30 Produce Degradation: Reaction Pathways and their Prevention
fungal mutants that lack cutinase can still infect produce [68,69]. Cruickshank et al.
[70] noted that the primary mode of cuticular penetration in tomato fruit is mechan-
ical and the cutinases may only play a minor role. In their study on fungal infection
of tomatoes, they treated the epicuticular wax layer with solvents or abraded the
cuticle surface before inoculating the fruit with Colletotrichum gloeosporioides.
There appeared to be a diffusion of solutes from the underlying cell layers that
stimulated the fungal growth. Infection occurred as penetration pegs eroded the
cuticle, thus allowing for further infection. The authors noted that as the tomato
ripened and the fruit skin turned yellow, the cuticle became resistant to cutinases.
However, the penetration peg was still able to pierce the cuticle of the fruit in spite
of the cutinase resistance. They concluded that cutinases might not be required for
fungal infection of tomato fruit [70].
Once the penetration peg has completely penetrated the cuticle, a fine hyphal
tube that spans the cuticular membrane starts to increase in diameter until it reaches
its mature size and secretes enzymes into the host tissue. In diseases such as soft
rot, enzymes seem to be by far the most important substances involved in spreading
disease. The fungi secrete one or more sets of enzymes that collapse and break down
the middle lamella and cell walls of parenchymatous tissue. Some pathogens can
infect produce and remain dormant for a time before infection resumes. For instance,
Prusky [71] found that Alternaria Rot in persimmon fruits stemmed from necrotic
leaf spots found in the orchard. The germinating conidia penetrated the fruit cuticle
directly. Subsequently, the hyphae developed intercellularly and produced dark,
quiescent infections that had renewed growth during storage.
Many examples in literature illustrate that the cuticle is a formidable barrier to
fungal invasion. Biles [72] found that peppers with thicker cuticle layers had greater
fungal resistance. Jenks et al. [43] found that mutant sorghum with a reduced cuticle
thickness had a higher susceptibility to fungal pathogens. Spotts et al. [73] studied
fungal infection in pear fruit and noted that in most cases fungi typically infest pear
by entering the fruit through a wound in the epidermis or stem. Pear cuticle is
particularly difficult for fungi to penetrate. The infection can occur during harvest
or when the fruit is exposed to contaminated water in dump tanks or in flumes used
in packinghouses. Wounded fruit can get mixed with good fruit in storage and may
ultimately infect other fruit.
2.5 ROLE OF CUTICLE IN HOST/PATHOGEN RESPONSE
Some produce, especially climacteric fruits, undergoes changes during ripening that
make them more susceptible to fungal infection. Fungal infection may be initiated
prematurely and then arrested until the host reaches the proper stage of maturity.
One such illustration involves avocado fruit inoculated with Colletotrichum
gloeosporioides at various stages of fruit ripeness [74]. In unripe fruit, the fungi
produced an appressorium and a penetration peg, which only penetrated the epicu-
ticular wax layer 4 days after inoculation. However, in ripe fruit, the penetration
pegs had penetrated subcuticular tissue within 48 hours of inoculation. The fungi
arrested further growth until the climacteric rise in respiration occurred. The resump-
tion of fungi growth resulted in an enlargement of the penetration peg and a rapid