An integrated approach to Pests & Diseases


James from The Gardens Group introduced himself and provided a brief background on the Group, including identifying the bases at Castle (Sherborne), Brimsmore (Yeovil) and Poundbury.   All attendees were handed vouchers and advised that all communications from the Group were now done electronically (newsletter plus Facebook).

Choose the right plant, which will make life easier – one that is

  • easy to grow
  • suitable for the location and purpose
  • one that has good disease resistance.

1.           Recommended Plants

James brought in a selection of his recommended plants and asked the Hortsoc members to identify each one

Cotoneaster ‘Cornubia’’ (AGM) – large leaved, shrub form, good shape with clusters of large red berries and good disease resistance.  It responds well to pruning.

Hypericums are a great choice, but select wisely e.g. Hidcote (AGM) is highly recommended although James suggested Hypericum ‘Miracle Summer’, which is more compact form.  It has yellow flowers and lovely autumn orange-red berries that are useful for floral decoration (Don confirmed his point!)

Caryopteris – ‘the blue spirea’,  relatively disease resistant. C. ‘Worcseter Gold’ (AGM)  has yellow foliage and blue flowers, requiring a sunny position.  He suggested a hard prune in the Spring to keep it in shape – twice the shoots, twice the flowers.

Osmanthus ‘Goshiki’ (AGM) – means five colours and looks similar to holly.  It is a good container plant and slow growing.

Sarcococca confusa (AGM) –  This is a lovely evergreen with tiny scented creamy white flowers that appear in winter and black berries follow these.

Abelia’ Kaleidiscope’ –  Pink summer flowering shrub with yellow / green variegated leaves and autumn colour.

Lavender  - James advised that it was robust apart from its physical needs.  Avoid poor drainage and lack of sun.  Prune twice-yearly – once after flowering (just below flower and in spring and down as far as the start of the shoots.  This will ensure good shape and will stop it becoming leggy.  Lavandula augustifolia  ‘Hidcote’ has one of the best blues.–Hidcote-  ‘Munstead’ is also good for a softer blue.   French lavenders need more protection.

Skimmia’ Rubella’ (AGM) (male) needs acid soil. If in the wrong soil it will show as yellow veining in the leaf.  Grow it in a container if the soil is limey soil and provide an ericaceous feed.

Pieris japonica ‘Carnival’ – a good container plant with variegated leaves with red flush in the spring and long lasting white flowers.

2.           Recommended Seeds

James advised that seed producers were now producing ranges of seed that focused on disease and pest resistance and were worth checking out.

He gave Unwin’s Gro-Sure range of black packets as an example.  These were also coated and so were easier to handle.

3.      Plant Husbandry

Once you have selected your plants then it is all about treating them well.  This included:

  • Watering
  • Feeding
  • Mulching
  • Pruning
  • Hygiene, including after pruning
  • Crop rotation


This is particularly important after there has been lots of rain and nutrients have been washed away

Products such as Miracle Gro are often used but this tends to be high maintenance since it requires regular feeds (often weekly).   Consider instead a general-purpose fertiliser such as Vitax Q4 (based on chicken poo), which is an extended release and includes lots of trace elements.  Place round the base of the plant every few weeks.  The range includes one for conifers and shrubs.

Blood and Bone meal was also slow release, but had a less immediate impact- more background / more natural way of fertilizing

Miracle Gro Ericaceous Soluble Plant could be used for plants that required an acid soil – if they have started to turn yellow then an application of a course of feed will begin to have an impact within 2-3 weeks.


Now is the time to do it, or in the spring.  James suggested that even bark would help reduce water loss by up to 70% during the summer.

Before mulching always clear the ground of infected leaves, for example, where there has been black spot on roses.  This will help avoid fungal spores re-infecting the soil.


Pruning is a good way of managing a range of problems.  Removal of the affected growth can be important.   Use a product such as Arbrex Seal and Heal for cut limbs.  This will help protect against the ingress of disease and harmful pests.

A general clematis pruning tip

Any clematis that flowers between midsummer’s day and September should be cut back hard in early spring. Those that flower before midsummer’s day only need a gentle tidying if it’s needed.

Crop rotation

Ensure move around each year.  Remember the four 4 groups that need to be rotated:

  • Alliums (onions, garlic, etc)
  • Brassicas (cabbages, broccoli, etc)
  • Root vegetables
  • Legumes (peas and beans)

4.      Cures – How to solve problems

 The market is moving increasingly away from chemicals – particularly as certain products are being proved unsafe and removed from the market.  There has been a corresponding increased interest in traditional and ‘friendly’ gardening (including plant choice, rotation, barriers and biological control).  The last resource in the portfolio is use of off the shelf products (organic and non-organic).

James considered other ways of dealing with pests without using blanket controls such as Pyrethrum Spray

leaving wild areas in or around your garden will encourage natural predators, including frogs, slow worms, etc. James advised that Dorset Wildlife Trust had been placing hedgehog boxes under compost bins, etc

  • diverse planting schemes
  • companion planting
  • introduction of predators and parasites
  • pheromone traps

Slugs & Snails – a range of options

Use organic slug pellets where possible.  These are based on ferric phosphate, which cause the slugs to go back underground and starve. The iron and phosphate they contain go back into the soil, both beneficial for plant growth.  Non-organic products rely on metaldehyde, which is toxic to wildlife and goes back into the food chain, affecting birds and hedgehogs.

Copper tape is also effective for pots or copper piping on raised beds creates an electrical barrier that slugs don’t like.

Slug Gone – pellets made of wool.  Okay for dressing of pots but not recommended for large areas.

Slug & Snail Traps – These can be effective and are usually alcohol (often beer) based.


5.      Biological Controls

Natural Predators

- Some pests are notoriously hard to control with chemicals. Consider use of natural predators for specific pests, such as vine weevils.  Nematodes may be used from April.  They work by swimming through soil and then burrow into grubs and die.   They are more effective with container grown plants of for confined spaces, for example for white fly or red spider mite in greenhouses.

Pheromone Traps

-       Fruit trees where the maggots cause damage to apples / plums.  The trap works by imitating the scent of female moths.  The male is drawn to it and gets caught on sticky trap.  Use these from late May to end of July.

-       Leek moths – The small yellow green caterpillars damage foliage and tunnel into stems. Control is either phermomone using the same principle as above or use of physical barriers (such as fleece on cloche frames, which may also be used against cabbage white and carrot fly.

Companion Planting

– Use to discourage pests, for example, herbs in amongst plants.   Examples suggested included use of chervil and lettuce; garlic generally, tagetes (Mexican marigold) and tomatoes.

Physical controls

-       Winter moths – apples and fruit trees – to be applied now (October)!  On old trees with fissures can use Vitax Fruit Tree Grease

-       Whitefly and other insects – In greenhouse use Yellow Sticky Traps.

-       Cabbage root fly – consider use of Cabbage Collars for your brassicas.  Place at base of stems.

-       Strawberry – use of straw or straw mats (can also be used for hostas) to prevent slug damage.  Broken eggshells and gravel are useful alternatives.

-       Slugs – melon rind or upside down grapefruit.

Mildew in greenhouses

- Ventilation and spacing is key.  Allow airflow.  Avoid watering foliage and water the ground only.

Chemical Barrage

-       ‘Friendly’ Treatments

Products containing plant (for example seaweed) or fish oils attack some pests and diseases by physical rather than chemical action.  On insects they work by blocking the pores; however their effectiveness is limited by the need for direct contact on the insect (scale insects, aphids, thrips whiteflies and spider mites).  For use on powdery mildews, black spot.  Suggest: Nature’s Answer Natural Fungus and Bug Killer

-       Tar Oil

Used to be something called tar oil that used to paint on trees but these products are no longer available for pesticide use, but are available for the sterilisation of greenhouse structures, seed trays and pots.   The organic version is based on fish oils and plant oils.  Suggest: Winter Tree Wash (Just Green), which is based on plant oils and prevents insect eggs.

-       Greenhouse sterilisers

Jeyes Fluid is best known but quite harsh.  Suggested alternative is Citrox Garden & Greenhouse Disinfectant.  It is citrus based and nicer to use.

Sulphur can be used to fumigate.  Leave for a day and ventilate well.

-       Chemical Products


Best selling brands often include Neonicotinoids, which work systemically by going through the plant.  There is increasing evidence that they get into the pollen and are having a devastating effect on bees.  If you need to use them then limit them to use of houseplants, where bees cannot be affected.

The Gardens Group, like many other retailers, has now removed a number of these products from its shelves because of the harmful impact.

Cat repellants –  Cats – Suggest: Pepper Dust (Bayer) but only good until it rains.  An alternative is Silent Roar, which is based on lion dung and works on territorial instincts of cats to avoid area.

Plant & Soil Treatment.  When planting anything new then suggest Rootgrow Mycorrhizal Fungi Rootgrow.  It is highly effective in encouraging secondary root growth and helping the plant to get established.  Note: not for use with acid-loving plants or brassicas.

6.      Shepton Horticultural Society Question Time

Q.        What can be done about mildew on trees?

A.        Very little!  Suggest consider pruning to assist air flow


Q.        Cherry tree has shriveled leaves and no fruit.  What is it?

A.        Not sure.  It could be bacterial canker that is entering via the blossom.


Q.        Wilting violas.  Many are fine; others are dying in small groups.  What is happening?

A.        Unable to identify without further information.


Q.        A 15-year-old viburnum appears to be dying.  What is wrong?

A.        All plants have natural lifespans.  This could be the case of reaching the end of this ones.  Suggested pruning out some of older wood.  Viburnums are good at regenerating from base.  They don’t tend to suffer much from pests.  Take out some of the older stems – about 1/3.  Another test of vigour is to scrape along the bark with your nail.  It should reveal green.  If brown instead then it is probably dead!


Notes by Simon Edwards