Common papaya diseases and pests plus how farmers can arrest spread

Papaya, also known as pawpaw, is an important agricultural export. Below, we look at some of the diseases and pests to watch out for

1. Papaya Anthracnose
Cause: Fungus (Colletotrichum gleosporoides)

Symptoms: Small water-soaked lesions on the fruit during ripening later becoming circular sunken lesions with light brown margins.

The fungus is spread by wind and rain while disease emergence is favoured by high temperature and humidity. The disease can have a serious impact on refrigerated fruit for export.

Management: Appropriate protective fungicides applied as a preventive measure are the best for managing anthracnose.

Benomyl or thiobendazole are amongst the more important fungicides used to reduce anthracnose of papaya.

These fungicides are used with or without hot water treatment after harvest. Dipping fruits in hot water at 48°C for 20 minutes reduces the incidence of the disease after harvesting.

Planting papaya as a multi-crop that is interspersed with non-hosts of C. gloeosporiodes such as citrus and coffee can help to minimise anthracnose disease incidence and severity.

Such planting configurations interfere with splash dispersal of the pathogen spores and thereby reduce the population size of the pathogen.

2. Black spot

Cause: Fungus (Asperisporium caricae)

Symptoms: Circular water-soaked or brown lesions on older leaves; centres of lesions become bleached as they mature; leaves curl and turn brown; raised lesions on trunks; sunken circular lesions on fruit.

Disease is spread by wind and rain and its emergence is favoured by cool weather interspersed with moisture from dew or rain.

Management: This is a minor disease for which control measures are seldom warranted, apart from general sanitary measures such as removal and destruction of disease crop debris.

Spraying with foliar protectant fungicides such as dithiocarbamates is effective.

3. Black rot

Cause: Fungus (Mycosphaerella caricae)

Symptoms: Black sunken rot on young fruits originating from stem end or contact with a leaf; young fruit withering and dropping from plant; small, brown sunken lesions with light brown margins on ripening fruit.

Fungus enters fruit through wounds.

Management: Protective fungicides similar to those used for black spot should be applied. Dipping fruits in hot water at 48°C for 20 minutes reduces the incidence of the disease.

4. Phytophthora fruit and Stem rot

Cause: Fungus (Phytophthora palmivora)
Symptoms: Water-soaked lesions on unripe fruit that oozes latex; withering fruit; water-soaked lesions on leaf scars of fruit bearing stem; mature fruit covered in white mycelium.

Fungus survives in soil and enters through wounds in the stem. The disease often emerges after strong wind damage.

Management: Use of appropriate protective fungicides such as mancozeb or copper sulfate.

Root rot in seedlings can be prevented by planting in holes filled with soil in which papaya has never been grown — by the time the roots extend out of the added soil the plant is no longer susceptible to the disease.

Preventive spraying against this disease is done about every 2 weeks in wet locations or seasons. In drier locations, the preventive sprays such as mancozeb or copper may be applied every 3 weeks, or less often.

Once Phytophthora appears in a field, the disease can be a major concern due to its ability to spread among plants and destroy fruits rapidly during windy, rainy periods.

In that case, curative, systemic metalaxyl fungicides may be used, such as Ridomil Gold Copper.

5. Bacterial canker and decline.

Cause: Bacteria (Erwinia spp)

Symptoms: Angular water-soaked lesions on leaves; lesions coalesce and spread along leaf veins; witling leaves, particularly at top of canopy; water-soaked lesion and cankers on stem; cankers girdle stem and cause plant to collapse; small water-soaked lesions on green fruit.

Bacteria survive in lesions and cankers.

Management: Copper has been widely proposed as offering some level of control of this disease.

However, its use is controversial. Some experts swear that it helps and others don’t agree at all. With wide spread infections, the recommendation is to remove all affected plants.

6. Bunchy top

Cause: Likely caused by Rickettsia bacteria transmitted by leaf hoppers.

Symptoms: Chlorosis of young leaves; water-soaked spots on petioles and stems; petioles rigid, horizontal and shortened; thickened leaf blades that cup downward; internodes shorten and growth stops resulting in a bunchy appearance to the plant.

Management: Use of tolerant varieties of papaya is currently the only method of control recommended but it has limited application.

Application of insecticide to reduce the incidence of leafhopper vectors may be beneficial.

7. Cercospora black spot

Cause: Fungus (Cercospora papayae)

Symptoms: Tiny black dots on fruit which enlarge to 3mm across; spots are slightly raised and although indistinct on unripe green fruit, become visible on ripening to yellow; lesions on leaves are irregular in shape and gray-white in colour; if infestation is severe, leaves may turn yellow and necrotic and drop from plant.

Disease usually enters orchard from infected papaya leaves in adjacent orchards.

Management: Applications of appropriate protective fungicides (mancozeb) at intervals of 14 to 28 days provide satisfactory control of the disease.

8. Papaya ring spot virus (PRSV)

Cause: Virus - transmitted by several aphid species.

Symptoms: Dark green rings on fruit which may be slightly sunken and become less distinct as the fruit ripens; fruits may have uneven bumps; leaves often exhibit a bright yellow mosaic pattern and new leaves are small and plant growth is stunted.

Management: Infected plants should be removed and destroyed to prevent spread of the virus; new planting materials should be free of the virus; intercropping papaya with a non-host such as corn can help to reduce the incidence of the disease in papaya orchards by providing aphid vectors with an alternative feeding site.

9. Papaya mealybug

Cause: Insects (Paracoccus marginatus) that have a wide host range; often tended by ants which farm them for their sugary honeydew secretions; transmit Cocoa swollen shoot virus.

Symptoms: Flattened oval to round disc-like insect covered in cottony substance on tree; chlorosis; plant stunting; leaf deformation; early leaf and fruit drop insects attract ants which may also be present; insect colony may also be associated with growth of sooty mold due to fungal colonisation of sugary honeydew excreted by the insect.

Management: Mealybugs can potentially be controlled by natural enemies such as lady beetles but are commonly controlled using chemicals.

Chemical pesticides may decrease populations of natural enemies leading to mealybug outbreaks.

Use of botanical insecticides such as neem oil (1 to 2%) or Fish Oil Rosin Soap (25g/litre of water) should be the first choice.

Chemical control is only partially effective and requires multiple applications. Application of chemical insecticides should be done as the last resort.

Recommended chemicals include profenophos 50 EC, chlorpyriphos 20 EC, buprofezin 25 EC, thiomethoxam 25 WG and imidacloprid 17.8 SL.

10. Scale insects (White peach scale)

Cause: Insect (Pseudaulacaspis pentagona)

Winged adult scale insects males mate with females which retain their eggs inside the body until they hatch.

Symptoms: Scale insects cause damage by feeding on twigs, branches and fruit, injecting toxins into the plant.

If the infestation is heavy, gumming may occur on the bark and twigs or entire branches can be killed; insects are flattened discs, or “scales” with no visible legs; scales produce a white waxy coating which eventually turns black (black cap stage).

Management: Populations are often kept in check by natural enemies, including predacious beetles and some wasps — although broad-spectrum insecticides may result in outbreaks of scale by killing off populations of beneficial insects.

Trees can be sprayed with horticultural oils which effectively kill scales without damaging natural enemies.

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A pictorial book on these papaya problems will be available @ www.egerton.ac.ke in October 2016

Prof Richard Mulwa is a Horticulture and Biotechnology Specialist at Egerton University.