360 Produce Degradation: Reaction Pathways and their Prevention
fungal diseases for fruits and vegetables in the U.S. along with fungicides used for
their control [4]. These tables also indicate which plant parts or stages are usually
afflicted.
Soil-borne fungi, such as Pythium, Phytophthora, or Rhizoctonia, and also some
seed-borne fungi (mainly Alternaria), may attack germinating seeds or young seed-
lings and cause “damping-off” disease, which is a general term for a sudden plant
death in the seedling stage caused by fungi [61]. To minimize potential losses, the
seeds can be treated with protectant fungicides, mainly with thiram or captan. Also,
the soil can be sterilized prior to planting using fumigants, such as methyl bromide,
metam-sodium, or chloropicrin. Infection of older plants does not usually lead
directly to their death, but they often develop stem and root rots, which result in
decreased plant vigor and productivity and may lead to further spread of the disease
to leaves, blossoms, and fruits [43]. Important examples include the devastating
carrot and parsley disease alternaria leaf blight (Alternaria dauci) [62], alternaria
leaf spot (Alternaria brassicae) on cole vegetables [63], and phytophthora blight
(Phytophthora capsici), which afflicts peppers and many other vegetables [64].
Soil-borne fungi can also directly attack plant parts in close proximity to the
soil, which is the case with most vegetables and some fruits (strawberries, grapes,
some citrus fruits) that have leaves and fruits close to the ground or creep on it, such
as cucurbits. In the case of fruit trees, their fruits and leaves are mainly affected by
air-borne fungi, such as Venturia, which causes pear or apple scab [65], or various
fungi responsible for powdery or downy mildew.
Fungal damage on leaves basically leads to the same consequences as insect or
mite feeding: reduced produce quality and size (and also reduced plant vigor, ulti-
mately leading to plant death) due to lowered photosynthetic capacity and produce
loss in the case of leafy or cole (cruciferous) vegetables. Infection of produce
products, such as fruits, roots, tubers, or bulbs, translates directly into yield loss,
which may occur both pre- and postharvest.
Often, presymptomatic stages of fungal infections are very difficult to detect and
the affected products may not be discarded during postharvest sorting, and thus the
disease may spread during transit and storage. In fruits, fungi can infect unripe fruit
and remain quiescent until conditions become favorable for growth. As the fruit ripens,
quiescence is broken and the fungus colonizes fruit tissues [66]. Fungal pathogens on
tropical fruits, such as Colletotrichum gloeosporioides on mango and papaya or Col-
letotrichum musae on bananas, may serve as typical examples of this behavior [67].
Postharvest application of fungicides can prevent the potential disaster caused
by disease spreading from infected to sound products and provide protection from
further fungal attack from another source. The most widely applied postharvest
fungicides for the control of various fungi in many produce types are thiabendazole
and o-phenylphenol [4,5]. Other examples include biphenyl (a vapor-phase fungicide
impregnated in paper wraps), used for postharvest protection of citrus fruits, and
diphenylamine (applied as a postharvest dip or in wax formulations), which serves
as a fungicide protectant and scald inhibitor (antiscalding agent) for pome fruits
[5,68]. Superficial scald is a skin disorder of certain apple cultivars (mainly Granny
Smith, Rome Beauty, Delicious, Winesap, and Yellow Newtown) that may develop
during storage due to the oxidation of alpha-farnesene [69]. Alpha-farnesene is a