Dealing with Garden Diseases

By Steve Malsher MCIHort

In last week’s blog, we looked at ways to deal with the garden pests that might be lurking in your garden at this time of year. This week, we will look at a few common diseases for this time of year. Customers often ask me what a plant disease is? The technical answer is: a bacterial, viral or fungal infection that has a detrimental effect on a plant. I thought I would look at one of each this week.

But first…

Although we have many weapons in our arsenal to deal with fungal infections, this is not the case for bacterial or viral infections. Prevention is better than a cure in all cases. So, here are a few steps to help.

  1. Hygiene, hygiene, hygiene! Keep your tools clean and disinfected as a matter of course. Clean and disinfect after a day’s use in the garden. As a professional horticulturist dealing with diseases, I often disinfect between pruning cuts, and certainly before going on to another plant.
  2. Remove possible vectors – weeds and pests that can carry the disease and infect your plant. A classic example is never to use Hawthorn as a hedge around an orchard, as it is a vector for Fireblight.
  3. Maintain the area around your plants to promote the best environmental conditions. A healthy plant in the right place will be better able to cope with the disease.
Dealing with Garden Diseases

Botrytis –  Grey Mould

Botrytis can be recognised by the dense, light grey fungal mass which develops after infection. Generally, it requires wounded tissue to infect the plant, which can explain its prevalence in plants that are de-leafed (Tomato), disbudded (Crysanthemum ssp.) or pest wounded (Strawberry).

Dealing with Garden Diseases
Dealing with Garden Diseases

Millions of spores are carried on the wind to the next plant, which is the most common way of dispersal. The infected plants then rot, making them inedible or unusable for aesthetic value.


Controls are mainly cultural.

  • Improve hygiene conditions, increase airflow around the plant and lower moisture levels where possible (i.e. in greenhouses).
  • Remove all infected leaves, stems and fruits, and ensure plants are not damaged.
  • Prune with clean, disinfected, sharp secateurs.

There are no chemical controls approved for home gardeners.

Dealing with Garden Diseases

Bacterial Canker on Prunus

Bacterial canker is a disease caused by two closely related bacteria that infect the stems and leaves of Prunus species such as plums, cherries and laurels. It is one of the few plant diseases caused by bacteria in the UK. Cankers begin to form in mid-spring and soon afterwards shoots may die back. Shotholes appear on foliage from early summer.


  • On stems and spurs: Sunken, dead areas of bark develop in spring and early summer, often accompanied by a gummy ooze. If the infection spreads all around the branch, it will die rapidly. However, it should be noted that gum production (gummosis) from the bark of Prunus species is actually quite common and, in the absence of dead, sunken bark, is likely to have resulted from causes other than bacterial canker (for example, physical damage or environmental stresses).
  • On leaves: Small brown spots appear, which are often round and fall out later to leave holes – as if the leaf had been hit by shotgun pellets, leading to the popular name of ‘shothole’
Dealing with Garden Diseases

Non-chemical control

Where possible, carry out all pruning in July or August when tissues are most resistant. This is also the best time to prune in order to minimise the risk of infection by spores of the fungus, causing silver leaf disease. Cut out all cankered areas, pruning back to healthy wood and paint promptly with a wound paint to protect the wound from re-infection. Burn or landfill the prunings.

The cherries ‘Merton Glory’, ‘Merton Premier’, ‘Merla’ and ‘Merpet’ and the plums ‘Marjorie’s Seedling’ and ‘Warwickshire Drooper’ have some resistance.

Dealing with Garden Diseases
Dealing with Garden Diseases

Tobacco Mosaic Virus (Often referred to as Cucumber Mosaic Virus)

The Tobacco Mosaic Virus is found in many parts of the world and affects tomatoes, cucumbers, peppers, potatoes, dahlias and of course, tobacco plants. It was the first plant virus to be identified and has been investigated in minute detail for the past 80 years.


The symptoms are listed below in the order in which they normally occur:

  1. Small patches, lighter than normal green, brown and yellow appear on the leaves. These are the earliest symptoms.
  2. Those patches on the leaves described above have been taken over by the virus and a side-effect is that the leaves may appear to be smaller than normal. This is caused by the plant’s photosynthesis process being disrupted.
  3. The whole plant may also be slow to develop after infection and is often smaller and generally less healthy-looking.
  4. Because this is a virus with no clear lifecycle, it can infect the plant at any stage of its life. If fruit develops, their skins will have bronze-coloured patches on them.
Dealing with Garden Diseases