Immunoprecipitation Reverse transcription Polymerase chain reaction RFLP Restriction fragment length polymorphism Introduction Whiteflies family Aleyrodidae are pests of herbaceous and woody plants in tropical countries and of greenhouse-grown plants in temperate climates. Whiteflies directly damage plants by sucking sap from leaves, inducing physiological disorders. Whiteflies can also affect hosts by producing a sticky secretion known as honeydew, which prevents leaves from functioning efficiently and acts as a substrate for fungi sooty molds. In addition to direct damage, whiteflies act as vectors of other plant pathogens, especially viruses that belong to genera Begomovirus, Crinivirus and Torradovirus.

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The Plant Health Instructor. His livelihood depends on his own 6-mu equals 1-acre or 0. He bought and planted apple trees three years ago. Before planting his trees, he had grafted them himself, by joining the scion or above-ground portion of the tree cultivar Fuji to a suitable rootstock. He pruned them at the proper time and fertilized according to recommendations for his region of Shaanxi Province. Everyone in his village admired his industriousness and thought he would earn a good profit from his fruit.

As a result, no one would buy these apples, so he had nothing to sell. Xilai Zhang was very upset and hopeless. Then the spots became larger and larger. Xilai used fungicide sprays in an attempt to control the spots, but in vain: The circular patches expanded, making the fruit unsaleable, even after Xilai had sprayed with several kinds of fungicides.

Xilai had no idea why fungicides have had no effect. After talking with Xilai yesterday, Fuwang realized that his own orchard was strikingly similar in several ways. For one thing, Fuwang and Xilai had bought apple scions for grafting from the same supplier. This made him feel a little anxious. Fuwang walked through his own orchard immediately.

His worry deepened: was he facing the loss of his orchard? His hope to raise the money to send his two sons to an excellent high school and university was starting to fade. He was directed to Dr. Han Qi, who is a plant pathology professor specializing in apple diseases.

He talked to both growers about their grafting practices. The viroid may have originated from infected stock plants — either the scion or the rootstock. Spreading the viroid from tree to tree during grafting may mean that many of your grafted trees were already infected when they were planted in your orchard.

So two things probably went wrong: a source of your grafted scions or rootstocks was contaminated to start with, then the viroid spread during the wounding that is needed to make the grafts. The apples will show symptoms year after year, and the health of the infected trees will gradually decline to the point that they cannot produce a commercial crop of fruit.

Was there no way to control it after the grafting was done? The diagnosis was much too late to help Fuwang. It puzzled him that during its earlier years, his orchard had appeared completely healthy.

This is called the incubation period. You know apple trees need around two years to grow before fruit develop, then when the tree begins to bear fruit around the third year, the stress created by the developing fruit can trigger an outbreak of symptoms. I spent almost my whole savings on my orchard. But I think I can help you to figure out a way to deal with your problem," Professor Qi consoled Fuwang.

In his office, Qi explained more details to Fuwang. It is one of the most serious viral diseases on apple, and it has been reported from apple orchards in China for almost 70 years. This disease affects apple and pear. The cultivar Red Fuji, which you planted, has been very widely planted in Shaanxi Province in recent decades, and is highly susceptible to this viroid. Whether insects can transmit ASSVd is uncertain; however, the main method of spread appears to be grafting.

Fuwang may have had just a few scions or rootstocks with ASSVd contamination to start with, but the grafting process can dramatically increase the number of infected trees that are planted in an orchard. As the fruit mature, the spots enlarge and cause yellowish rings on the fruit surface.

Some even transition from yellow to brown Figure 5. Some fruit are also abnormally small and misshapen Figure 6. It frequently results in large economic losses. What can I do? In reply, Professor Qi offered some ideas to help Fuwang devise a strategy to cope with this viroid crisis. What are the main ways in which Apple scar skin viroid is transmitted? In the present case, that means doing everything possible to keep ASSVd out of the orchard.

There is no cure for diseases caused by viruses or viroids once they have infected a host plant. As a result, using virus-free and viroid-free planting materials is the most effective way to prevent these diseases from spreading long distances. Many countries, in collaboration with commercial tree producers, have adopted practices that require or recommend purchasing only planting stock that has been reliably certified to be free of viruses and viroids.

The planting stock is tested repeatedly for presence of known viruses and viroids; material that tests positive at any stage or production is discarded and destroyed. In addition, many countries have instituted strict quarantine programs, in which imported plants are inspected, and their certifications are checked, to prevent entry of infected plants into these countries.

Planting viroid-free trees is a major step toward minimizing the risk of ASSVd. Cultural practices Grafting means combining the scion upper part of the combined plant and the rootstock lower part — into a single plant. However, the wounds caused by grafting cuts can introduce viroids and viruses into plant cells.

Therefore, disinfecting grafting tools frequently when making grafting cuts is a way to prevent viroid and virus transmission from infected tissues to healthy tissues. Almost all commercial apple trees are pruned — that is, selected shoots and branches are removed - annually in order to control tree size and allow light to penetrate throughout the tree canopy, which optimizes fruit quality.

During pruning in an orchard with some viroid-infected trees, it is advisable to prune the healthy-looking trees first, then the ones with symptoms, in order to minimize the risk of spreading the viroid from infected trees to healthy trees. An alternative approach is to remove ASSVd-infected trees in orchards as soon as it becomes clear that they are infected, to avoid possible spread of the viroid to neighboring trees by means of pruning wounds.

Insecticide sprays The significance of insects in transmitting apple scar skin viroid in apple orchards has not been determined. However, there is evidence that ASSVd can be transmitted by the greenhouse whitefly Trialeurodes vaporariorum to plants such as bean and cucumber under experimental conditions. As a result, spraying insecticides could be considered a precautionary strategy given the uncertainty about the risk of ASSVd transmission by pest insects, but this practice also would impose an added expense for apple producers.

Typical commercial apple orchards in China are small in scale - less than 0. When it comes to harvest season, neighbors gather together to help each other pick apples Figure 7 and 8. Viruses and viroids that attack plants Viruses are tiny pathogenic disease-causing particles that can be seen only with an electron microscope.

Plant cells have cell structure like cell walls, cell membrane and nucleus. But viruses lack cell structure and must parasitize cells of a host organism in order to survive. Viruses are much smaller than bacteria, and exist as particles of nucleic acid DNA or RNA encased within a protein shell.

Viroids are even smaller than most viruses; they consist of tiny single-stranded molecules of ribonucleic acid RNA , and lack an outer layer of protein. In perennial multi-year cropping systems like apples, viroids can spread rapidly by means of contaminated pruning and grafting tools, farm implements, clothing, or human hands. After viroids are inoculated onto a new host, they will attack and invade the cells. After new copies of viroids form, they can break out of host cells and infect nearby cells.

So-called clean plant networks have been developed to ensure that plant propagation material is pathogen-free and available. Trained staff conduct regular visual inspections and laboratory testing to provide this assurance. For example, apple growers can purchase young trees that are certified to be free from viruses and viroids. Adoption of clean plant networks by growers has greatly reduced the risk of diseases such as apple scar skin viroid in many countries — provided that all growers participate in the system.

Although instituting clean plant networks in apple production would sharply increase the cost of young apple trees, they have the potential to eliminate virus and viroid diseases as serious economic risks. It can combine desirable qualities of two plants - the scion upper part of the combined plant and the rootstock lower part — into a single plant. For example, a goal for apple grafting would be to combine large, flavorful, commercially popular fruit scion with small tree size rootstock.

Grafting requires making cuts into both the scion and rootstock to create a zone where the two segments can grow together. Figure 10 shows two scions inserted into a cleft in the rootstock. Generally, apple growers prune their trees in late winter, but summer pruning is recommended in some circumstances. What management practices should you implement in order to minimize the risk of ASSVd infection in this new orchard?

Acknowledgements We thank Dr. Nicholas T. Viroids and Satellites, chapter Kim, H. R, Lee, S. H, Lee, D. H, Kim, J. S and Park, J. H, David A. R, George L. The Apple Industry in China. Wang, Y. N, Hu, T. L, Liu, S. X, Zhao X. S, Yang J. Y, Cao K. RNA Biology, 12 10 : —



A faint band corresponding to the ubiquitin ligase mRNA was also observed when using healthy cucumber phloem. Most commercial pear cultivars show no symptoms when infected with ASSVd. Some RNA species possess stem loop structures similar to tRNAs, and the existence of other unknown RNAs with such structures, which can confer increased stability, is probable. Acta Horticulturae ; Published online Sep 1. There was a problem providing the content you requested ASSVd isolates were maintained on cucumber and bean.


Apple scar skin viroid naked RNA is actively transmitted by the whitefly Trialeurodes vaporariorum.




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