Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
42 Postharvest Diseases of Fruits and Vegetables
fungi pathogenic to carrots (especially that of Botrytis cinerea and
Sclerotinia sclerotiorum) was weaker when the relative humidity was close
to saturation than when it was at the 90-95% level (van den Berg and
Lentz, 1968). Since these enzymes play an important part in the
development of the pathogen within the host tissues, the pectolytic activity
levels of the pathogens under the various humidity conditions might
constitute a significant factor in the development of decay.
3. THE STOREROOM ATMOSPHERE
The gas composition of the storeroom atmosphere directly affects the
development of fungal mycehum in the plant tissue, as it also affects spore
germination on fruit and vegetable surfaces. Yet, hke the other
environmental conditions, the storeroom atmosphere, too, simultaneously
affects the host, the pathogen and their interrelations. Since the
atmospheric gas composition can significantly affect the ripening and
senescence processes of the host, and since the physiological state of the
host can affect its susceptibility to disease, the atmospheric gas composition
indirectly affects decay development in the tissue (Barkai-Golan, 1990). The
direct and indirect effects of the atmospheric gases (the levels of O2, CO2
and their combinations) on the development of pathogens, and the use of a
suitable atmosphere composition for inhibiting disease in stored fruits and
vegetables is discussed in the chapter on Means for Maintaining Host
Resistance - Modified and Controlled Atmosphere.
D. CONDITIONS PERTAINING TO THE HOST TISSUES
1. ACIDITY LEVEL (pH)
The low pH that characterizes many fruits (below pH5) is probably an
important factor in their general resistance to bacterial decay agents, but
it furthers the postharvest development of various fungi. In fact, fungi
cause most of the decays in harvested fruits, whereas bacteria are
important mainly in vegetables. Unlike the fruit, the various other plant
organs such as roots, tubers, stems or leaves, where the pH level is
generally higher and ranges between 4.5 and 7.0, might often be attacked
by soft-rot bacteria (Lund, 1983; Bartz and Eckert, 1987). Exceptions to
that rule are some "fruit-vegetables", such as tomatoes, bell peppers and
cucumbers, in which bacterial decay is quite common. The pH of soft rot
lesions on tomato fruits is higher (pH>5.0) than that of surrounding
healthy tissue (pH= 4.3 - 4.5), suggesting that the bacteria are capable of
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