Diseases of fruit plantation medicinal and aromatic crops (2+1)

Papaya ring spot virus

Causal organism: Papaya ring spot virus
Symptoms

• Yellowing and vein-clearing of young leaves are the earliest symptoms on papaya.
• It is followed by a very conspicuous yellow mottling of leaves and sometimes severe blistering and leaf distortion. Dark-green streaks and rings also appear in the leafstalks and stems.
• The disease derives its name from the striking symptoms that develop on fruits. It consists of concentric rings and spots or C-shaped markings, a darker green than the background-green fruit colour.
• Symptoms persist on the ripe fruit as dark orange-brown rings.
• Vigour of trees and fruit set is usually reduced depending on the age of the plant when infected. Fruit quality, particularly flavour is also adversely affected.
• Other key symptoms are the intense yellow mosaic on leaf lamina and numerous "oily" streaks on petioles.
• The leaf canopy becomes smaller as the disease progresses due to development of smaller leaves and stunting of the plant.
• Fruit yield and levels are markedly lower than fruit from healthy plants. Leaf and fruit symptoms are most intense during the cool season.
• Leaves often develop a shoe-string appearance caused by the extreme reduction of leaf lamina similar to that caused by broad mites.
• Papaya trees of all ages are susceptible and generally will show symptoms two to three weeks after inoculation.
• Trees infected at a very young stage never produce fruit but rarely die because of the disease.
• There are, however, some strains which cause wilting and sometimes death of young trees.


Spread

 
• The virus transmitted from plant to plant by any species of aphids, in non-persistant manner.
• Many species of aphids are capable of transmitting this virus and it takes only a few seconds of feeding time for an aphid to acquire the virus onto its mouthparts.
• It then spreads the virus to other plants during brief feeding probes.
• Papaya ring spot virus is not spread by other insects and it does not survive in soil or dead plant material.
• The virus can also be spread by the movement of infected papaya plants and cucurbit seedlings.
  
• Once infected, plants cannot be cured by spraying pesticides or removing affected plant parts.
 
Biology

These viruses typically have long flexuous, rod-shaped particles about 800-900 nm long and are transmitted by different species of aphids in a non persistent manner. Papaya ring spot virus is grouped into two types, PRV-p and PRV-w. The former type infects both papaya and cucurbits while the latter type infects cucurbits only but not papaya. In fact, PRV-w causes major damage to cucurbits and was previously called as watermelon mosaic virus I. Both the types cause local lesions on Chenopodium quinoa and C. amaranticolor.

Epidemiology

• Secondary infection spreads rapidly and an orchard can become totally infected in three to four months.
• This situation occurs in young orchards located close to infected plants and when the populations of winged aphid flights are high.
• Papaya ring spot virus can be rapidly spread by several aphid species in a non persistent manner.
• Though many cucurbits are susceptible to PRV-p, they do not serve as an important alternate host. Instead, the dominant strain in cucurbits is PRV-w.
• Therefore, the spread of the virus (PRV-p) into and within an orchard is primarily from papaya to papaya.
• There is no evidence that PRV can be transmitted through seeds from infected papaya or cucurbits.
• The development of the disease in an orchard follows the general pattern of viruses that are spread by aphids in a non persistent manner.
• The amount of primary infection increases as the distance from infected papaya trees decrease.
Management
Non-Chemical method of Control
• Efforts to control papaya ring spot disease in papaya have include roguing, breeding for tolerance to PRV-p, cultural practices and cross protection. None of these methods, when treated individually, can provide complete control of the disease. In most cases, the best control is achieved by using a combination of following approaches.
• Quarantine measures: Introduction of PRSV-P into the major growing areas of papaya should be avoided.
• Roguing infected plants: Early detection of infected plants and prompt removal can contain an outbreak. However, rouging is unlikely to be effective once the disease becomes established in a plantation. .
• The most important is to establish orchards with virus free and new orchards should be established away from infected orchards.
• Orchards should not be established by inter-planting seedlings among trees that are infected with PRV-p. Additionally, disease incidence can be reduced by planting non-host crops such as corn, around the orchard and even between the rows.
• In this case the aphids flying into the papaya orchard would first land and feed on the alternate crop, lose their ability to transmit the virus to papaya due to the non persistent mode of transmission
• Aphids can be controlled by application of Carbofuran (1 kg a.i./ha) in the nursery bed at the time of sowing seeds followed by 2-3 foliar sprays of Phosphamidon (0.05%) at an interval of 10 days starting from 15-20 days of sowing.