PAPAYA - Tropical Fruits - page 319

(Table 11.6). In Taiwan, papaya is restricted to annual rather than perennial
production and cultivation and is often carried out in large net-houses to keep
out the disease-carrying, non-persistent aphid vectors.
The symptom on the fruit is dark green, concentric rings, hence the name
for the disease. On the younger stem and on the petioles, elongated, green,
water-soaked streaks appear. Infected plants also show chlorosis in the young
leaves, and the upper portion of the crown appears prominently yellowed.
Fruits are also low in sugar content with poor fl avour.
There are no control methods once the plant is infected. Eradication,
sanitation methods and isolating the papaya-growing area by a papaya-free
buf er zone allow papaya to continue to be grown. An orchard or growing
area isolated far enough from disease sources remains uninfected because by
the time aphids can move from the virus source to the healthy orchard, they
are non-infective due to the non-persistent characteristic of the virus in the
aphid. Also, curcurbit plants should not be interplanted in papaya orchards as
they are alternate hosts to the virus. A cross-protection method using a mild
strain confers some immunization, enabling plants to be 82% more productive
than unprotected trees in fi eld trials. Breeding and selection has resulted
in development of tolerant varieties such as 'Carifl ora', 'Tainung No. 5',
'Sinta' and 'Eksotika'-derived breeding lines. The genetically modifi ed variety
'Rainbow' transformed with the virus coat protein gene has shown excellent
resistance to the disease in Hawaii. Transforming of papaya with the viral
coat protein has been successful; however, each virus race has a specifi c coat
protein, so dif erent coat protein genes are needed in dif erent areas.
INSECTS AND MITES
Many insects have been reported on papaya, but most are unimportant and
the damage is negligible or easily controlled. A few may cause severe problems
in localized areas (Table 11.7). Aphids, such as Aphis gossypii and Myzus
persicae , are important only as vectors of PRSV. Fruit fl ies are troublesome in
the export trade as papayas may need to receive a disinfestation treatment for
fruit fl y eggs and larvae before export. Rarely are fruit at the mature-green to
colour-turning stage infected; egg lay becomes a problem when fruits have
25% or more skin colour (Seo et al. , 1982).
Several species of mites are known to infest papaya (Table 11.7), the
commonest being the red spider mite ( Tetranychus cinnabarinus ) and the broad
mite ( Hemitarsonemus latus ). The mite population increases dramatically
during the dry and cool spells, and they can be located on the undersurface
of emerging leaves and petioles. In severe infestations, the leaves are shed
prematurely and the shoot terminals suf er die back. They also cause extensive
scarring (netted corky appearance) on the surface of the fruits when they feed
on the epidermal tissues.
Thrips ( Thrips parvispinus) are common on papaya fl owers during the
blooming season. However, during the early stages of plant growth, they feed