The effect of the entomopathogenic fungus Beauveria bassiana on growth, physiological responses and control of aphid (Myzus persicae) infestation on Lactuca sativa L.

Abstract

Green peach aphid (Myzus persicae) is among the most destructive pests of agricultural and horticultural crops. This sap-sucking insect species reduces crop productivity and is a vector to many phytopathogens, and are often difficult to control with conventional synthetic insecticides. The quests for alternative environmentally-friendly insecticides for controlling aphids are intensifying. The use of entomopathogenic fungus presents an enticing opportunity to control sap-sucking insects such as M. persicae. Endophytic fungus such as B. bassiana is a natural parasite of insects and is a safer alternative to synthetic chemicals that are potentially toxic to human and environmental health. The objectives of this study were to (i) assess the Beauveria bassiana colonization on Lectuca sativa plants, (ii) assess pathogenicity of B. bassiana against M. persicae in the laboratory, (iii) assess the effect of B. bassiana on antioxidant contents of L. sativa, (iv) assess the effect of B. bassiana on proximate components of L. sativa, (v) assess the effect of Beauveria bassiana inoculation on secondary metabolites contents of lettuce, and (vi) assess the effect of B. bassiana inoculation on M. persicae infestation level on lettuce plants in a greenhouse. The B. bassiana strain (SM3) used in this study was first evaluated in an insect mortality bioassay to determine its pathogenicity and suitability for further study. Insects in each treatment group were exposed to one of four fungal conidial concentrations: 0, 1 x 106, 1 x 107 and 1 x 108 conidia mL-1. The strain was pathogenic against M. persicae, with an LC50 of 1.1 to 1.6 x 106 conidia mL-1. Potted-lettuce plants were allocated to one of four conidial concentrations (0, 1x 106, 1 x 107, and 1 x 108 conidia mL-1), following a complete randomized design, with a singlefactor. The effects of fungal inoculation on plant growth, plant physiology, plant secondary metabolites, and aphid infestations were assessed. After 21 days post-treatment, fresh leaves were picked off plants and taken to the laboratory to assess whether the fungus colonized the plant tissue. Leaf sections from the harvested leaves were surfaced sterilized. The leaves were then placed on selective solid potatoes dextrose agar (PDA) plates of half strength of 19.5 g/1000 ml, containing 0.04 g streptomycin, and 0.02 g ampicillin sodium salt. The plates were incubated at 25 ± 2 oC. Based on the mycelia outgrowth from the leaf sections, the fungus colonized up to 76% of plants in 1 x 108 conidia mL-1 suggesting that tissue colonization by the conidia of B. bassiana was high. The fungus did not significantly affect the growing parameters (P>0.05); however, there was a significant difference in crown size and plant height (P<0.001). The tissue contents of the micronutrients Mn, Fe, Cu, and B increased significantly (P<0.05) with the fungal treatment. On the other parameters assessed, antioxidant activities of the extracts and protein content of lettuce varied with conidial treatments. Interestingly, the values of antioxidant activities, carbon (C) content, and protein content in the control (no fungus) and highest conidial treatment were consistently higher than the moderate conidial treatments. The fatty acids and chlorophyll contents were not significantly (P>0.05) influenced by conidial inoculation. Chapter three results showed that the fungus did not affect insect infestation levels (P>0.05). However, there was a statistical difference among the treatments on total polyphenol content (P<0.001). Flavonols were not significantly affected (P > 0.05) by the fungal inoculation. A wide range of volatile compounds was detected using GC-MS analysis. Some are well-known insect repellents and antifeedents, such as Limonene and 3-octanol. The 3-octanol and 2,4-Di-tert-butyl-phenol were significantly (P<0.01) more concentrated in the fungal treated plants than the control plants. Generally, while B. bassiana inoculation significantly affected total polyphenol and micronutrient (Mn, Fe, B, Cu) contents, it did not significantly affect flavonol level nor insect infestation levels. In conclusion, the B. bassiana strain used in this study successfully colonized the lettuce plants. Overall, the fungus had minimal effects on plant growth and protection against aphid infestations. Exposure to the fungus did not significantly induce increased macronutrient contents in the plant tissue of nutrients and some volatile chemical constituents. This study provides insights into the effect of fungal inoculation on the growth, physiology, and aphid infestation of lettuce plants. The study also highlights the need for further studies to better understand the endophytic fungus-host plant- herbivorous insect relationship.