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Diseases
The diseases of papaya include those caused by viruses and fungi. Papaya grow-
ing areas have to combat a number of major and minor diseases, and the extent of
damage depends upon agroecological regions. Fungal diseases are a major problem
in papaya production. The common strategy for fungal disease management is pre-
vention through the application of fungicides at regular intervals. Proper timing,
deposition and coverage of the fungicide application are critical for the effective
control. Once the disease is established, the fungicides have little impact on control.
Viral diseases have been recognised as the greatest threat to the papaya industry. It
is thought that at least two viral diseases are involved in papaya and it has been sug-
gested that the diseases are spread in part by the tapping of green fruits for their latex
(the source of papain). Most serious of all is the papaya ringspot virus (PRSV) which
affects both plant and fruits.
10.1 Papaya Ringspot Virus
The term papaya ringspot (PRS) was first coined by Jensen in 1949 to describe a
papaya disease in Hawaii. The inevitable entry of Papaya ringspot virus (PRSV)
into the Puna district on Hawaii Island was discovered during the first week of May
in a papaya field in Pahoa, 1–3 miles from the major papaya growing areas in Puna
(Hawaii). Diseases such as papaya mosaic (caused by Papaya mosaic virus (PapMV))
and watermelon mosaic (caused by Watermelon mosaic virus-1) were shown recently
to be caused by PRSV. The PRSV causes a major disease of papaya and cucurbits
and is found in all areas of the world where papaya and cucurbits are cultivated.
The primary host range of PRSV is limited to papaya (Caricaceae) and cucurbits
(Cucurbitaceae), with Chenopium amaranticolor and C. quinoa (Chenopodiaceae)
serving as local lesion hosts. The virus is grouped into the papaya infecting biotype
(PRSV-p) which affects papaya and/or cucurbits, and the cucurbit infecting biotype
(PRSV-w) which affects cucurbits only. PRSV belongs to family Potyviridae, genus
potyvirus and is an aphid-transmissible RNA virus that commonly infects papaya,
causing serious disease and economic loss. Virions of PRSV are rod-shaped and
flexuous measuring 760–800 × 12 nm with a monopartite single-stranded positive
sense RNA as its genome. Like other potyviruses, PRSV is transmitted in a non-
persistent manner by several species of aphids. Genetically engineered papaya has
been used to successfully control the disease caused by PRSV in Hawaii. The virus,
PRSV, is transmitted non-persistently by aphid vectors and does not multiply in the
vector. The disease cycle can start with aphids feeding on infected papaya for as little
as 15 s and subsequently feeding on a healthy papaya. There is no incubation period.
The virus does not persist in the vector so transmission to another plant has to occur
rather rapidly (Gonsalves et al. 2010).