Papaya Cultivation; Chapter 4 - Disease & Pest

Papaya ring spot virus is a well-known virus within plants everywhere. The first signs of the virus are yellowing and vein-clearing of younger leaves as well as mottling yellow leaves. Infected leaves may obtain blisters, roughen or narrow, with blades sticking upwards from the middle of the leaves. The petioles and stems may develop dark green greasy streaks and in time become shorter. The ring spots are circular, C-shaped markings that are darker green than the fruit itself. In the later stages of the virus, the markings may become gray and crusty. Viral infections impact growth and reduce the fruit's quality. One of the biggest effects that viral infections have on papaya is the taste. Right now, the virus is uncontrollable.

The papaya mosaic virus is very destructive because it completely destroys the plant until only a small tuft of leaves are left. The virus affects both the leaves of the plant and also the fruit itself. Leaves show thin, irregular, dark-green lines around the borders and clear areas around the veins. The more severely affected leaves are irregular and linear in shape. The virus can infect the fruit at any stage of its maturity. Fruits as young as 2 weeks old have been spotted with dark-green ring spots about 1 inch in diameter.

Rings on the fruit are most likely seen on either the stem end or the blossom end. In the early stages of the ring spot, the rings tend to be many closed circles, but as the disease develops the rings will increase in diameter consisting of one large ring. The difference between the ring spot and the mosaic viruses is the ripe fruit in the ring spot has mottling of colors and mosaic does not. There are few more disease caused by Root rot virus, Collar rot virus, Cucumber mosaic virus etc.

Fungi Anthracnose is known to specifically attack papaya especially the mature fruits. The disease starts out small with very few signs, such as water-soaked spots on ripening fruits. The spots become sunken, turn brown or black and may get bigger. In some of the older spots, the fungus may produce pink spores. The fruit ends up being soft and having an off flavor because the fungus grows into the fruit. 

The fungus powdery mildew occurs as a superficial white presence on the surface of the leaf in which it is easily recognized. Tiny, light yellow spots begin on the lower surfaces of the leaf as the disease starts to make its way. The spots enlarge and white powdery growth appears on the leaves. The infection usually appears at the upper leaf surface as white fungal growth. Powdery Mildew is not as severe as other diseases. 

The fungus phytophthora blight causes damping-off, root rot, stem rot, stem girdling and fruit rot. Damping-off happens in very young plants by wilting and death in plant. The spots on established plants start out as water-soaked lesions at the fruit and branch scars. These spots can get bigger and cause the death of the plant. The roots can be severely and rapidly infected, causing the plant to rapidly brown and wilt away collapsing within days. The most dangerous feature of the disease is the infection of the fruit because it cause harm to people who consume it. The biggest evidence that the fungus is present are the water-soaked marks that appears first along with the white fungus that grows on the dead fruit. After the fruit dies it shrivels and falls to the ground.

Pests The papaya fruit fly is mainly yellow with black marks. The female papaya fruit fly has a very long, slender abdomen with an extended ovipositor that exceeds the length of its body. The male papaya fruit fly looks like the female with the differences of a hairy abdomen and no ovipositor. Long slender eggs are laid inside of the fruit by the female papaya fruit fly. The larva is white and looks very much like the regular fruit fly larvae. The female is capable of lying up to 100 or more eggs and are laid during the evening or early morning in groups of ten inside young fruit. They usually hatch within 12 days of being in the fruit where they’ll feed on the seeds and interior parts of the fruit. When the larvae mature (usually 16 days after being hatched) they eat their way out of the fruit, drop to the ground, and pupate just below the soil and emerge within one to two weeks as mature flies. The flesh of the papaya must be ripe in order for the fly to migrate towards the surface of the fruit because unripe papaya juice is fatal to them. The papaya will turn yellow and drop to the ground if it is infected by the papaya fruit fly. 

The two-spotted spider mite is a 0.5 mm long brown or orange-red but a green, greenish yellow translucent oval pest. They all have needle-like piercing-sucking mouthparts and feed by piercing the plant tissue with their mouth parts usually the underside of the plant. The spider mites spin fine threads of webbing on the host plant and when they remove the sap, the mesophyll tissue collapses and a small chlorotic spot forms at the feeding sites. The leaves of the papaya fruit turn yellow, gray or bronze. If the spider mites aren’t controlled it can cause the death of the fruit.

The papaya whitefly lays yellow oval eggs that appear dusted on the undersides of the leaves. They eat the papaya fruits leaves therefore damaging the fruit. There, the eggs developed into flies in three stages called instars. The first instar has well-developed legs and is the only mobile immature life stage. The crawlers insert their mouthparts in the lower surfaces of the leaf when they find it suitable and usually don’t move again in this stage. The next instars are flattened, oval and scale-like. In the final stage as the pupal the whiteflies are more convex, with large conspicuous red eyes.

(Click For Chapter - 5)