Plant growth promoting rhizobacteria (PGPR)

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Fusarium and Pythium spp. as pathogens of sorghum

One of the major concerns to agricultural food production worldwide is diseases caused by phytopathogenic fungi of which Fusarium and Pythium attack most of the economically important crop plants (Gohel et al., 2006). Sorghum (Sorghum bicolour (L) Moench) is ranked second among the five most important cereal crops in Eastern Africa (FAO, 1999). It is believed that sorghum was probably initially domesticated in central Africa in the region of Ethiopia and Sudan. From its initial cultivation in Africa, the crop was introduced into Asia, America and Australia (Forbes et al., 1986). Sorghum is an economically important crop providing food and fodder in the semi arid tropics of the World. In Ethiopia, it is widely grown in the Southern part of the country especially in the dry land areas with high temperature and low rainfall. The grain yield currently estimated in the continent is relatively lower than those in other parts of the World (FAO, 1999). Low sorghum yields are mainly due to insect pests and diseases caused by phytopathogenic fungi and bacteria. Most of the fungal pathogens reported on sorghum worldwide occur in Eastern Africa including Ethiopia (Huluka and Esele, 1992).
Several species of fungi are known to cause various types of diseases in sorghum. Pythium and Fusarium spp. are among the most common phytopathogenic fungi that cause seedling and root rot
diseases in sorghum (Forbes et al., 1986; Horne and Frederickson, 2003). Pythium spp. survives in
the soil as oospores and germinates in response to seed and root exudates in wet soil. They germinate either directly by producing germ tubes or indirectly by producing zoospores. The pathogens can then rapidly penetrate host cells and tissues that lack secondary wall thickenings (Forbes et al., 1986). Evidence has been presented that Pythium ultimum Trow var. ultimum causes a chronic root and seed rot of grain sorghum and negatively affects grain yield in a continuous sorghum production system (Davis and Bockus, 2001). Pythium graminicola Subramanian is also reported to cause root rot in sorghum (Horne and Frederickson, 2003). Pythium root rot in sorghum is characterized by various symptoms including necrosis of seedling leaf tip and blade, collar rot root rot and streaking of the vascular system leading to the death of the plant (McLean and Lawrence, 2001).
Sorghum is also attacked by root and stalk diseases caused by a number of Fusarium spp. The primary inoculum of Fusarium spp. consists of conidia and mycelia that have over seasoned in crop
debris and the propagules are not capable of surviving more than three months in the absence of plant debris (Claffin, 1986). The fungi are widely distributed in host root tissues under field conditions and respond to stress in the plant by taking advantage of preferential growth conditions to incite diseases (Leslie, 1990). In one experiment for example (Leslie, 1990), F. moniliforme and F. proliferatum have been recovered from root tissues in 71 % sorghum samples. In tropical and temperate regions, F. moniliforme is the major causative agent of seed rot; seedling blight; root and stalk rot (Claffin, 1986; Horne and Frederickson, 2003). F. moniliforme is also involved in causing grain mold of sorghum, a serious disease which became a major constraint to sorghum improvement and production worldwide (Navi et al., 2005). Prominent in the root and stalk rot of sorghum are also other Fusarium spp. such as F. oxysporum, F. graminearum, F. tricinctum, F.solani and F. equiseti (Claffin, 1986). Mahalinga et al. (1988), reported that F. oxysporum and F. pallidoroseum are potentially pathogenic on some sorghum genotypes causing a negative effect on seed germination and seedling growth. Fusarium spp. from sorghum, millet and maize have recently become the subject of in depth research due to serious production losses in these crops as a result of stalk rot, ear rots and grain mold infections by these fungi (Leslie et al., 2005). According to this report, Fusarium isolates recovered from sorghum and millet are identified as F. moniliforme. This is an indication that sorghum is attacked by a wide spectrum of Fusarium spp. leading to a serious loss in yield. Losses due to seedling blight and root and stalk rot caused by Fusarium spp. vary from 5-10% and may approach 100% in localized areas (Claffin, 1986).

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CHAPTER 1- GENERAL INTRODUCTION
1. 1. Fusarium and Pythium spp. as pathogens of sorghum
1. 2. Plant growth promoting rhizobacteria (PGPR)
1. 3. Bioformulations and application of rhizobacteria as biocontrol agents
1. 4. Current status and future prospects of using rhizobacteria as biocontrol and growth promoting
agents
1. 5. Objectives of the study
1. 6. References
CHAPTER 2 – IN-VITRO AND GREENHOUSE SCREENING OF RHIZOBACTERIA FOR
BIOLOGICAL CONTROL OF FUSARIUM OXYSPORUM THAT CAUSES ROOT AND CROWN
ROT OF SORGHUM (Sorghum bicolor (L.) Moench)
Abstract
2. 1. Introduction
2. 2. Materials and methods
2. 3. Result
2. 4. Discussio
2. 5. References
CHAPTER 3- SUPPRESSION OF PYTHIUM ULTIMUM ROOT ROT IN SORGHUM BY RHIZOBACTERIAL ISOLATES FROM ETHIOPIA AND SOUTH AFRICA
Abstract
3. 1. Introduction
3. 2. Materials and methods
3. 3. Results
3. 4. Discussion
3. 5. Reference
CHAPTER 4 – STUDY OF THE MODES OF ACTION OF RHIZOBACTERIAL ISOLATES
EFFECTIVE IN THE SUPPRESSION OF FUSARIUM OXYSPORUM AND PYTHIUM
ULTIMUM
4. 1. Introduction
4. 2. Materials and methods
4. 3. Results
4. 4. Discussion
4. 5. References
CHAPTER 5 – GROWTH PROMOTION IN SORGHUM (Sorghum bicolor (L.) Moench) BY
RHIZOBACTERIAL ISOLATES FROM THE RHIZOSPHERE OF SORGHUM AND GRASSES
IN ETHIOPIA AND SOUTH AFRICA
CHAPTER 6 – SELECTION OF THE MOST EFFECTIVE BIOCONTROL AND GROWTH
PROMOTING RHIZOBACTERIAL ISOLATES IN SORGHUM
CHAPTER 7- GENERAL DISCUSSION

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