Are your Christmas Raspberries under Threat?

Punnets of fresh Raspberries (CC Public domain photo by Couleur)

If you search for Raspberry on the internet, your first page results are more like to turn up Raspberry Pi than the fruit. A lot of the early computer companies were named after fruit: Apricot, Tangerine and Acorn. Our focus is on the European Raspberry, Rubus idaeus. We associate Raspberries with summer and yet they are available all year round, including as a Christmas treat. Given the number of pests and diseases that threaten its cultivation, how much longer will this be the case?

Origin

Raspberries have been harvested from the wild for thousands of years. They are native to Europe, Asia and North America. The old English name was raspis. This is believed to refer to the rough, slightly hairy and rasping surface of the fruit in comparison to the smoother surface of blackberries.

Rosales, Rosaceae, Rosoideae and Rubus

Rubus is a genus in the Rosaceae or Rose family in the Rosales order. Interestingly, most Rosales species have a symbiotic relationship with root fungi (Ectomycorrhizae) and often also with nitrogen-fixing bacteria. Rosaceae is a complicated family with complex reproduction, making it difficult to put precise figures on the number of genera and species. Christenhusz et al. suggest between 54 and 90 genera with about 2,950 species. The family is further divided into 3 subfamilies: Rosoideae, Dryadoideae and Spiraeoideae. The Rubus genus has around 250 species in the Rosoideae.

Cultivars and Hybrids

Modern cultivars have emerged through hybridisation and backcrossing with the American Raspberry, Rubus occidentalis. This has often been done to improve disease resistance. There are also hybrids with other fruits, for example Loganberries, Rubus loganobaccus (a cross between Raspberries and Blackberries).

Wild Fruit

The ancient Greeks were probably the first to cultivate the fruit. Raspberries were said to grow abundantly on the slopes of Mount Ida, hence the origin of the epithet idaeus in its Latin name. Unusually for a wild fruit, the Rubus idaeus is found to have outstanding flavour in the wild. It only really became a crop in the Middle Ages and cultivars did not really emerge until the late eighteenth century.

Always a Red Fruit?

We associate the colour red with Raspberries. This is not always the case, although the genus name Rubus derives from red and actually relates to the Blackberry. Raspberries can be red, yellow, white or varying shades in between. A unique feature of Raspberries is that the receptacle stays on the branch when the fruit is picked, leaving a hollow centre that is soft to eat. This is in strong contrast to Blackberries for example.

Rubus idaeus, Raspberries in close-up showing the hollow centre after picking
CC Image by Beverly Buckley from Pixabay 

Pests and Diseases

Unfortunately, delicious though they are, Raspberries are prone to a number of pests and diseases. Pests can carry viruses from plant to plant. Climate change is likely to worsen this situation. Just a few of the problems mentioned by the RHS follow below:

Pests

  • Aphids – transmit plant viruses, distort growth and excrete sticky substance on which sooty moulds grow.
  • Glasshouse red spider mite (Tetranychus urticae) – feeds on sap and causes mottled leaves and early leaf loss.
  • Raspberry leaf and bud mite (Phyllocoptes gracilis) – causes pale yellow blotches on upper leaf surface.
  • Leafhoppers – suck on leaf tissue and remove green chloroplasts from cells, thus preventing the proper functioning of leaves.
  • Raspberry beetle (Byturus tomentosus) – grubs damage fruit.

Diseases 

  • Powdery mildews – fungal disease of foliage spreading white, dusty coating.
  • Raspberry cane blight  (Paraconiothyrium fuckelii, syn. Leptosphaeria coniothyrium) – causes extensive dieback of canes.
  • Raspberry rust (Phragmidium rubi-idaei) – yellow pustules on leaf surface that can cause premature shedding of leaves.
  • Raspberry spur blight (Didymella applanata) – fungal disease that weakens canes, kills buds and reduces yield.
  • Raspberry viruses – a variety of viruses with varied symptoms, some affect other plants too. Raspberry mosaic can be caused by at least 5 different viruses – the main ones are Rubus yellow net virus and Raspberry leaf spot virus.
Rubus idaeus, Raspberries succumb easily to mould as the fruit bruises easily
Rubus idaeus, Raspberries succumb easily to mildew as the fruit bruises easily
© Karen Netto (Andrews)

Controls?

Both chemical and non-chemical controls are often ineffective. Gardeners are advised to purchase virus-resistant plants and destroy infected canes immediately.

Culinary Inspiration

Despite all its problems, the Raspberry remains a firm favourite with top dessert chefs, inspiring all sorts of syrups and sauces. What would we do without Raspberries for jams, juices, confectionery and liqueurs?

© Karen Netto (Andrews)

References and Further Reading

Case, Frances (Ed.) (2008): 1001 Foods You Must Try Before You Die. A Global Guide to the Best Ingredients. Cassell Illustrated. Quintessence Publishing. London. (p.28).

Christenhusz, Maarten J. M. & Fay, Michael F. & Chase, Mark W. (2017): Plants of the World. An Illustrated Encyclopedia of Vascular Plants. Kew Chicago. (pp.262-267).

Davidson, Alan (2006): The Oxford Companion to Food. Second Edition. Edited by Tom Jaine. Oxford University Press. (pp.654-655).

RHS (2019): Raspberry viruses. RHS Gardening. Last accessed 19 December 2019.

RHS (2019): Rubus idaeus. Common Raspberry. RHS Gardening. Last accessed 17 December 2019.

van Wyk, Ben-Erik (2005): Food Plants of the World. An illustrated guide. Timber Press. Portland, Oregon. U.S.A. (p.325).

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