3 Pest control and decay
3.1 Pest species
3.2 Pest Control
A serious disease during the seedling stage is seedling rot or phytophthora causing soft rot and wilting of seedling, which also affects mature trees. It can be controlled by good drainage and sanitation control that is by removing all infected plants.
Anthracnose cause dark depressed lesions on ripe fruits, which then become soft, dark coloured and unattractive. (Fig. 37 Depressed lesion by anthracnose in maradol pawpaw).
Fig. 37 Depressed lesion by anthracanose in maradol pawpaw
It can be controlled by spraying Dithane M-45 (a broad spectrum fungicide that is composed by Ethylenebisdithiocarbamate 80 %, manganese 16 %, and Zinc 2 %) on the leaves and fruits every ten days. The fungus Collectotrichum gloeosperoides can be controlled by recommended fungicides. The root knot nematode affects the roots causing galls and damage to the root system.
The most serious of the pests is the fruit flies, which cause damage to the fruits. Cultural control is practised by removing all ripe and infested fruits. Damages caused by the leaf sucking pests such as aphids and mites can be controlled by recommended insecticides (Agrolink, 2002).
a) Pests
A major hazard to pawpaws in Florida and Venezuela is the wasp-like pawpaw fruit
fly, Toxotrypana curvicauda. The female deposits eggs in the fruit, which
will later be found infested with the larvae. Only thick-fleshed fruits are
safe from this enemy. Control on a commercial scale is very difficult. Home
gardeners often protect the fruit from attack by covering with paper bags, but
this must be done early, soon after the flower parts have fallen, and the bags
must be replaced every 10 days or 2 weeks as the fruits develop. Rolled newspaper
may be utilized instead of bags and is more economical. India has no fruit fly
with ovipositor long enough to lay eggs inside pawpaws.
An important and widespread pest is the pawpaw web-worm, or fruit cluster worm, Homolapalpia dalera, harbored between the main stem and the fruit and also between the fruits. It eats into the fruit and the stem and makes way for the entrance of anthracnose. Damage can be prevented if spraying is begun at the beginning of fruit set, or at least at the first sign of webs. The tiny pawpaw whitefly, Trialeuroides variabilis, is a sucking insect and it coats the leaves with honeydew which forms the basis for sooty mold development. Shaking young leaves will often reveal the presence of whiteflies. Spraying or dusting should begin when many adults are noticed. Hornworms (immature state of the sphinx moth– Erinnyis obscura in Jamaica, E. ello in Venezuela, E. alope in Florida) feed on the leaves, as do the small, light-green leafhoppers. Other pests requiring control measures in Australia include the red spider, or red spider mite, Tetranychus seximaculatus, which sucks the juice from the leaves. In India and on the island of Maui, plant and fruit infestation by red spider has been a major problem. This pest and the cucumber fly and fruit-spotting bugs feed on the very young fruits and cause them to drop. In Hawaii, the red-and-black-flat mite feeds on the stem and leaves and scars the fruit. The broad mite damages young plants especially during cool weather.
In the Virgin Islands scale has been most troublesome, apart from rats and fruit-bats that attack ripe fruits. In Australia, 5 species of scale insects have been found on pawpaws, the most serious being oriental scale, Aonidiella orientalis, which occurs on both the fruit and the stem. So far, it is confined to limited areas. In Florida, the scale insects Aspidiotus destructor and Coccus hesperidium may infest bagged fruit more than unbagged fruit. Another scale, Philaphedra sp., has recently been reported here.
Indian scientists have observed that immature earthworms, Megascolex insignis, are attracted by and feed on rotting tissue of pawpaw plants. They hasten the demise of plants afflicted with stem rot from Pythium aphanidermatum and may act as vectors for this fungus. Root-knot nematodes, Meloidogyne incognita acrita, and reniforin nematodes, Rotylenchulus reniformis, are detrimental to the growth and productivity of pawpaw plants and should be combatted by pre-planting soil fumigation if the nematode population is high.
b) Diseases
Hawaii, partly because of its distance from other pawpaw-growing areas, is less
afflicted with disease problems than Florida and Puerto Rico, but still has
to combat a number of major and minor maladies. Most serious of all is the mosaic
virus, on plant and fruit, which is common in Florida, Cuba, Puerto Rico, Trinidad,
and first seen in Hawaii in 1959. It is transmitted mechanically or by the green
peach aphid, Myzus persicae, and other aphids including the green citrus
aphid, Aphis spiraecola, in Puerto Rico. Two forms of mosaic virus are
reported in Puerto Rico: the long-known "southern coast pawpaw mosaic virus",
the symptoms of which include extreme leaf deformation, and the relatively recent
"Isabela mosaic virus" on the northern coast which is similar but without leaf
distortion. Both forms occur in some northcoast plantations. There is no remedy,
but measures to avoid spread include the destruction of affected plants, control
of aphids by pesticides, and elimination of all members of the Cucurbitaceae
from the vicinity. Mosaic is sporadic and scattered and not of great concern
in Queensland.
Pawpaw ringspot virus, prevalent in Florida, the Dominican Republic and Venezuela, is occasionally serious in the Waianae area on the dry leeward side of Oahu. It is transmited by the same vectors. Mosaic and ringspot viruses are the main limiting factors in pawpaw production in the Cauca Valley of Colombia.
In Florida, virus diseases were recognized as the greatest threat to the pawpaw industry in the early 1950's. The first signs are irregular mottling of young leaves, then yellowing with transparent areas (Fig. 38 Pawpaw "mancha anular virus" ("monkey hand" symptom), De Los Santos, 2000), leaf distortion, and rings on the fruit. If affected plants are not removed, the condition spreads throughout the plantation. Fruits borne 2 or 3 months after the first symptoms will have a disagreeable, bitter flavor.
Fig. 38 Paw paw "mancha anular" virus ("monkey hand" symptom)
At the Agricultural Research and Education Center of the University of Florida in Homestead, the late Dr. Robert Conover established a test plot of pawpaws grown from seed of 95 accessions from a number of countries and 94 collections in Florida in the hope of finding some virus-free strains. Most of the introductions were highly susceptible to pawpaw ringspot virus; local strains showed some resistance. Highest tolerance was shown by a dioecious, round-fruited, yellow-fleshed strain brought from Colombia by Dr. S.E. Malo several years ago. The fruits weigh 3-5 lbs (1.36-2.27 kg).
It is thought that at least 3 virus diseases are involved in pawpaw decline in East Africa and it has been suggested that the diseases are spread in part by the tapping of green fruits for their latex (the source of papain).
Bunchy top is a common, controllable mycoplasma disease transmitted by a leafhopper, Empoasca pawpawe in Puerto Rico, the Dominican Republic, Haiti, and Jamaica; by that species and E. dilitara in Cuba; and by E. stevensi in Trinidad. Bunchy top can be distinguished from boron deficiency by the fact that the tops of affected plants do not ooze latex when pricked. In the subtropical part of Queensland, but not in the tropical, wet climate of northern Queensland, pawpaw plants are subject to die-back, a malady of unknown origin, which begins with shortening of the petioles and bunching of inner crown leaves. Then the larger crown leaves quickly turn yellow. Affected plants can be cut back at the first sign of the disease and if the cut stem is covered to avoid rotting, the top will be replaced by healthy side branches. The problem occurs mainly in the hot, dry spring after a season of heavy rains.
Anthracnose, which usually attacks the ripe fruits and is caused by the fungus Colletotrichum gloeosporioides, was formerly the most important pawpaw disease in Hawaii, Mexico and India, but it is controllable by spraying every 10 days, or every week in hot, humid seasons, and hotwater treatment of harvested fruits. A strain of this fungus produces "chocolate spot" (small, angular, superficial lesions). A disease resembling anthracnose but which attacks pawpaws just beginning to ripen, was reported from the Philippines in 1974 and the causal agent was identified as Fusarium solani.
A major disease in wet weather is phytophthora blight. Phytophthora parasitica (Fig. 39 Pawpaw rotting stem by phytophthora sp. De los Santos et al., 2000) attacks and rots the stem and roots of the plant and infects and spoils the fruit surface and the stem-end, inducing fruit fall and mummification. Fungicidal sprays and removal of diseased plants and fruits will reduce the incidence. P. palmivora has been identified as the chief cause of root-rot in Hawaii and Costa Rica.
Fig. 39 Pawpaw rotting stem by phytophthora sp.
Root-rot by Pythium sp. is very damaging to pawpaws in Africa and India. P. ultimum causes trunk rot in Queenisland. Collar rot in 8- to 10-month old seedlings, evidenced by stunting, leaf-yellowing and shedding, and total loss of roots, was first observed in Hawaii in 1970, and was attributed to attack by Calonectria sp. Collar rot is sometimes so severe in India as to cause growers to abandon their plantations.
Powdery mildew, caused by Oidium caricae (the imperfect state of Erysiphe cruciferarum the source of mildew in the Cruciferae) often affects pawpaw plants in Hawaii and both plants and fruits elsewhere. Sulfur, judiciously applied, is an effective control. Powdery mildew is caused by Sphaerotheca humili in Queensland and by Ovulariopsis pawpaw in East Africa. Angular leaf spot, a form of powdery mildew, is linked in Queensland to the fungus Oidiopsis taurica. Corynespora leaf spot, or brown leaf spot, greasy spot or "pawpaw decline" (spotting of leaves and petioles and defoliation) in St. Croix, Puerto Rico, Florida and Queensland, is caused by Corynespora cassiicola, which is controllable with fungicides.
A new pawpaw disease, yellow strap leaf, similar to YSL of chrysanthemums, appeared in Florida during the summer in 1978 and 1979.
Black spot, resulting from infection by Cercospora pawpawe, has plagued Hawaiian growers since the winter of 1952-53. It causes defoliation, reduces yield, blemishes the fruit, and is unaffected by the hot-water dip. It can be prevented by field use of fungicides. Rhizopus oryzae is most commonly linked with rotting fruits on Pakistan markets. R. nigricans is the usual source of fruit rot in Queensland. Injured fruits are prone to fungal rotting caused by R. stolonifer and Phytophthora palmivora. Stem-end rot occurs when fruits are pulled, not cut, from the plant and the fungus, Ascochyta caricae, is permitted entrance. This fungus attacks very young and older fruits in Queensland and also causes trunk rot (Fig. 40 Fungus attack in maradol pawpaw, De los Santos et al., 2000). In South Africa, it affects the cultivar'Honey Gold ' which is also subject to spotting by Asperisporium caricae on the fruits and leaves. Both of these diseases are controllable by fungicidal sprays.
Fig. 40 Fungus attack in maradol pawpaw
Infection at the apex by Cladospoiium sp. is manifested by internal blight. A pre-harvest fruit rot caused by Phomopsis caricae pawpawe is troublesome in Queensland and was announced from India in 1971. A new disease, pawpaw apical necrosis, caused by a rhabdovirus, was reported in Florida in 1981.
Pawpaws are frequently blemished by a condition called "freckles", of unknown origin; and mysterious hard lumps of varying size and form may be found in ripe fruits. Star spot (grayish-white, star-shaped superficial markings) appears on immature fruits in Queensland after exposure to cold winter winds. In Uttar Pradesh, an algae, Cephaleuros mycoidea, often disfigures the fruit surface.
In Brazil, Hawaii and other areas, a fungus, Botryodiplodia theobromae, causes severe stem rot and fruit rot. Trichothecium rot (T. roseum) is evidenced by sunken spots soon covered by pink mold on fruits in India. Charcoal rot, Macrophomina phaseoli, is reported in Pakistan. Young pawpaw seedlings are highly susceptible to damping-off, a disease caused by soil-borne fungi– Pythium aphanidermatum, P. ultimum, Phytophthorap palmivora, and Rhizoctonia sp.,–especially in warm, humid weather. Pre-planting treatment of the soil is the only means of prevention.
Pawpaws generally do poorly on land previously planted with pawpaws and this is usually the result of soil infestation by Pythium aphanidernwtum and Phytophthora palmivora. Plant refuse from previous plantings should never be incorporated into the soil. Soil fumigation is necessary before replanting pawpaws in the same field (Morton, 1987).
Pests and diseases: Thrips, mites and white flies as well as In red spider and fruit spotting bugs are potential problems in some areas. The plants may also be attacked by mildew, anthracnose (Fig 36 anthracnose in maradol pawpaw), root rot and various virus diseases Fruit flies often ruin the fruit in Florida and Hawaii. Nematodes can attack the roots and are often a factor in the decline of individual plant. Gopher damage can be avoided by planting in wire baskets. Pawpaw plants should probably be replaced every 4 years or so (CRFG, 1998).
c) Insects, Diseases, And Nematodes
There are three insect pests of pawpaw which can occasionally be damaging in
Florida. In order of importance they are: 1) pawpaw fruit fly (Toxotrypana
curvicauda), the larvae (maggots) feed on the seeds and interior tissues
of the fruit. 2) pawpaw webworm (Homolapalpia dalera), the worms produce
a web around the fruit and stem and feed on the tissue under the web. 3) pawpaw
white fly (Trialeuroides variabilis), adults and immature stages of this
sucking insect feed on the underside of the younger leaves. Sooty mold, a black
mildew, which grows on the excretions of the whitefly, is often associated with
infestations.
The major fungus diseases in Florida are: 1) Anthracnose (Colletotrichum gleosporiodes), together with other fungus rots (Table 8), may cause considerable spoilage of the fruit, particularly during rainy weather. 2) Powdery mildew (Oidium spp.) is recognized by the whitish mildew growth on leaves and on stems of seedlings and can be troublesome during the winter and spring. For more information and current control recommendations, please contact your local County Agricultural Extension Agent.
Nematodes, such as root-knot, can be very damaging to pawpaws, particularly in sandy soils (Fig 41 Pawpaw "Mayate prieto" worm, De Los Santos et al., 2000). Since effective chemical control is difficult and very costly, the homeowner should try preventive measures such as rotating planting sites, or maintaining plants in the most vigorous condition possible. Mulching helps plants withstand nematode attack under some conditions (Malo, 2001).
Fig. 41 Pawpaw "Mayate prieto" worm
a) Control
Remove infected plants as soon as these are detected. Regular inspection and
roguing can contain the disease in many instances.
A quarantine area was proclaimed in 1991 in an effort to restrict the movement of pawpaw ringspot. Pawpaw plants and cucurbit seedlings are not able to be moved out of south-east Queensland without written approval of an inspector.
Within south-east Queensland do not move seedling pawpaw or cucurbit plants from a property where pawpaw ringspot is known to occur (Persley, 2002).
Table 8 Diseases, symptoms and control measures in pawpaw fruits.
b) Physiological and Physical Disorders
Skin abrasions result in blotchy coloration such as green islands (areas
of skin that remain green and sunken when the fruit is fully-ripe) and accelerate
water loss. Abrasion and puncture injuries are more important than impact injury
for pawpaw.
Chilling injury symptoms include pitting, blotchy coloration, uneven ripening, skin scald, hard core (hard areas in the flesh around the vascular bundles), water soaking of tissues, and increased susceptibility to decay. Increased alternaria rot was observed in mature-green pawpaws kept for 4 days at 2°C, 6 days at 5°C, 10 days at 7.5°C, or 14 days at 10°C. Susceptibility to chilling injury varies among cultivars and is greater in mature- green than ripe pawpaws (10 vs. 17 days at 2°C; 20 vs. 26 days at 7.5°C).
Heat injury Exposure of pawpaws to temperatures above 30°C (86°F) for longer than 10 days or to temperature-time combinations beyond those needed for decay and/or insect control result in heat injury (uneven ripening, blotchy ripening, poor colour, abnormal softening, surface pitting, accelerated decay). Quick cooling to 13°C (55°F) after heat treatments minimizes heat injury (Kader, 2000).