Garden Pests & Diseases

Garden Pests & Diseases

In recent years home gardening has changed:

  •  Gardens are smaller.
  •  Gardening is a major hobby for less people.
  •  Gardeners in general are less knowledgeable.
  •  There are less specialist garden centres and more mass merchandisers selling plants.
  • There is a greater fragmentation of suppliers and brands of plants and products.
  • Quality garden information is hard to find, especially regarding fertilisers and pest & disease control.


If you are going to put time and effort into your garden, you want it to be successful. At the same time you want to treat the soil and the environment with respect so that your gardening efforts will be successful for a lifetime and more. The following notes are science based and proven to be effective, and sustainable. They combine the good parts of organic gardening with the cost effective science proven pragmatism of conventional gardening. Maximum use of preventative cultural practices are recommended which leads to less need of pesticide intervention. Where intervention is necessary – make sure you have correctly identified the problem, select the most cost effective and appropriate product for the crop requiring treatment – (there are very few effective products approved for use on many edible crops).


Disease Prevention

We explore the components of a garden in which can be altered to prevent disease.



Ensure the soil is free draining by raising the planting areas and adding compost or similar organic matter. Good drainage prevents soil fungus diseases which attack roots or crown. If lime has not been used in the compost, add an annual dressing. For acid loving plants use Gypsum in place of lime)


Crop rotation

Rotation of crops in the vegetable garden is a very effective method of preventing the build- up of soil fungus diseases such as damping off fungi of germinating seeds; club root of brassicas; sclerotinia of lettuces, beans, and tomatoes; fusarium, verticillium, and rhizoctonia of tomatoes and many other vegetables.


Disease resistant species / varieties

This method of disease prevention has not been well communicated to gardeners. Today there are many excellent varieties of vegetables and annual flowers available that have high resistance to many diseases and some insects.
Generally they are hybrids and cost a little more, but very well worth it for the improved performance and superior taste. 



Use a balanced NPK fertiliser where the N (nitrogen) and the K (potassium) are about equal.
Soft growth from excessive nitrogen application is more susceptible to disease, than slightly harder growth produced when the potassium is at least as high or slightly higher that the nitrogen.


Water with a soak hose rather than sprinklers

Regular overhead watering of plants with sprinklers maintains a high humidity and soft growth, which provide perfect conditions for development of fungal and bacterial diseases.
Watering with weeping soak hoses makes better use of water by placing it directly on the soil without waste, and minimises humidity and soft growth.


Practice good hygiene

A major source of infection of diseases is infected leaves and plants left either on the plant or on the soil surface. Maintain good hygiene in the garden by removing diseased leaves / plants and disposing in green waste. Do not add to the compost unless large heaps of high temperature compost is being made. Regularly dip secateurs in bleach or disinfectant when pruning to avoid spreading disease. If reusing punnets or trays for seed raising or cuttings, wash and dip in bleach before each use.


Beneficial Predators

There are many naturally occurring organisms that attack pests and diseases of plants. These include – insects, mites, fungi, bacteria, and viruses. Some attack directly and some are vectors. Man has assisted by developing and releasing colonies to boost numbers (eg. predator mites) or in some cases a culture which is sprayed on like a pesticide, (eg Btk- Grosafe Bactur or Kiwicare bio Caterpillar killer). Beneficial predators can give satisfactory control in commercial growing when used in a pest management programme, but rarely give sufficiently good control in home gardens. Btk is used by organic growers for caterpillar control and some conventional gardeners use it for caterpillar control during harvesting of edible crops as it has a nil withholding period. There have been one or two cases where insects have been controlled naturally by predators – Army caterpillar is a case in point.


Traps & Barriers

Sticky yellow traps are useful in confined spaces such as a greenhouse, but barely contribute outdoors. Pheromone traps such as Codling moth traps are useful for light infestations. If the traps are catching a lot, it indicates the infestation is heavy, in which case the traps are a good indicator that a codling moth spray should be applied.


Netting a crop to prevent moths or beetles laying eggs on a crop, although costly, is effective for pests such as the potato/tomato psyllid, as it is difficult to control with pesticides. It is sometimes used for white butterfly control. One negative side effect has become apparent – aphids still seem to find a way of laying young through the net, and once under the net, the lack of natural predators, allows the aphid population to build rapidly to serious levels. Hence when using net barriers, keep a watch for aphid build up and apply an insecticide if required.


Companion Planting

There is little peer reviewed scientific evidence that companion planting is effective. Yes there are anecdotes, but not backed by science. Recently I read that marigolds repel aphids, but then spotted several trays of marigold plants in a garden centre really thick with aphids. I was then informed by a believer that marigolds attract aphids away from other plants – this was the opposite theory to many articles I have read. Garlic is similar – it is stated as repelling aphids and mites but from a lifetime of gardening I find garlic is the first plant in spring to be attacked by aphids, and border security inspections readily show mites as a major pest on imported garlic.


There is a little truth in that some species of marigolds supress nematodes (microscopic eel worms) in the soil. This is backed by science. Companion planting is attractive and makes a good story but don’t bet the house on it as an effective pest and disease control measure.

Pesticide Intervention

We explore the when its the right time to use a pesticide and what one to use.

Correctly identify the problem

In many situations, a pesticide is applied where it is not required, or the wrong pesticide is used. Start by ensuring correct identification. Many diseases have similar symptoms – black or brown spots. Several insects leave holes in leaves, and several cause yellowing.

Downy mildew and anthracnose of roses are often identified as black spot. Fungicides used for black spot do not control downy mildew which is very common in spring and autumn.
Diamond back moth is more common than white butterfly on vegetable brassicas, but rarely identified as such.

Identification is made easy with the book – ‘Garden Pest & Disease Control’ by Bill Brett.
See at the end of this bulletin for details.


Check the best / appropriate time to treat the problem

Once identified, check the life cycle and other details to ascertain if treatment is necessary, and what time of year is best. Full details on almost every pest and disease requiring treatment in NZ is found in the book ‘Garden Pest & Disease Control’ by Bill Brett.


Use the correct approved product for the particular crop

There are an increasing number of unregistered pesticides finding their way onto the market, using loop holes in the law. There are also exemptions from registration if the pesticide is not intended for food crops. However, with the aid of some ‘weasel’ words and a few anecdotal blogs, and the unregistered pesticide is being used on food crops, without toxicity checks, or residue levels being set, or active ingredient levels declared.

Use only pesticides registered by MPI under the ACVM Act 1997, – this will be on the label.

BioGro certified organic is not a substitute for registration. Bio Gro certified organic only certifies free of synthetic chemicals – it does not certify- ‘safe’, ‘can be used on food crops’, ‘effective’, or ‘fit for purpose’.
Only a very small number of home garden pesticides are approved for use on edible crops.
The NZ registration status of virtually every home garden pesticide, and the crops on which its’ use are approved, are found in the book ‘Garden Pest & Disease Control’ by Bill Brett.
See at the end of this bulletin for details.


Observe withholding period on food crops

Registered products approved for use on edibles have a withholding period set – the time that must elapse between the last spray and harvesting of the crop. This is on the label and must be observed. Modern pesticides are of lower toxicity and many now have a ‘nil’ or very short withholding period. This period can also be found for all approved home garden products available in NZ, in the book ‘Garden Pest & Disease Control’ by Bill Brett.
See at the end of this bulletin for details. Read the pesticide label before use – it is a legal document.


Pest & Disease Control Information

The book ‘Garden Pest & Disease Control’ by Bill Brett, is available from independent specialist garden centres, university book shops, specialist book stores, and Paper Plus. It is also available on line from
The book published Dec 2016 covers all aspects of pest and disease control –

  •  Identification – the largest range of garden problems published in a NZ book, described in detail and illustrated with some 240 photos.
  • All science proven methods of prevention
  •  A detailed description of all pests and diseases commonly found in NZ gardens.
  •  A detailed description of all brands of all pesticides distributed in NZ


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