Diversity, Factors Influencing Spread and Population Build Up of Potato Cyst Nematodes and Potential of Phytochemicals in Their Management in Kenya

Abstract

Potato cyst nematodes (PCN) of the genus Globodera are one of the most significant constraints to potato production in Kenya. Limited research has been undertaken on the causal agent, practices that influence population build-up and spread of the pest as well as the economic importance of PCN populations in the country. Therefore, this study was carried out between 2019 and 2022 with the aim of determining the diversity of PCN populations in the country; site specific factors that contribute to spread and build-up in population density; relative yield loss caused by the pest; and the efficacy and active ingredients of selected botanical pesticides in PCN management. The diversity of potato cyst nematode populations was determined using morphometrical and simple sequence repeats markers. Morphometric analysis proved ineffective in differentiating Globodera. pallida from Globodera rostochiensis. Results of the molecular analysis showed that all the samples from the three study counties (Nyandarua, Nakuru and Meru) gave amplicons of 434bp that are specific for G. rostochiensis. The results showed that G. rostochiensis populations from the three counties were successfully discriminated by Gr 67, Gr79, Gr90 and GRM2 SSR markers (PIC ≥ 25). The fixation index (Fst) ranged from 0.021 to 0.048 indicating that the samples had low genetic variation. To determine the factors contributing to spread and population build-up of potato cyst nematodes (Globodera spp.) in Nyandarua county, a survey was carried out in Upper Highland 3 (UH3), Upper Highland 2 (UH2) and Lower highland 4 (LH4). A total of 65 randomly selected farmers were interviewed using a structured questionnaire and soil samples collected. Results showed that cyst counts were higher (302 cysts/ 300 g) in farms with more than one crop cycle compared to farms where one crop cycle was grown per year having 38.5 cyst/300 g soil. Over 80% of the farmers used farm saved seed tubers and grew cultivar Shangi that is susceptible to PCN. The study showed that cysts adhered on farm tools, seed potato and foot wear. Assessment of the relative yield loss caused by potato cyst nematodes at varying infestation levels was determined through field experiments. Potato cyst nematodes reproduced more on Desiree and Shangi varieties than on variety Manitou indicating that the two are suitable PCN hosts. Tuber yield losses were significantly reduced in potato varieties treated with PCN control products over the untreated control plots. A positive correlation (r= 0.4640, r= 0.4802) (p≤0.05) between final cyst counts and reproduction index during long and short rains respectively was observed. Additionally, final cyst counts were significantly (p≤0.05) and negatively (r=-0.4987, r=-0.5085) correlated with total tuber yield during long and short rains season. Assessment of the efficacy of selected botanical pesticides in potato cyst nematode management was done through in vitro, screenhouse and field experiments. Plant extracts from Mexican sunflower, garlic, ginger, Mexican marigold, spring onion, sodom apple, eucalyptus leaves, eucalyptus bark, green tea leaves and onion bulb were effective in inhibiting egg hatch, reducing egg viability, and increasing mortality of second stage juveniles (J2s). Application of hexane extracts from ginger, garlic and Mexican sunflower resulted in a significant (P<0.05) increase in juveniles mortality and loss of egg viability resulting in potato yield increase under screenhouse and field conditions. The ginger extract, at a concentration of 100 mg/ml, had an inhibitory effect on multiplication of the nematodes and was rated as the most potent in reducing numbers of PCN. Results from the survey work led to the conclusion that G. rostochiensis was the predominant species within the study area with limited genetic variation among the populations studied. Factors that were contributing to spread and build-up of potato cyst nematodes include cropping practices and soil types. Mexican sunflower, ginger, and garlic extracts have potent nematicidal effects against PCN J2s and eggs due to phytochemicals present in the extracts. The three botanical extracts significantly increased potato yields by 112.2 and 80.6 % during the short and long rains, respectively. Ginger extract, applied at 100 mg/ml is recommended as the most effective extract against PCN in order to improve potato yield. There is need for an integrated approach incorporating all appropriate strategies such as planting of disease-free tubers, resistant varieties, field sanitation as well as enhancing farmers’ knowledge on management of the pest to reduce the spread and build-up of PCN.