If after checking diagnostic and specific resources you think you may have encountered a notifiable disease or organism, notify authorities and follow their instructions. Do practice good biosecurity to prevent spread, but do not try to eradicate it unless so instructed.

In forest trees in Scotland notify Scottish Forestry via TreeAlert or by email, or if in any other plants notify the Horticulture and Marketing Unit at SASA. Outwith Scotland consult the Plant Health Information Portal for other UK reporting contacts.

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.

In addition to Wilts and Vascular Diseases, pests of roots and wood can cause structural instability. Decaying roots or the presence fruiting bodies of bracket or other wood rot fungi can indicate heart rot, and structural instability in trees.

• Consult an arborist if trees may pose a hazard to humans.
• Wood rot fungi are an essential part of natural ecosystems, and are to be expected on any fallen wood. Most of these are non-pathogenic or only weak pathogens, and are not a cause for concern.
• Wood and root rots in living trees can, however, signal a larger problem or a more aggressive pathogen.
• Prevention is the best way to minimise wood and root rots.
  • Follow biosecurity guidance to reduce opportunities for introducing root and wood rotting pathogens on soil, plants for planting, or tools and equipment.
  • Most wood rot pathogens are weak pathogens but will opportunistically infect wounds or otherwise stressed plants, so minimising mechanical damage will help keep plants healthy.
  • Maintaining good spacing among plants, good drainage, and otherwise un-stressed plants can help prevent root and wood rots.
  • Wood rot fungi are expected to be found on already-dead wood, and usually are not a cause for concern. If, however, an otherwise healthy tree has been heavily damaged or killed by root and wood rots such as Armillaria spp., it can be helpful to remove the stump and as much of the root mass as possible.
  • Wood and roots infected with root and wood rot pathogens should not be chipped and used for mulch or in home compost.

There is very little to be done to treat plants once they are infected with a virus; virus control hinges on prevention.

• When disease is of concern, infected plants should be removed and destroyed by burning or bagged in landfill. Contact SEPA for guidance on burning.
• Follow good biosecurity practice to help prevent virus infections.
• Sterilising tools with virus-preventing disinfectants is crucial where virus is suspected or when working with virus-prone species.
  • Soaking in a solution of 20% nonfat dry milk is among the most effective methods of killing viruses on surfaces.
  • Soaking in a dilution of commercial bleach (1 part bleach to 7 parts water, or ~0.6% sodium hypochlorite) is even more effective at killing viruses, but can corrode metal.
• Controlling or avoiding vectors, such as aphids and nematodes, can help prevent virus transmission.

Most diseases associated with roots and stems are ultimately problems of water or sugar conductance. Pests and pathogens that degrade roots or block xylem prevent water uptake. Those that infect collars and cause trunk cankers degrade the phloem, interfering with sugar transport. These diseases can appear as either a slow decline, cause rapid death of the entire plant or one or more sections, cause rotting of roots, or crown thinning and wilting from new growth downward. Most diseases associated with wood-boring insects are wilts; fungi that grow in the galleries carved by the insects’ larvae interfere with water conductance. Visible signs and symptoms of wilts and vascular diseases can include oozing cankers on the stem or collar, insect exit holes or galleries, fungal fruiting bodies, or knots or galls on roots.

• Check specific threats for statutory listings, and promptly report any suspected cases or any unusual outbreaks.

Avoidance

• Follow biosecurity guidance to keep pests and pathogens out.
• When purchasing plants, buy from trusted, local suppliers, repot to fresh compost on arrival, disposing the old potting media into landfill, and keep plants secluding for several weeks. This gives any latent diseases that may have been suppressed by chemicals, or are becomung established, time to emerge.
• Be especially careful with projects restoring sensitive lands or threatened species. Where there are severe or specific disease threats it may be wise to keep new plantings physically separated from established populations. See for example the Forestry Commission Scotland guidance for planting juniper and the California Oak Mortality Task Force guidelines for restoration and fieldwork.
• Select sites appropriate for the species, especially with regard to drainage. Disease is most likely to occur where water collects, and many of the pests and pathogens behind these diseases are transmitted through water and mud. New plantings in the runoff zone from existing disease centres are highly likely to become infected. On the other hand, drought can compound root and stem problems, and a plant already stressed by disease can be killed off by drought, or vice versa.
• Maintaining airflow and separation among plants will likewise help to reduce the conditions that favour disease, especially root and bole rots. Paradoxically, increased airflow can increase the likelihood of individual plants contracting diseases with propagules that spread in wind or blown rain.

Management

• The appropriate response when disease occurs will be determined by the identity and severity of the disease.
• Oozing cankers can have many causes, and a canker or the presence of wood rot fungi in the absence of other signs of decline, or only slow decline may not require a management response.
• However, keep in mind that an infected plant will be a source of inoculum to plants around it. In cases of severe disease, the plant should be removed and destroyed by burning or in landfill. In most cases, the roots should be excavated and removed. The soil around the roots will most likely contain infectious propagules, so excavation equipment must be carefully cleaned and the soil prevented from moving. For some pests and pathogens the risk associated with disturbing the soil may be greater than the benefit of excavation; check specific guidance for these.

Treatment

• There are few chemical treatment options that are appropriate for root and stem diseases in the natural environment. Avoidance and management of disease, including removing infected materials, are the best course of action.
• In some cases, phosphite compounds (also known as phosphonate) can be useful to provide systemic protection against both oomycete (eg Phytophthora species) and fungal pathogens. These can be used to protect individual plants, but treatments must be repeated, and protection wears off after the treatment is discontinued. Phosphite compounds and other fungicides can also suppress symptoms of infection allowing pathogens to be transmitted undetected, and so should not be used on plants in propagation contexts.
• Check specific threats for more detailed advice.

Wood-boring insects

• Several wood-boring insects are of great concern for arrival and spread in the UK. It is important that any suspected cases are reported promptly, so that their identity can be confirmed and control measures can be enacted quickly. Be sure that authorities have a chance to identify the insects; do practice biosecurity measures to contain them but do not use insecticides or other treatment measures on notifiable pests unless instructed by national plant health authorities.  
• Wood-boring insects are generally attracted to stressed trees. Managing stands of trees to reduce stress will reduce susceptibility. New stands should be planted far from infestations, while it is necessary to inspect thinned or otherwise felled trees for infestations, and if found peel bark to expose brood chambers.
• Biocontrol agents may be available, depending on the pest species.