Abstract
Possible endophytic colonisation of rice cultivar parts (leaves, stem and roots) by Beauveria bassiana isolates and their potential as biocontrol agents against Sesamia calamistis Hampson (African pink stem borer) were investigated. Five promising B. bassiana isolates were evaluated for their endophytic colonisation, the effectiveness of the inoculation methods and the efficacy of the isolates as biocontrol agents against S. calamistis. The plant part colonised is often dependent on the inoculation method. Colonisation of plant parts was assessed at 30 and 60 days after seed inoculation and foliar spray. For the pathogenicity activity, third instar larvae of S. calamistis were fed with rice stems that were previously inoculated with endophytic isolates of B. bassiana. Both inoculation methods led to the colonisation of the rice cultivar tissues, but were affected by the interactions of cultivars x isolates x inoculation methods. The colonisation of the cultivar plant parts varied over time (30- and 60-day intervals), and was affected by the inoculation method used. For both inoculation methods, highly significant differences were observed in the roots and the leaves over time (p=0.0001). However, with seed treatment, there was no significant difference in levels of colonisation in stems by the isolates x time (p=0.32). The B. bassiana isolates were pathogenic on the third instar larvae of S. calamistis, causing mortalities of more than 50% at 28 days after treatment. However, the virulence of the isolates varied. According to the isolates and the inoculation methods, B. bassiana formed an endophytic relationship with rice plants, and produced various mortality rates.
 Significance:
 
 Beauveria bassiana could be a potential biocontrol agent of rice stem borer, S. calamistis as there is no report of endophytic isolates of B. bassiana for the control of rice borers.
 Currently there is no commercially registered biocontrol agent against rice borers; hence further studies into B. bassiana could lead to the registration and commercialisation of B. bassiana as a bio-pesticide for rice stem borers.
Highlights
Rice (Oryza spp L.) is one of the world’s most important crops, providing food for more than half of the world’s population.[1,2,3] Rice and wheat (Triticum spp L.) together contribute about 21% of the total energy consumed by humans.[4]
The rice plants were colonised by B. bassiana isolates using both inoculation methods
No colonisation by B. bassiana isolates was observed in the tissues of the control plants, with either inoculation method (Tables 1 and 2)
Summary
Rice (Oryza spp L.) is one of the world’s most important crops, providing food for more than half of the world’s population.[1,2,3] Rice and wheat (Triticum spp L.) together contribute about 21% of the total energy consumed by humans.[4] In West Africa it has become the main source of calories for low-income households.[5] Two Oryza spp. are cultivated globally: Asian rice (Oryza sativa L.) and African rice (Oryza glaberrima S.), for which the cultivation is limited to tropical West Africa.[6] Rice is grown and consumed in more than 40 countries on the African continent.[7] Its consumption has increased rapidly in Africa, making it the second largest source of carbohydrates in sub-Saharan Africa.[4] Imports of rice account for nearly 40% of the total rice consumption of the region.[8,9]
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