BIOASSAY OF PHYTOTOXIC METABOLITES OF FUNGI ASSOCIATED WITH Solanum lycopersicum L. (TOMATO) LEAVES

Abstract

Solanum lycopersicum (tomato) is an essential vegetable crop consumed worldwide. Major limiting factors in its production include fungal foliar diseases. Phytotoxic fungi play critical roles in the pathogenesis and expression of disease symptoms in the plant. An understanding of the phytotoxins produced in tomato leaves will enhance its optimal production. However, there is dearth of information on tolerance of tomato varieties to phytotoxins associated with their leaf diseases. Therefore, this work was aimed at investigating the phytotoxins produced by pathogenic fungi associated with tomato leaves.

Infected leaf samples (3 per plant, 30 plants per farm) of Kerewa variety were randomly collected at the expression of disease symptoms from 3 farms in Alapoti, Ogun State. Samples were cultured on Potato Dextrose Agar for fungal isolation. All isolates were identified using morphological and microscopic characteristics. Pathogenicity test was conducted based on Koch’s postulates. Pathogenic fungi were cultured in Czapecks Dox broth using rotary shaker (96 rpm) for 28 days. Phytotoxins were extracted separately with Ethyl acetate and Diethyl ether and the yields determined and extracts measured in milligram (mg). Portions of the extracts were analysed by gas chromatography-mass spectrometry for identification of constituents. The pathogenicity of the extracts was evaluated using in-vitro and in-vivo leaf bioassays on eleven varieties of tomato (Kerewa, Ibadan local, LEMT3, LEMT25, LEMT39, LEMT47, LEMT49, Assila, Gem Pride, ROMA-VF and UC-82-B). Data were analysed using ANOVA and means were separated with Fisher’s Least Significant Difference (α ≤ 0.05).

Identified symptoms on the leaf samples were chlorosis, leaf spot and wilt. Fungi isolated from diseased tomato leaves were Aspergillus aculeatus, A. niger, A. tamarii, A. ustus, A. versicolor, Epicoccum nigrum, Fusarium oxysporum, Phialophora melinii, Phomopsis sp. and Trichodema asperellum. Fusarium oxysporum and Phomopsis sp. were found to be the causal organisms of the leaf infections. Diethyl ether and ethyl acetate extracts of Phomopsis sp. produced 50.0mg and 41.0mg, respectively. Fusarium oxysporum extracted with ethyl acetate produced 54.5mg, while diethyl ether gave 39.0mg. Compounds identified from the extracts from Phomopsis were 1,2-Benzenedicarboxylic acid and Benzeneacetic acid, while 5-Butyl 2-Pyridinecarboxylic acid, 1,2-Benzenedicarboxylic acid and 3-butyl-pyridine were from Fusarium oxysporum. Treatment of tomato varieties with phytotoxins from Phomopsis sp. for in-vitro assay showed LEMT39 and LEMT49 to be susceptible, while LEMT25 was highly resistant. For extracts from Fusarium oxysporum, LEMT3, LEMT25, LEMT39 and LEMT47 were susceptible; Kerewa and Ibadan local were highly resistant. For in-vivo leaf bioassay on Assila, there were significant differences by the fractions causing leaf spot and wilt and no significant difference for chlorosis. There were significant differences in the effect of the fractions causing wilt, spot and chlorosis in Gem Pride and Ibadan local, while for ROMA-VF, there was significant difference in leaf spot and none in wilt and chlorosis. On UC-82-B, wilt and chlorosis were significant in leaves treated with the extracts, while leaf spot was not significant.

The phytotoxins produced by the fungal pathogens induced foliar diseases on the tomato. Cultivation of varieties tolerant to these toxins is thus encouraged.