Host-plant, induced mutagenesis and silicon derived resistance to aphids in common bean, (phaseolus vulgaris l.) and cowpea, (vigna unguiculata (l.) walp genotypes.
Date
2023
Authors
Zimba, Kennedy J.
Journal Title
Journal ISSN
Volume Title
Publisher
The University of Zambia
Abstract
Common bean (Phaseolus vulgaris L.) and cowpea (Vigna unguiculata (L.) Walp) are important food crops across Sub-Saharan Africa. In Zambia, common bean and cowpea yields are typically lower than potential yields due to the impact of plant diseases and invertebrate pests. Black bean
aphid (Aphis fabae Scopoli) and cowpea aphid (Aphis craccivora Koch.) negatively impacts common bean and cowpea productivity directly by ingesting plant assimilates and indirectly by vectoring viral diseases. Aphid management has, for a long time, relied on the use of synthetic pesticides but with variable success due to the development of resistance in many target organisms. Synthetic pesticides may also be harmful to human and environmental health as well as negatively impacting non-target organisms. Current breeding programs aim to develop common bean and cowpea cultivars with improved yield and tolerance to pests. The major objective of this study was to identify and characterise resistance to aphids in common bean and cowpea genotypes while also assessing the potential of silicon to complement mutation derived aphid resistance. Specific objectives of this study were to; (i) identify and characterise resistance to black bean aphid in selected breeding lines and mutation derived common bean genotypes, (ii) identify and characterise resistance to cowpea aphid in mutation derived cowpea genotypes under laboratory and field conditions and, (iii) evaluate the performance of cowpea aphid on silicon treated mutation derived cowpea genotypes under laboratory and field conditions. Five common bean varieties (Rozi Koko, Mwezi Moja, Majesty, KK25 and AO-1012-29-3A) and four mutation-derived genotypes (CA 3, CA 15, CA 24 and CA 38) were evaluated for resistance to black bean aphid. Bean cultivars, Kabulangeti and Carioca (variety from which all mutants were derived) were used as controls for bean varieties and mutant genotypes, respectively. Several parameters of aphid resistance traits were assessed. Aphid deterrence was assessed by settling preference while physical barriers to aphid feeding were evaluated by nymph survival. Reduction in palatability of phloem sap was evaluated by nymph development and mean relative growth rate. Electrical penetration graph recordings of feeding behaviour were performed in order to localise aphid resistance factors. A complete randomised design was adopted for all laboratory bioassays. Nymph development was significantly longer (13.6 days) on AO-1012-29-3A compared to Kabulangeti (11.2 days) despite the fact that there were significantly fewer glandular trichomes on this line, suggesting the presence of aphid resistance factors. Cowpea genotypes derived from three susceptible varieties (Bubebe; Lutembwe and Msandile) after mutagenesis by gamma irradiation were assessed. Eleven genotypes were evaluated: six (BB 3-9-7-5, BB 7-9-7-5, BB 8-1-7-5, BB 10-4-2-3, BB 14-16-2-2 and BBV) from Bubebe, three (LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12) from Lutembwe and two (MS 1-8-1-4 and MS 10-7-2-1) from Msandile. Aphid resistance was evaluated by recording aphid colony growth, mean relative growth rate, intrinsic rate of natural increase, doubling time and feeding behaviour when reared on each genotype. A complete randomised design and randomised complete block design were used for laboratory bioassays and field experiments respectively. Where colony growth was recorded, aphid population was significantly reduced by; 48.5 % on BB 7-9-7-5, 69.3 % on LT 3-8-4-1, 67.5 % on LT 4-2-4-1 and 78.7 % on LT 11-3-3-12 compared to their respective parents. LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12 also resulted in lower aphid mean relative growth rate, intrinsic rate of natural increase and doubling time compared to the parent. Slower colony growth, mean relative growth rate, intrinsic rate of natural increase and doubling time on LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12 and slower colony growth only on BB 7-9-7-5 suggests the presence of mutation derived resistance to cowpea aphid. Characterisation of feeding behaviour on LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12 showed that resistance is mediated by epidermal and mesophyll-based factors. Silicon accumulation capacity of these genotypes as well as aphid performance parameters, as described above, were assessed. A complete randomised design and split-plot design were used for the laboratory bioassays and field experiments respectively. Where silicon was applied, significantly higher concentrations were recorded for Lutembwe and LT 11-3-3-12 plants compared to untreated plants. Silicon application to Lutembwe resulted in slower aphid colony growth and lower mean relative growth rates compared to untreated plants, suggesting the occurrence of silicon-induced resistance. Electrical penetration graph recordings of aphid feeding on silicon-treated Lutembwe plants showed that aphid resistance is mediated by phloem-based factors. Silicon application to LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12 plants did not enhance aphid resistance. This may be due to mutagenesis disrupting their ability to effectively accumulate silicon or because the partial resistance of these mutant genotypes masked any benefits from silicon application to these plants. Overall, AO-1012-29-3A is a promising common bean parent line with useful genetic attributes that may be used to develop aphid resistant common bean varieties. BB 7-9-7-5, LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12 are promising genotypes that should be evaluated further for genetic improvement of cowpea against the cowpea aphid. Silicon application or use of mutation-derived genotypes may be effective tools with which to manage cowpea aphid on cowpea, but there appears to be little benefit of combining these approaches. These findings have important implications for developing an integrated pest management framework for aphid pests on common bean and cowpea.
Description
Thesis of Doctor of Philosophy in Plant Science (Entomology)