Effects of stages of maturity on the susceptibility of tomato fruits to postharvest fungal pathogens

Abstract

The tomato is popularly grown in Kenya as a fruit vegetable, and hence provides employment and income to
rural communities. Besides, it is a source of minerals and vitamins and therefore improves the quality of stew
and salads. However, adequate amounts of tomatoes of good quality are rarely available due to production
constraints, notably lack of varieties that are resistant to pre- and postharvest rots, not prone to spoilage
during postharvest handling, and do not have a “short shelf life”. Postharvest tomato rots are principally
caused by fungi and bacteria. Other factors that determine the magnitude of postharvest losses include: the
fruit maturity stage at harvest, the fruit cultivar and the postharvest pathogen identity. Hence, a project was
designed to find out the effect of colour maturity stage at harvest on the susceptibility of tomato fruits cultivar
Cal J to the postharvest fungal pathogens (Fusarium oxysporum f. sp. lycoperisici, Fusarium solani, Alternaria
alternata and Geotrichum candidum). Tomato fruits, cultivar Cal J, were harvested at the green, yellow and red
colour maturity stages and inoculated with the 4 fungi (Fusarium oxysporum f. sp. lycoperisici, Fusarium
solani, Alternaria alternata and Geotrichum candidum) that were isolated from diseased fruits sampled from
tomatoes sold in a Nairobi market. The pathogenicity of the fungi was assessed by comparing the lesion
development and amount of damage caused on inoculated tomato fruits. The susceptibility of the tomato fruits
to the fungi at different stages of maturity was determined by comparing lesion development and fruit damage
on the mature green, yellow and red cultivar Cal J tomato fruits. The fungus G. candidum caused significantly
(P≤0.05) highest damage with respect to lesion diameters and fruit damage (97.2%) compared to the other fungi:
F. solani (82.4%), F. oxysporum f. sp. lycopersici (62.6%) and A. alternata (52.2%) on mature red fruits. The
damage caused by A. alternata was significantly (P≤0.05) smallest. This is probably because G. candidum
produces enzymes that effectively degrade the tomato tissues while those produced A. alternata are not as
effective. Similarly, G. candidum caused significantly (P≤0.05) highest (97.2%) damage on the mature red fruits
while the damage (20.5%) caused on the green fruits was significantly (P≤0.05) lowest. The trend prevailed for
the other fungi. This could be due to higher concentration of biochemical defence mechanisms in green tomato
fruits, which reduce as the tomato fruits ripen. Storage trails indicated that the “shelf-life” of the mature green
Cal J tomato fruits was up to 3 times longer than that of the mature red ones, confirming that Cal J tomato fruits
harvested at the green maturity stage have a significantly (P≤0.05) longer “shelf-life” than those harvested at
the other maturity stages. The results, from the study, makes it necessary to advice farmers to: harvest
tomatoes at the green maturity stage, avoid postharvest handling process that inflict damage; plant tomato
varieties that withstand mechanical damage, and are resistant to postharvest infection; and prevent contact
between tomato fruits with sources of fungal inoculums.