Management of green peach aphid, Myzus persicae in amaranth using host plant resistance and seed treatment

Abstract

Leafy Amaranthus is an important food crop due to its high nutritional value, healthy benefits and improvement of farmers’ livelihoods mostly in Sub-Sahara Africa. The green peach aphid, Myzus persicae (Sulzer) is a major pest of leafy amaranth Sub-Sahara Africa Myzus persicae causes damage through direct feeding and by transmitting plant viruses. Management of the green peach aphids is often based on application of insecticides through conventional spray equipment. However, costs and quantities involved is uneconomical and their application is problematic for small holder farmers. Furthermore, aphids have developed resistance to various chemical groups of insecticides. Aphid-tolerant varieties and seed treatment could offer a potential means of control that would reduce the need for insecticide sprays. However, there is limited knowledge of host resistance mechanisms and seed treatment in new Kenyan amaranth varieties and-aphids interaction This study therefore sought to evaluate the potential of utilizing host plant resistance and seed treatment with three specific objectives namely: (i) to identify farmer practices in the management of aphids; (ii) to determine variation in resistance among new amaranth varieties to the green peach aphids (iii) to evaluate the effect of seed treatment on performance of the new amaranth lines and the green peach aphids. A survey was conducted on 600 randomly selected households in Kisii, Kisumu Vihiga and Kiambu Counties using semi-structured questionnaire to identify farmer practices used in the management of aphids in amaranths. In addition, eight focus group discussions and 16 key informants’ interviews were conducted in the four counties. Data collected from the survey were tabulated and analyzed using descriptive statistics to get an overview on farmer’s management practices for aphids in amaranth. Data of the focus group discussions and key informants’ interviews were discussed basing on management practices used by amaranth farmers to reduce aphid infestation. The survey showed that, 94.3% of the respondents mentioned aphids, (Hemiptera: Aphididae) as a major pest of leafy amaranth, and 96.8% ranked aphids as number one insect pest of leafy amaranth in all the four counties. The most important aphid symptom on leafy amaranth was leaf curling mentioned by 43.2% and 49% in the respondents of Kiambu and Kisumu Counties respectively who also mentioned aphids as the most important insect pest of leafy amaranth. A majority of the farmers (34%) used insecticides for aphid control in leafy amaranth. Non-insecticide methods were used at a lesser extent by 7.6% of the respondents. None of the farmers interviewed mentioned biological control or host plant resistance as aphid control options in leafy amaranth. The effect of M. persicae on seven leafy amaranth varieties (Abuku 1-7) was conducted under a high tunnel at Jomo Kenyatta University of Agriculture and Technology (JKUAT). The number of aphids and plant growth parameters including leaf damage score, specific leaf area (SLA) and yield were determined. Data subjected to analysis of variance using R. version 3.43 in the statistical program “R” from the Ime4 package. Tukey’s honestly significant difference (HSD) test at 0.05 level of significance was used to separate the means. The number of aphids, leaf damage score, SLA total leaf weight, uninfested and infested leaves were significantly different among leafy amaranth varieties; A. blitum selection (Abuku 1 and 2) A. hybridus selection (Abuku 3,4,5, and 6), A. hypocandracus selection (Abuku 8) and within varieties of the same species (P < 0.001). Abuku 1 and 2 had the lowest populations of M. persicae, while Abuku 5 had the highest population. A significantly greater leaf damage score was noted in Abuku 5 and a lower in Abuku 2. A greater SLA was noted in Abuku 8 and the lowest in Abuku 2. The impact of seed treatment on seed germination was evaluated in the laboratory, while that of M. persicae populations on growth and yield of the various varieties were conducted in a high tunnel at JKUAT. A significantly higher germination of seeds was noted with Abuku 1 and 2 seeds treated with a combination of thiamethoxam (20 g/kg), metalaxyl-M (20 g/kg), and difenoconazole (2 g/kg) (1.25ml ai/250g seed). There was significant difference of seeds germinating after 24 h and 3 months of seed treatment (P < 0.001). A higher germination of seed was noted with Abuku 1 and 2 seeds of 24 h and 3 months of seed treatment. A significantly higher number of live aphids was noted in Abuku 5. Seeds treated with a combination of thiamethoxam (20 g/kg), metalaxyl-M (20 g/kg), and difenoconazole (2 g/kg) (1.25ml ai/250g seed) had significantly more live aphids. A significantly greater fresh leaf yield was noted with Abuku 3, 4, 5 6 and Terere seeds treated with a combination of thiamethoxam (20 g/kg), metalaxyl-M (20 g/kg), and difenoconazole (2 g/kg) or a combination of imidacloprid (233g/l), pencycuron (50g/l), and thiram (3 ml ai/250 g seed). Abuku 3 untreated and seeds treated with a combination of thiamethoxam (20 g/kg), metalaxyl-M (20 g/kg), and difenoconazole (2 g/kg) (1.25ml ai/250g seed) had a significantly greater fresh leaf yield and SLA respectively. In summary, the population of aphids were more in Abuku 5 than in other Abuku varieties and Terere. The number of germinating seeds was also more effective in Abuku 1 and 2 than other varieties with or without seed treatment. However, the yield was significantly less in Abuku 1 and 2 and more abundant in Abuku 3 with and without seed treatment. In conclusion, utilization of environmentally friendly approaches such as M. persicae tolerant varieties and seed treatment can improve management of aphids in amaranth production by small holder farmers.