The use of hot water treatment by small holders for the control of alternaria alternata, the cause of black mould disease of tomato

There are many vegetable fruits recognized in Nigeria, but tomato, a vegetable fruit is a major food component, an ingredient utilized by every house hold and constitutes the national food security programme. The record confirmed that Nigeria produces approximately 1.8 million metric tons of fresh fruits for domestic consumption, with national demand of about 2-3 million tons per annum with a demand gap of about 500,000 metric tons. Tomato production is an important source of income to farmers unfortunately diseases such as Alternaria alternata greatly increase food losses by an approximately 20-30% and methods of using synthetic chemical compounds can be costly and dangerous if applied by an unskilled operator and are often not available at the time when required. As a result this study focused on the effect of hot water dipping as a non-chemical method to control the black mould disease caused by Alternaria alternata on red tomatoes. Hot water dip at 50ᴼC for 5 or 10 min was carried out on Alternaria alternata spore suspension (in-vitro), the results showed a significant (P≤0.05) reduction in germination of spores after 48 h. The in-vivo hot water treatment was carried out in three groups, viz-a-viz; first group consists of 30 and 50⁰C and tomato fruits were heated in hot water for 30 and 60 min respectively. The second group was 30, 40 and 50⁰C and fruits were dipped in hot water for 20 min. In the third group the temp was at 40, 45 and 500C and fruit were dipped in hot water at these temperatures respectively for 10 min. Furthermore, the hot water temp was increased to 50 and 55⁰C and inoculated fruits were immersed for 5 min in separate hot water bath. In this trial the result showed that dipping artificially inoculated fruit at 50 or 55⁰C for 5 min significantly reduced (P≤0.05) decay development caused by A. alternata. Conidia germination was more sensitive than mycelia growth to 50⁰C, but inhibition of both processes increased with the duration of time of treatment. The in-vitro hot water treatment of Alternaria alternata spores at 50⁰C i for 30 min significantly reduced the spore germination and mycelia elongation of the fungal pathogen in 48 h. The in-vitro result obtained was attributed to the direct effect of heat on the spore germination as well as mycelia growth resulting in the reduction of the growth of the fungus on the inoculated red fruit. Splitting was observed on the pericarp (skin) at the point of inoculation of fruits before hot water treatment at 55°C for 5 min. The hot water treatment of the tomatoes had the following effects on the attributes of quality: the Brix degrees measurement showed a negligible difference in 40 °C or 50°C compared with the control for 30 min heat treatment after 24 h storage. Also there was no effect of heat on the total soluble solid likewise, the firmness measurement on flesh of tomato showed no significant difference when compared with the control. In this study the change in colour after heat treatment was not statistically significant. Similarly, in the taste test there appears no real difference recorded in the attributes of juiciness, flavour and overall acceptance except that the skin of the tomato was recorded “softer” by some of the taste panellists. This study has shown that prestorage hot water treatment may be a useful non-chemical method of controlling A. alternata postharvest disease pathogen without adverse consequence on the fruit quality.