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Introduction
- Plant diseases comprise a major factor in the culture
of papaya.
- Adequate control measures are necessary to intensify the production
of papaya.
- There are over 17 diseases caused by fungi, viruses, mycoplasma
and nematodes, which affect papaya plant all over the world.
- Of these about 5-6 diseases like collar rot, damping off,
anthracnose, mosaic and leaf curl are serious problems in
many papaya growing areas of the world.
- They can reduce yield greatly and impair marketability of
fruits.
- They must be checked and losses due to them must be minimized
in order to increase the economics of papaya production.
Diseases
- Stem rot - Pythium aphanidermatum and Rhizoctonia solani
- Powdery mildew - Oidium caricae
- Anthracnose - Colletotrichum papayae
- Leaf spots - Phyllosticta sulata Cercospora papayae
- Fruit rots - Many fungi
- Mosaic - Papaya mosaic virus
- Leaf curl - Nicotiana virus 10
- Minor diseases - Papaya ring spot virus
Minor diseases
- Damping off - Pythium spp.
- Blight - Phytophthora nicotianae var. parasitica
- Papaya leaf reduction - Papaya leaf reduction virus
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Anthracnose
- The disease is prevalent wherever the papaya is grown
and becomes more prominent after reaching the consumer.
Symptoms
- The spots on fruits first appear as brown superficial
discolouration of the skin which develop into circular, slightly
sunken areas and 1 to 3 cm in diameter.
- Gradually the lesions coalesce and sparse mycelial growth
appears on the margins of the spots.
- Under humid conditions, an encrustation of salmon pink spores
are released.
- Infection at early stages of fruit results in mummification
and deformation.
- Necrotic spots are produced on the leaves and stems.
- Errupted acervuli in concentric rings are seen on the petioles.
- Although the disease usually appears on the ripening portion
of the fruit, they occasionally infect its green portions
first causing small lesions.
- Soon after penetration of the fungus, the latex from the fruit
oozes out in sticky mounds of horns.
- Lesions enlarge very slowly and rarely larger than 12 mm in
diameter as long as the fruit remains green.
Fungi
- Gloesoporium papayae P. Henn. and Colletotrichum papayae
(Syn. Colletotrichum gleosporioides Penz.).
Mode of spread and survival
- The incipient infection is carried by the fruits from
the field.
- The disease is spread through wind-borne conidia.
- Conidia are also spread by rain splashes.
Epidemiology
- Severity of the disease on foliage is increased under
conditions of excessive moisture.
- Old leaves are generally more susceptible.
- Lesions develop more slowly on the immature fruits than on
the mature fruits.
Management
- Spray any one of the following Carbendazim 1g/litre at
45 days interval.
- Chlorothalonil 2 g/litre at 10 to 15 days interval Mancozeb
2 g/litre at 10 days interval.
- Two sprays of systemic fungicides or four sprays of non-systemic
fungicides controls the disease.
- Fumigation with benzylisothiocynanate controls post harvest
spots and rots.
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Fruit rots
Rhizopus stolonifer (Fr.) Lind
- It occurs in different parts of India.
- The rot is characterized by production of irregular, water
- soaked lesions.
- They gradually enlarge and are covered with white and dark
brown fungal growth and sporangiophores.
- Finally the fruit collapses, becomes watery and emit a foul
odour.
- The fungus infects wounds Fruit fly punctures after harvest
increase the disease incidence.
- The rot rapidly destroys the entire fruit and quickly spreads
to other fruits.
- To control the disease dip the fruits with DCNA (2,6-dichloro
- 4 nitroaniline) at 1,000-2,000 ppm.
- Hot water treatment of fruits at 49oC for 20 min checks the
rot.
Ascochyta caricae Pat
- The fungus causes both fruit and trunk rot.
- It attacks half-grown or mature fruits which show small, circular,
water-soaked spots.
- The lesions become sunken and brownish-black.
- The rot usually spreads outwards irregularly.
- Rotting is severe at 30oC and 100 per cent humidity.
- Spray the fruits with Bordeaux mixture @ 5 g in 1 litre of
water at intervals of 21 to 30 days from the time of setting
to control the disease incidence.
- Submerge the fruits in water at 48 to 49oC for 20 min for
effective control.
Botryodiplodia theobromae Pat
- It incites stem-end rot and a surface fruit rot.
- It usually induces a wider and softer water-soaked margin
and greater internal discolouration.
- Sporulating lesions are black and have a rough surface because
of erumpent, confluent pycnidia.
- The disease development is more rapid on ripe and half ripe
fruits than on green fruits.
- The rot begins as dark green, water-soaked spots.
- Later, the affected portion becomes shrivelled and turns dark
brown.
- The spots are surrounded by a dark green water soaked area.
- Infection of fruit stalk results in fruit drop.
Fusarium solani (Mart.) Sacc.
- Causes dry fruit rot reported from West Bengal.
- The affected fruit shows small, stem-end or surface lesions.
- Later, profuse white hyphae are seen on the rotted area.
- Hot water treatment of fruits at 49oC of 20 min controls the
disease.
Alternaria alternata (Fr.) Keissler
- It incites a rot which has been reported from Bihar.
- It is characterised by grey-brown, circular to semi-circular
patches covered with mycelium and conidiophores.
Phomopsis caricae-papayae Petrak and Cif
- This disease fungus was reported from Madhya Pradesh.
- Affected fruit develops a water-soaked spot which increases
in size.
- The whole area becomes soft and pulpy.
- The rotten area turns dark brown to black and get depressesd
and cracks at a later stage.
Macrophomina phaseolina (Tassi) Grid
- It was reported from Punjab.
- The rot is responsible for 5 to 20 per cent losses during
summer.
- The disease is characterised by small, water-soaked spots
which appear as circular specks on the fruit surface.
- The spots rapidly enlarge and become sunken
- The pathogen advances deep into the fruit and causes rotting
and disintegration.
- Affected fruit pulp turn brown to black.
- The attacked tissues harden and become dotted with sclerotia
of the pathogen.
- Maximum decay of fruits is noticed at 30oC with 100 per cent
relative humidity.
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Leaf Curl
Symptoms
- The disease is characterised by severe curling, crinkling
and distortion of the leaves accompanied by vein clearing
and reduction of leaf lamina.
- The leaf margins are rolled downward and inward in the form
of inverted cap.
- The veins get thickened and turn dark green.
- The leaves become leathery and brittle and petioles are twisted.
- Affected plants bear only a few flowers and fruits.
- In advanced stages, defoliation takes place and growth of
the tree is stunted.
Causal agent
- Tobacco leaf curl virus, Nicotiana virus 10.
Mode of spread and survival
- The virus infects tobacco, tomato, sunnhemp, cape gooseberry,
chilli petunia, hollyhock, Zinnia, Datura stramonium L. and
several other weeds and ornamental plants.
- The disease is transmitted by grafting and white fly, Bemisia
tabaci. It is not sap transmissible.
Management
- Destroy the infected plants even in the nursery.
- In the orchard, rougue and destroy the affected plants.
- In addition, spray with monocrotophos @ 5 ml in 10 litre of
water or dimethoate @ 3 ml in 10 litre of water to control
the insect vector and reduce the disease in the field.
- Further, the diseased plants in the crops of tomato and tobacco
growing in the vicinity of papaya plantation helps to check
the disease under control.
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Leaf Spots
Phyllosticta sulata Chowdhury
- The disease occurs in Assam and it is prevalent during
Sepember-January.
- The leaf spots are variable in size.
- Some are small and round 1.0 mm in diameter and others are
irregular, oval or elongated having a size of 3 to 15 mm in
length and 2 to 11 mm in breadth.
- The spots are almost white in the centre and often bounded
by a yellowish or brown colour margin which gradually merges
into the normal green colour of the leaf.
- The central portion of the leaf spot is thin and papery and
ultimately becomes brittle and falls.
- The disease spreads through wind - borne spores.
- The disease is prevalent during September-January.
- The disease is controlled by three or four spraying of Bordeaux
mixture 1.0 per cent at monthly intervals.
Cercospora papayae Hansford
- The spots are sub - circular to irregular, 3 to 8 mm in
diameter.
- It is ash coloured on the upper surface, immarginate and indistinct
in lower surface.
- The fungus produces fasicles which are 8 to 10 stalked.
- Conidiophores are medium brown to paler, more narrower towards
the tip, multiseptate, not branched and geniculate and 3.5
to 6.0 x 50 to 200 um.
- Conidia are hyaline, acicular, variously curved, multiseptate,
large truncate, tip sub-acute and shortest one is almost cylindric
and 20 to 75 x 3 to 5 um.
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Mosaic
- It is noticed in Bolivia, India, Peru, the U.S.A. and
Venezuela.
- The mosaic disease was first reported in India from Mumbai
and Pune in 1947.
- It is prevalent in Andhra Pradesh, Bihar, Karnataka Madhya
Pradesh, Maharashtra, Orissa, Punjab, Rajasthan, Utter Pradesh
and West-Bengal.
- This disease may cause as much as 90 per cent damage in severe
cases.
Symptoms
- The disease produces typical mosaic symptoms showing chlorosis
with dark green blisters on the leaves.
- The lamina is reduced and malformed.
- The stem, petiole and fruits develop enlongated water-soaked
areas showing concentric or circular rings or lesions.
- The diseased plants show decline and marked reduction in growth.
- As the disease advances, older leaves fall down and a small
tuft of younger leaves is left at the top which are also malformed
and have typical mosaic symptoms.
- Diseased plants of fruits develop innumerable circular, water-soaked
lesions with spots in the centre.
- In severe cases, the fruit size is severely reduced with deformed
shape.
- There is no reduction in the flow of latex.
Fungus
- Oidium caricae Noack. It is an obligated parasite.
- The mycelium is hyaline, septate and haustoria develop in
epidermal cells.
- Conidia are hyaline, granular and 14 to 19 x 28 to 30 um.
Mode of spread and survival
- The fungus spreads through wind-borne conidia.
Epidemiology
- Maximum disease incidence is recorded during September-November
with a peak in October.
- The disease is more severe when the atmospheric temperature
was between 16.4 and 22.90C, relative humidity between 65
and 86 per cent with 6.2 h of sunshine.
- The relative humidity and sunshine at 14.30 h, the most important
factors for disease development.
Management
- Spray or apply any one of the following chemicals:
- Spray wettable sulphur 1 g/litre per cent at 10 days interval.
- Apply systemic fungicides like:
- Bayleton 1 g/1 litre (or)
- Carbendazim 1 g/litre at monthly interval
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Powdery Mildew
- The disease is prevalent in Delhi, Karnataka and Maharashtra
Symptoms
- Diffuse mats of external, white mycelium develop on both
the leaf surfaces but are more common on the underside of
the leaves.
- Lesioned areas become chlorotic and sometimes are surrounded
by a dark margin.
- Production of conidia causes mycelial mats to appear powdery.
- Flower stalks and fruits are also affected.
- The fungus also attacks the stem of the young seedlings when
grown under reduced light conditions.
- The typical powdery growth is found on the stem of the plant.
- Severe attack leads to death of top portion of the seedlings
Fungus
- Oidium caricae Noack. It is an obligated parasite.
- The mycelium is hyaline, septate and haustoria develop in
epidermal cells.
- Conidia are hyaline, granular and 14 to 19 x 28 to 30 um.
Mode of spread and survival
- The fungus spreads through wind-borne conidia.
Epidemiology
- Maximum disease incidence is recorded during September-November
with a peak in October.
- The disease is more severe when the atmospheric temperature
was between 16.4 and 22.90C, relative humidity between 65
and 86 per cent with 6.2 h of sunshine.
- The relative humidity and sunshine at 14.30 h, the most important
factors for disease development.
Management
- Spray or apply any one of the following chemicals:
- Spray wettable sulphur 1 g/litre per cent at 10 days interval.
- Apply systemic fungicides like:
- Bayleton 1 g/1 litre (or)
- Carbendazim 1 g/litre at monthly interval
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Ring Spot
- This virus disease was first reported in 1949.
- It occurs in China, France, Germany, India, Italy, Maxico,
Taiwan and the U.S.A.
Symptoms
- The disease is characterized by vein clearing, puckering
or bulging of the leaf tissues between the secondary veins
and veinlets on the upper surface of the terminal leaves.
- The margins and distal parts of young leaves roll downwards
and inwards.
- The virus induces mosaic mottling, dark green blisters, necrosis
of chlorotic areas, leaf distortion which result in shoe-string
symptoms and stunting of the plants.
- On the stem of young plants, mosaic or mottle symptoms also
show dark green spots and oily or water-soaked streaks.
- The fruits are smaller, deeply lobed and lopsided and show
circular and concentric rings.
- Diseased fruits contain low sugar content. Latex quality from
diseased plants is poor.
Causal agent
- Papaya Ring Spot Virus (PRSV) (Syn. Papaya distortion
ring spot virus).
- The virus particles are rod shaped with 760 to 800 nm in length
and 12 nm in width.
- Thermal inactivation point of the virus lies between 54 and
60oC and it loses its infectivity at 10-3 dilution.
- The virus is viable upto 10 hours at 27 to 30oC.
Mode of spread and survival
- The virus is sap transmissible.
- Among insect vectors Aphis gossypii and Myzus persicae are
the most effective vectors.
- It is also transmitted by Aphis citricola, Aphis craccivora
and Rhopalosiphum maidis.
- The virus is transmitted in a non-persistent manner.
- It is neither soil-borne nor seed-borne.
Management
- Rougue and destroy the disease affected plants.
- Spray with insecticides like monocrotophos 5 g/litre to control
the insect vector effectively.
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Stem rot/Foot rot
- The disease is widespread in Africa, Hawaii, India and
Sri Lanka.
- In India, it usually appears from June-August.
- This is a very serious disease prevalent throughout the country
and may lead to complete failure of the crop if it appears
in the early stages of growth.
Symptoms
- Water soaked patches appear on the stem at the ground
level.
- These patches enlarge and girdle the base of the stem.
- The diseased tissues turn dark brown or black and rot.
- The terminal leaves turn yellow, droop and wilt.
- Fruits shrivelled and drop off.
- Due to disintegration of parenchymatous tissues at the bse
of the stem the entire plant topples over and dies.
- If the back is opened the internal tissues appear dry and
give a honey comb appearance.
- Rotting may spread above and below on the stem and down to
the roots.
- The roots deteriorate and may be destroyed.
Fungi
- Pythium aphanidermatum (Eds.) Fitz. and Rhizoctonia solani
Kuhn
Mode of spread and survival
- The fungi survive in the form of oospore or sclerotia
in the soil.
- The seedlings raised in the infected soil carry the disease
to the field.
Epidemiology
- One week old seedlings are more susceptible than one year
old trees.
- Stem rot caused by Pythium aphanidermatum is commonly noticed
in 2 to 3 year old trees.
- This fungal disease usually appears during rainy season and
severity increases with the intensity of rainfall.
- Optimum temperature of 36oC is favourble for disease development.
- Rhizoctonia solani is severe during dry and hot weather.
Management
- Seed treatment with captan at 4g/kg or chlorothalonil
reduces the damping off caused by Rhizoctonia solani.
- Raise the seedlings on well drained nursery area.
- Uproot and burn the diseased seedlings carefully.
- In the field, remove the diseased plants and do not use the
same pit for replanting.
- Drench the base of the stem with any one of the following:
i Bordeaux mixture 2 g/lit.
ii Captan 2 g/lit.
iii Copper oxychloride 2.5 g/lit.
iv Metalaxyl 1 g/lit.
v Tridemorph 1 1/2 g/lit.
vi Chlorothalonil 2 g/lit.
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