PAPAYA - Tropical Fruits - page 316

collar and root-rot and sometimes stem canker and fruit rot in papaya. Papaya
is predisposed to the disease in areas with a high water table, poor aeration
and persistent high rainfall. Phytophthora with Pythium also cause seedling
'damping-of ', a serious disease during the nursery stage. Phytophthora root
rot is a major concern in all areas where the same land is used repeatedly
for production. Depending upon soil type and rainfall, the replant problem
may become very serious within two or three successive plantings, with
seedling mortality up to 45% (Nakasone and Aragaki, 1973). Alternatively,
virgin soil (soil never planted to papaya) is placed into each planting hole in
a replant fi eld; this allows the seedling to become established before the roots
penetrate beyond the virgin soil and they are then less af ected by root rot
fungus. Development of cultivars with strong tolerance through breeding
is slow, with the cv. 'Waimanalo' being such a product. Seedling damping of
immediately after germination is caused by a complex of organisms. The use
of sterilized germination medium with good aeration and control of moisture
are preventative measures.
Fruit diseases generally occur postharvest. Incidence of some fruit diseases
can be minimized by fi eld sanitation and fi eld application of appropriate
fungicides, while others can be reduced by careful handling of fruit, as these
organisms infect through wounds. Anthracnose, a preharvest infection fruit
surface rot, and stem-end rot, a harvest wound rot, are the two most common
postharvest rots of papaya. At the early stage of the infection, anthracnose,
C. gloeosporioides , is manifested as tiny dark spots on the skin of fruit, and if
left untreated, they form larger, depressed lesions when the fruits start to ripen.
In more severe cases, especially when the fruits turn full colour, the lesions
may coalesce with each other and the pinkish spores of the fungi are evident
in the depressed lesions. The infected fruit becomes soft, dark coloured and
unattractive.
Bunchy or malformed top is a relatively new disease of papaya. The
disease is a complex involving both thrips and a normally saprophytic fungus
Cladosporium oxysporum . Severely infected plants remain stunted and are
slow to recover. Symptoms are typifi ed by the appearance of new fl ushes
of leaves that are malformed, with leaf spots and 'shot-holes' (ranging from
1 to 3 mm wide). Short, light-yellow streaks (due to feeding by thrips) are
usually interspersed with the spots. The 'Eksotika' and 'Solo' varieties are very
susceptible at the juvenile stage.
The papaya virus disease, papaya ringspot virus (PRSV) has become the
limiting factor for commercial production in many areas. This disease has
been referred to as papaya mosaic virus (PMV) and distortion ringspot virus
(DRV). The fact that PRSV is suspected to be a number of a dif erent viruses
is due to the variations in the expressed symptoms associated with dif erent
races. Papaya ringspot virus (PRSV) has severely restricted papaya production
in Thailand, the Philippines and Taiwan and has also been recorded as the
major papaya disease in India, Hawaii, the Caribbean, Malaysia and Vietnam