Control

Plant diseases and their treatments can be categorised into broad types, and similar responses are recommended within these types.

 

Note

The advice provided in this site is intended as a guide only.
 
In undertaking any suggested method for destroying diseased or infected trees or plants that involves applying any chemical treatments, it is important to read the manufacturer’s instructions for use and associated safety data.
 
Take all necessary safety precautions if burning infected plant material and never burn material where there may be any risk to life or property. You should always consult SEPA prior to burning material to ensure you have any required permissions or licences.
 
You should seek further advice from the relevant authorities listed in this site if you are in any doubt as to how to treat or destroy infected plants or trees.

About This Page

If you can identify the cause of a plant disease, it is always best to check specific resources for tailored advice to control it. This is particularly true for notifiable pests. It is often the case however that notifiable diseases can be ruled out, yet the precise cause is not known or falls into a broad category such as 'fungal root rot' or 'leaf rust'. For this situation we have compiled generalised advice for the control of different kinds of plants diseases. This advice will usually apply to most diseases within the category and may be taken as general guidance, but note that exceptions will abound!

Getting Started

If you are uncertain of the type of problem affecting a plant, follow these steps:

  1. Determine the general cause and the extent of the disease symptoms. Look for insect damage such as leaf galls or insect exit holes in the bark, or for signs of fungi or other organisms such as brackets on boles of living trees or leaf spots. Note that some damage can be caused by abiotic factors (such as frost). A field guide such as the CABI Field guide for the identification of damage on woody sentinel plants can help to pinpoint causes of disease.
  2. Use Recommended Sources to try to identify the causal organism and to determine if it is a common and or/native species, and whether it is disease-causing or just a saprophyte (living of the dead plant that something else has killed).
  3. Search the Threats page by Host for diseases on the Pest Risk Register to determine whether you need notify authorities. If you think it might be a notifiable organism notify authorities and follow their instructions – do not try to eradicate it unless so instructed.
  4. If you can identify a causal organism, consult the listed resources for specific guidance on control. For generalised advice, check these Prevention – Biosecurity  and Control pages to learn ways to help keep the disease from spreading, and what kinds of control responses are available for non-notifiable diseases.

Disease Types

  • Localised feeding or infections can cause lesions or discolouration on leaves, flowers and fruit, leaf loss, or in severe cases progressive dieback from branches that can eventually kill the plant.

    Branch dieback

    Branch dieback refers to dieback that originates in leaves or twigs and progresses to localised death, and in severe cases can kill the plant. These diseases are often accompanied by fungal cankers on affected branches.

    They can be difficult to distinguish from problems of water conductance originating in the roots or stems, which can cause wilting, shepherds crooks, thin crowns, or dieback progressing from new growth inwards.  

    • Branch dieback can often be controlled by out pruning dead or diseased material. Make cuts at least 20 cm below the visible lesion, and disinfect tools between cuts. Collect and remove debris and fallen leaves to prevent this material from re-infecting nearby plants. Dispose of the infected branches and debris by burning or landfill. Contact SEPA for guidance on burning.
    • Check specific threats for the best time of year or weather conditions to prune, in order to avoid creating wounds at times when they are most likely to be infected. For most pathogens, this means cold, dry weather is best for pruning.

  • Problems limited to leaves and twigs

    Some degree of leaf feeding by insects, leaf spot and/or rust is normal and unavoidable, and may not require management response unless it is caused by a reportable high-priority pest, or where plants are being managed for production, e.g. fruit or wood. In the case of reportable pests, follow regulatory agency guidance.


    Where disease is severely affecting plant health, some control may be possible.

    • Pruning and removing diseased leaves can help, especially if done early in the season before symptoms are very widespread. This is generally helpful only when a small number of leaves are symptomatic; removing a lot of leaves can harm the plant more than it helps.
    • Likewise, physical removal of insect pests can be very effective for smaller outbreaks, and can be the most effective control option for insects including scales, which are susceptible to chemical controls for only a limited part of their lifecycle.
    • Many rust diseases require two plant species to complete their life cycles (e.g. Melampsoridium hiratsukanum, alder rust, overwinters on larch); the severity of these alternating-host rusts can sometimes be reduced by removing one or the other of the two hosts.
    • Maintaining distance between plants of the same species can help reduce disease, as can planting in appropriate sites to reduce overall stress. Poor nutrition can make plants especially susceptible to foliar fungal diseases, particularly those that cause necrotic lesions (spots of dead tissue). On the flip side, a nitrogen-rich environment can favour biotrophic fungal pathogens including rusts.

    The use of chemical control should be carefully considered before being introduced to the natural environment.

    • Insecticides may be available for treatment of severe insect problems. Unintended effects on non-target insects should be carefully considered before treating, and plants in flower should never be treated with insecticides to avoid danger to bees.
    • Phosphites and other fungicides may be available for some branch and foliar pathogens, and certain types provide systemic protection against a variety of disease causes.
    • However, fungicides can also suppress symptoms of infections allowing pathogens to be transmitted undetected, and should not be used on plants in propagation contexts. Check the specific threat and sources such as the RHS for advice.