An integrated approach to pest management in field pea, Pisum sativum (L.), with emphasis on pea aphid, Acyrthosiphon pisum (Harris)
This study comprises investigations into pea aphid, Acyrthosiphon pisurn, and ascochyta blight damage on field pea, the evaluation of plant resistance levels in both breeding lines and cultivars, the identification of plant resistance and the underlying mechanisms, and cultural, chemical and biological control methods. Varietal resistance studies indicated that there were differences between the Ethiopian and the South African strains of pea aphid with regard to their survival and reproduction on the field pea genotypes evaluated. The field pea entries performed very well against the former strain compared with the latter. Three lines (Holetta Local-90, 305PS210687 and 061K-2P-2/9/2) performed well across both strains. Field pea lines exhibiting tolerance, antixenosis and antibiosis resistance to A. pisum were identified under greenhouse conditions. Some lines showing high levels of antibiosis to nymphal feeding were also found in both strains. This kind of resistance mechanism may promote insect biotype development through increased selection pressure on the pest population. Strain variation was also evident in tolerance, antixenosis and antibiosis resistance. The South African strain was the least aggressive across all entries. Of the 30 varietiesllines (including a local susceptible cultivar from Ethiopia) evaluated for resistance to isolates of Mycosphearella pinodes, Oregon Sugar Pod II had a 1.9 blight severity and was scored as resistant, three genotypes (Green Feast, Sugar Queen and line 304WAll01973) were scored as intermediate (2.1 - 3.0 severity factor) and the remaining 26 genotypes were scored as susceptible (3.1 - 4.0 severity factor) or highly susceptible (4.1 - 5.0 severity factor). In all scoring dates, significant differences occurred among genotypes, isolates and genotype x isolate interactions. However, the genotype x isolate interaction contribution to total variation was much lower than that of genotypes and isolates separately. The isolate of the Denbi site in Ethiopia was slightly more virulent than those of the Holetta and Kulumsa sites. Assessments regarding the potential of biological control of pea aphids using a predatory beetle (Hippodamia variegata) and entomopathogenic fungus (Beauveria bassiana) indicated that predator-treated plots supported significantly lower aphid numbers from the third week onwards, when compared to the fungus-treated and infested control plots. The degree of mycosis caused by Beauveria on pea aphids was 14.3% in week three after inoculation and the figure dropped to 2.5% in week 5. Percentage yield loss due to pea aphid in predator-treated plots was 8.3 % compared with 16.0 % in fungus-treated plots. Field pea intercropped with Ethiopian mustard sustained less pea aphid and ascochyta blight incidence, compared to faba bean, wheat and field pea mono crop at all locations studied. The land equivalent ratio for this particular mixed crop system exceeded 1.0, indicating that the mixed crops selected were efficient for yield and monetary outcome. The increase in efficiency was ascribed to the barrier effect of mustard plants in the intercrop set-up, which was significant in reducing pea aphid population size and disease severity. The effect of fertilizer application and sowing date on pea aphid and ascochyta blight severity was location specific. At the Holetta site in Ethiopia disease severity and pea aphid infestation were significantly reduced in fertilized plots compared with unfertilized plots, while it was only the disease that showed significant difference at the Denbi and Kulumsa sites. This indicates the importance of fertilizer application as a cultural control strategy for this disease. Neither early nor late sowing resulted in reduced aphid infestation and disease infection at any of the locations. Significant interactions between variety, sowing date and fertilizer for ascochyta blight was observed, indicating that the effect of one factor was influenced by the other two factors. For aphid population density and yield, the three factors had little or no effect on each other at the Denbi and Kulumsa sites. Cultivar Markos was moderately resistant to ascochyta blight and it gave higher yield compared to Mohanderfer and the varieties used by farmers. Neem seed kernel extract application was superior to Multineem", a commercial product, against pea aphid development and reproduction. The neem preparations significantly reduced the number of molts, longevity and fecundity of A. pisum in a concentration-dependent manner. The effect on young adults exposed to neem was not as drastic as in the case of immatures. Acute and chronic toxicity effects on pea aphid were noted showing that azadirachtin is an effective inhibitor of population growth of pea aphid both on treated plants and when topically applied to the insect. Host plant resistance and natural chemical (neem) pest control in large scale farming systems, or integrated with cultural and biological control in low-input subsistence farming systems provides effective management strategies for pea aphid and ascochyta blight in field pea. From this study, possible implementation of IPM in field pea is presented and includes aspects of varietal resistance and biological, cultural and chemical control.