112 Papaya
10.1.1 Symptoms
The earliest symptoms on papaya are yellowing and vein-clearing of the young
leaves. In papaya, leaves develop prominent mosaic and chlorosis on the leaf lamina,
and water-soaked oily streaks on the petioles and upper part of the trunk. Severe
symptoms often include a distortion of young leaves which also result in the devel-
opment of a shoestring appearance that resembles mite damage (Figure 10.1). Trees
that are infected at a young stage remain stunted and will not produce an economical
crop. Fruits from infected trees may have bumps similar to that observed on fruit
of plants with boron deficiency and often have ‘ring spots’, which is the basis for
the disease’s common name. A severe PRSV isolate from Taiwan is also known to
induce systemic necrosis and wilting along with mosaic and chlorosis (Gonsalves
1998). The disease derives its name from the striking symptoms that develop on
fruit. These consist of concentric rings and spots or C-shaped markings, a darker
green than the background-green fruit colour. Symptoms persist on the ripe fruit as
darker orange-brown rings. Vigour of trees and fruit set is usually reduced depend-
ing on the age of the plant when infected. Fruit quality, particularly flavour, is also
adversely affected. If affected plants are not removed, the condition spreads through-
out the plantation. Fruits borne 2 or 3 months after the first symptoms have a dis-
agreeable or bitter flavour in taste.
Papaya mosaic symptoms were first reported in 1962 together with another virus
that became known as PRSV, a potyvirus. The two viruses were differentiated in
1965 and 1967 using particle lengths, serology, host range, inclusion bodies and
aphid transmissibility. PapMV causes mild mosaic symptoms on leaves and stunting
of the plant. Presently, it is also known as PRSV. Recently, both the viruses were
reported at molecular level to be the same as potyvirus.
A mosaic symptom is more serious on young plants. The disease symptoms appear
on the top young leaves of the plants. The leaves are reduced in size and show blister
like patches of dark-green tissue, alternating with yellowish-green lamina (Figure
10.2). The leaf petiole is reduced in length and the top leaves assume an upright
position. The infected plants show a marked reduction in growth (Noa-Carrazana
et al. 2006). The fruits borne on diseased plants develop water soaked lesions with a
central solid spot. Infected fruits are elongated and reduced in size.
FIGURE 10.1 Symptoms of PRSV on papaya leaves and fruits.