Effects of inorganic fertilizer on tomato – bacterial wilt interactions in high tunnel tomato varieties

Abstract

Tomato (Solanum lycopersicon L), is one of the most common and important vegetable crops in the world with over 18,477 hectares under tomato production in Kenya. Tomato production is however, greatly threatened by bacterial wilt (BW). Bacterial wilt, caused by Ralstonia solanacearum is a serious disease in the tropics and subtropics, causing severe losses in many agricultural crops. To control its infection, several measures have been deployed but with minimum success. Alternative integrated strategies are therefore needed for the management of this pathogen. This study sought to test the hypothesis that particular rates of inorganic fertilizers influence certain secondary metabolites in the tomato that affect the plant’s response to bacterial wilt. The hypothesis was tested by: (i) Investigating the farmers’ knowledge, attitude and practices (KAP) towards R. solanacearum in high tunnel tomato production in Kiambu County, central Kenya. This was done by conducting a focused group discussion (FGD) for 32 farmer groups which was followed by soil sampling from the high tunnels in six sub counties in the period July – September 2016 using a checklist with open ended questions. (ii) establishing site specific soil chemical properties influencing population density of R. solanacearum in the high tunnels in Kiambu County, which was done by analyzing the chemical properties of the soil samples collected and relating that to the BW populations from the same soil samples as quantified; (iii) evaluating the pathogenicity of R. solanacearum under different inorganic fertilizer rates in high tomato varieties grown under high tunnels by conducting potted trials with different levels of NPK fertilizer treatments, namely, 0 g, 2.5 g, 5 g, 10 g and 20 g per 2 kg of sterilized 2:1 red soil and on two tomato varieties; and (iv) identifying the chemical components released by tomato plants grown under different inorganic fertilizer rates done by extracting and quantifying phenolics from the various treatments at the International Centre of Insect Physiology and Ecology. Results from the KAP survey showed that about 78.1% of high tunnels were in use for 1-2 years and 62.5% of farmers taking part in the study could identify symptoms caused by BW, which resulted in crop loss of 50%–100%. About 82.6% had the soil in which they produced tomato in the tunnels analyzed for nutrition and presence of pathogens, but the majority (71.7%) never followed recommendations on how to amend their soils. These findings indicate that more research and information are required so that farmers can optimize high tunnel production of tomato under tropical conditions. Analyses of soil samples collected showed that BW was present in all the Sub counties, with Gatundu North sub county testing up to an average concentration of 12.551 × 109 cfu/ml of R. solanacearum, which was 3.8 times higher than Juja sub county. Results showed that there was a strong positive correlation between R. solanacearum with soil nitrogen (78%) and phosphorous (87%), and a weak negative correlation with soil pH (42%), electrical conductivity (8%), zinc (23%) and copper (4%). When pathogenicity of R. Solanacearum was examined on Anna F1 and Tylka F1, results showed that no significance difference (P = < 0.05) existed between the two varieties in terms of BW attack and yield. When treatments were separated for treatments, 5 and 10 g yielded best (no significance difference at P=0.05). Wilting was highest (87.13%) in the control and least (30%) at 10 g. Qualitative tests results revealed that alkaloids, flavonoids, tannins, saponins and terpenoids were all present in all the treatments. In addition, when quantified using the Folin Ciocalteu method, total phenolics were found to be highest in 10 and 20 g respectively with no significance difference (P=<0.05) between them, with concentrations of 126.46 and 132.75 milligram gallic acid equivalent (mg GAE)/g dry weight of extract (mg/gGAE) respectively. Least concentration was found in the control with a concentration of 45.71 mg/gGAE. A strong negative correlation (r = 0.7002) was found between wilt incidence and the total phenolics present in the treatments meaning that wilt incidence decreased with increase in total phenolics. This indicates that plant defense against the bacterial wilt pathogen is influenced by phenolic content in the plant. In conclusion, proper application of inorganic fertilizers at affordable and non-toxic rates (10g/hole) for tomato production offers a promising bacterial wilt management alternative because of the ability of enhancing the plant defense system. As such, identification and subsequent understanding of the functions of the specific chemical exuded by tomato plants due to fertilizers applied would not only address the existing knowledge gap, but also offer hope for the development of a new approach or the improvement of existing bacterial wilt control strategies.