as Fusarium sp which was confirmed at the International Mycological Institute (IMI), Egham London, as Fusarium pallidoroseum. The cultural and morphological characteristics were same as those described by CMI. During the pathogenicity tests 11 and 18 days old castor seedlings inoculated with the three methods of inoculation (spray, smear and soil) were most susceptible. High disease severity was observed in castor seedlings inoculated with high inoculum density.
The effect of varying concentrations of fungicides namely benomyl, benomyl + thiram, metalaxyl-M + thiomethoxam + difenoconazole, tricyclazole and carbendazim + mancozeb on mycelial growth, sporulation, spore sizes and mycelial regrowth of Fusarium pallidoroseum was evaluated in vitro and with the exception of metalaxyl-M + thiomethoxam + difenoconazole in the in vivo. All the concentrations of the fungicides tested significantly inhibited mycelial growth and sporulation. Benomyl, benomyl + thiram, tricyclazole had the greatest inhibitory effect on mycelial growth, sporulation and mycelial regrowth of the fungus. Mancozeb gave the least inhibitory effect on Fusarium pallidoroseum in vitro, while carbendazim + mancozeb and metalaxyl-M + thiomethoxam + difenoconazole gave partial inhibition. Mancozeb controlled disease better than all the other fungicides during in vivo evaluation.
In the field study conducted during the 2006 wet season at Samaru, the effect of three sowing dates on incidence and severity of leaf blight caused by Fusarium pallidoroseum on castor was low with no significant differences between the sowing dates.
From the study, the following conclusions can be made:-
- Leaf blight and wilt of castor is induced by Fusarium pallidoroseum.
- The younger plants (11 days old) were more susceptible to
- At high inoculum density of 3.6 x1010the disease severity was higher at 7
- The six chemicals evaluated in vitro had inhibitory effect on mycelial growth, sporulation, spore size and mycelial regrowth of pallidoroseum and their effect increased with concentration. For the in vivo trial all the fungicides at recommended rate gave better results by reducing leaf blight severity on castor plants than at half the recommendedrate.
- Sowing late, August 11 resulted in poor seed germination thereby affecting bean yield.
From the above conclusions, Fusarium pallidoroseum on castor in Samaru can be controlled using fungicides. Other types of disease management like cultural control practices involving sowing dates and spacing, breeding for resistant varieties can also be evaluated so as to develop an effective control package for the disease.
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