Managing Plant Diseases and Viruses

The RHS, Defra, APHA and Forest Research have announced their ‘Check a Sweet Chestnut‘ program, which calls on the public to monitor sweet chestnut trees for known problems and diseases to help map the spread of new organisms to the UK.

Sweet Chestnut Trees are commonly found in parks and woods across the UK, and are currently affected by two devastating problems: chestnut blight, and the oriental chestnut gall wasp.

Viruses, diseases and disorders effect many plants in different ways, and are often tied to non-native species entering the UK.

What are Plant Viruses?

Plant viruses cause a wide range of discolourations and distortions in leaves, shoots, stems and flowers, but rarely kill the plant. With very few control options, virus infection in garden plants can be a nuisance for gardeners.

International expansion in the trade of an ever-increasing diversity of plant species and cultivars is helping to spread an increasing number and diversity of plant viruses around the globe. During plant production, viruses are mainly spread from plant-to-plant via vegetative propagation, although viruses are also frequently moved globally via seed. Understanding the diversity of pathogenic viruses associated with horticultural plants is an ongoing challenge for plant pathologists.

Leaf with plant disease

While plant viruses are generally thought of as disease-causing entities, recent work has shown that many viruses actually play an important and beneficial role for plants, especially in extreme environments in which they are involved in conferring tolerance to drought, cold and hot soil temperatures. Some viruses actually increase the desirability of a plant, for example Abutilon mosaic virus, which causes an attractive mosaic pattern in the leaves of variegated forms of the flowering maple.


Symptoms vary depending on the virus and the plant, but you may see the following symptoms:

  • On leaves: Pale green or yellow (chlorotic) patterns including spots, streaks, mottle, mosaic and oakleaf patterns, ring spots, vein clearing (the leaf veins themselves become pale or colourless) or vein banding (the areas immediately adjacent to the veins are paler or a different colour). Ocassionally the areas affected by these various symptoms are other colours, such as red or purple. There may be spots of brown, dead (necrotic) tissue. You may also see narrowing of leaves, stunted growth or distortions, sometimes with very reduced surfaces between the veins (puckering), resembling damage by hormone weedkillers. Leaves may be also be rugose (wrinkled), rolled, or bent down (epinasty).
  • On shoots: Tufts of stunted stems (known as witches’ brooms, although these can also have other causes), yellow or brown streaks or spots on stems.
  • On flowers: Small or distorted flowers. Streaks of a second colour (most often white) in the petals (‘breaks’).
  • On fruit: Distortion and irregular colour patterns such as marbling or ringspots, and malformed contents.


There are no chemical controls for plant viruses. It is not practicable to control the virus insect vectors such as aphids with the non-persistent insecticides available to gardeners.

  • Infected plants can act as a reservoir for infection. You may prefer to destroy the plant to prevent further spread.
  • Keep gardens weed-free, as weeds can be reservoirs of virus infection for garden plants
  • Wash hands and tools after handling suspected infected plants.
  • Do not propagate from any plant suspected of virus infection.
  • Raise plants from seed where possible, as only a minority of viruses are transmitted through seed.
  • If buying plants rather than seed, try to source certified virus-free material (e.g. for fruit crops).

What are Plant Diseases?

Infectious plant diseases are caused by bacteria, fungi, or viruses, and can range in severity from mild leaf or fruit damage to death.

Chestnut Blight

Chestnut blight is a type of fungal disease that was first confirmed on sweet chestnut trees in the UK in 2011. The disease poses a significant threat to approximately 12,000 hectares of woodland which has sweet chestnuts as the majority tree species, predominantly located in southern England.

  • The foliage wilts and dies as a result of girdling cankers lower down the stem.
  • A striking contrast between green healthy and diseased orange bark on young stems. Underneath the bark buff-coloured fungal growth may be present.
  • Cankers spread throughout the tree surface, killing the tree.
Plant disease - chestnut blight

Young sweet chestnut trees may succumb to the disease within a year, meanwhile mature trees may take years before eventually dying.

Downy Mildew

Downy mildew is a disease of the foliage, caused by a fungus-like organism. It is spread from plant to plant by airborne spores. It is a disease of wet weather as infection is favoured by prolonged leaf wetness.

A range of common edible and ornamental plants can be affected, including carrots, foxgloves, grapevines, busy Lizzie, onions, pansies, poppies, rhubarb, and spinach.

Plant disease - downy mildew
  • Discoloured blotches appear on the upper leaf surface. These may be pale green, yellow, purple or brown, depending on the plant affected.
  • A mould-like growth appears on the underside of the leaf, corresponding to the blotch on the upper surface. This growth may be white, grey or purple, depending on the species.
  • Severely affected leaves may shrivel and turn brown or turn yellow and fall prematurely.

Fusarium Wilt

Fusarium wilt is a soil-borne fungal disease. It causes the water-carrying vessels to become blocked, so that the plant wilts and often dies.

Fusarium wilts are caused by pathogenic strains of Fusarium oxysporum, which are usually very host-specific. The disease can be very damaging on Callistephus (China aster) and Dianthus (carnations and pinks).

  • The plant’s growth is stunted, and the leaves yellow and wilt.
  • The xylem vessels turn red, and can be seen inside the stem as lines or dots.
  • White, pink or orange fungal growth appears on the outside of affected stems, particularly in wet conditions.
  • The roots or stem begin to decay – affected China asters often have blackened, rotting stem bases.
Plant disease - fusarium wilt

How can I Reduce the Spread?

  • Wear footwear and outerwear that can easily be kept clean.
  • Clean footwear and outerwear to ensure they are visually free from soil and organic debris.
  • If you are revisiting an infected tree, plan to visit highest-risk sites last.
  • Clean your hands with sanitising gel (or soap and water) and tools (eg tape measure) with bacterial wipes or via a spray bottle with a bleach solution (1: 9 bleach/water) between touching trees,  =to prevent spreading pests and diseases between trees.
  • If taking samples, for example a leaf to examine for signs of a gall on the leaf closely later, clean and disinfect cutting tools and hands after each sample with antibacterial wipes or gel.
  • Keep any samples in sealed containers.

For more information on plant diseases and biosecurity, visit the RHS website.

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