What is it and what can be done about it? By Anna Mouton.
What is plum marbling?
Plum marbling is an infectious disease of Japanese plums caused by plum viroid I. Viroids are small circular strands of RNA — short for ribonucleic acid — that multiply in the plant’s cells. Think of a viroid as essentially a large molecule that sometimes causes disease by disrupting normal cell functions.
What are the symptoms?
Plum marbling takes its name from the irregular skin coloration or marbling seen in infected plums. Marbling is only visible once fruit have developed colour. It is more striking in some cultivars than others. Marbling may be inconspicuous on very dark cultivars.
Infected trees can also bear plums that are small and hard with an irregular shape and uneven surface. Plums may have so-called corky flesh which renders the fruit inedible. Both yellow-skinned and red-skinned cultivars can suffer corky flesh.
Plum marbling and corky flesh may not be obvious at the time of harvest. Sometimes reduced size is the only visible indicator of infection. The risk is that affected fruit will be sent to market where the consumer will experience either corky flesh or a fruit that has failed to fully ripen.
Symptoms of plum marbling only occur on fruit. Other parts of infected trees contain the viroid but show no abnormalities.
Table 1: Examples of symptom variation in different cultivars
|Severe corky flesh. Uneven fruit surface.
|Red blush. Small irregular fruit. Severe corky flesh.
Can marbling symptoms be caused by other diseases?
Yes — but keep in mind that plum viroid I is the principal cause of marbling and corky flesh in Japanese plums in South Africa.
Plum marbling may be confused with a disease called plum pseudo-pox. Plum pseudo-pox is caused by apple chlorotic leaf spot virus. Infection with apple chlorotic leaf spot virus is rare in South African orchards. Plant material used to propagate certified nursery trees is tested to ensure freedom of this virus.
Hop stunt viroid has been reported to cause symptoms that are similar to marbling in other countries. Hop stunt viroid occurs in South African plum orchards but there does not appear to be an association between infection and symptoms of marbling in our country.
Laboratory tests are available to distinguish infection with plum viroid I from other infections.
How is plum marbling transmitted?
Plum marbling is transmitted through infected plant material. Plum viroid I is present in all parts of infected plants including leaves and stems. Experiments have shown that plum viroid I can be transmitted to healthy trees by inoculating them with buds from infected trees.
It is highly likely that healthy plant material can become diseased when grafted onto infected rootstocks. Trials are underway to investigate the risks of top-working infected trees.
Growers have reported that plum marbling spreads slowly or not at all within orchards. Plum viroid I could nonetheless be mechanically transmitted from infected to healthy trees by routine practices such as pruning. A current research project is looking into possible mechanical transmission of plum viroid I.
There is no evidence that plum marbling is spread by insects or other vectors.
Recent research found that plum viroid I is not transmitted through seed. Scientists collected seed from trees infected with plum viroid I and raised seedlings. They sampled the seedlings five months after germination and all tested negative for plum viroid I.
How can marbling be controlled?
There is no treatment for plum marbling. Infected trees may show fewer symptoms when unstressed, but they remain diseased — symptoms will return as soon as the tree encounters tough environmental conditions.
Orchards should be monitored close to harvest as symptoms are only visible in fruit that has coloured. Symptomatic trees should be marked. The presence of plum viroid I can be confirmed by laboratory testing at SAPO Trust or the disease clinic at Stellenbosch University.
Trees that test positive are best removed and burnt. Keep in mind that affected plums do not always display obvious symptoms at harvest. Harvesting from infected trees can lead to the sale of poor-quality fruit.
Infected trees can be a source from which disease spreads mechanically by means of pruning and other practices. Disinfect pruning equipment with diluted household bleach — add 150 millilitres of bleach to 850 millilitres of water. This disinfectant can corrode blades, so clean and oil pruning equipment at the end of work. Avoid mechanical pruning.
Prevent the introduction of plum marbling by planting certified material. Testing for plum viroid I is now performed voluntarily by all plant improvement organisations and will soon be a requirement for certification. Parent material held in nucleus and foundation blocks has been tested and shown to be free of plum viroid I. Parent material in mother blocks has also been tested, and infected plants have been removed.
Plant material that is certified under the Deciduous Fruit Plant Certification Scheme is clearly identified by a blue label. The parent material of these nursery trees has been tested for plum viroid I, apple chlorotic leaf spot virus, apple mosaic virus, prune dwarf virus, and Prunus necrotic ringspot virus, all of which affect stone fruit.
Plum marbling has been around for many years but until recently the cause was unknown. The discovery of plum viroid I has increased our understanding of the disease and our ability to control it — but we can’t expect to have all the answers overnight. Read more about current research on plum marbling in Getting To Grips With Plum Marbling.
All images supplied by Hano Maree | Stellenbosch University.
Contact details for testing
021 887 6823
Contact person: Lize Burger
Department of Plant Pathology
Contact person: Sonja Coertze
021 808 4798
021 808 4223