This fungal dieback disease seems to have increased in significance on garden conifers in recent years. Weakened plants, or those suffering from physical damage, are particularly susceptible to attack.
What is Pestalotiopsis?
Species of the fungus Pestalotiopsis are often found associated with leaf spots and diebacks, not just of conifers but of a very wide range of woody plants. They are usually regarded as being weakly pathogenic, that is they require a plant to be weakened or damaged in some way in order to colonise it. However, once within the plant tissues they can sometimes cause quite extensive damage.
Pestalotiopsis is often found colonising the shoots of conifer hedges that have suffered initial damage from aphid infestation. Wet weather favours dispersal of the spores and their ability to infect the plant.
You may see the following symptoms:
- Leaves turn yellow and then brown, often progressing back from the tip of a shoot
- A constricted, girdling area of browning sometimes develops on a twig, with the entire twig dying beyond this point
- Numerous black, ‘pinhead’-sized fruiting bodies, just visible to the naked eye, develop within the affected plant tissues
- Under wet conditions, thin black tendrils of spores ooze from the fruiting bodies
N.B. The development of brown patches and shoot dieback in conifer hedges can have a number of different causes (e.g. aphid damage, Pestalotiopsis, pruning at the wrong time of year, or various interactions between these factors). Laboratory examination of samples of affected shoots may be required to determine the precise cause(s).
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods,
cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
Try to avoid or prevent any of the factors that can lead to plant damage or stress. For example:
- Prevent or control pests such as cypress aphid
- Ensure that newly-planted conifers establish well
- Water plants during periods of extended drought
- Avoid unnecessary physical damage
- Prune hedges at the correct time of year (hedge pruning times)
- Avoid pruning during prolonged periods of wet weather (which are suitable for spore germination and infection), or during drought conditions (which can itself result in dieback)
If dieback due to Pestalotiopsis is diagnosed, pruning out the affected shoots will reduce the number of spores available to
set up new infections. However, remember that many conifers will not produce new growth if they are cut back hard into old growth.
There are no fungicides available to amateur gardeners with specific recommendations for use against Pestalotiopsis. However, the fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are labelled for the control of other diseases on ornamental plants, and could therefore be used legally on conifers (at the owner’s risk) to try and control Pestalotiopsis.
There is no specific information available as to the efficacy of these products against the fungus, however. It is possible that repeated sprays would be required where the disease is present, particularly during unsettled weather.
The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Resolva Rose 3 in 1, Rose Shield Bug & Fungus Killer, Roseclear Ultra Gun 2, Rosegarde) and triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun).When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.
Pestalotiopsis species are regarded as weak pathogens, requiring a plant to be physically damaged or stressed (which itself can lead to tissue damage) in order to colonise the plant tissues. Very occasionally, the fungus is thought to be capable of attacking intact, healthy shoot tips.
Factors that can predispose a plant to colonisation by Pestalotiopsis include:
- Pest damage – cypress aphid (Cinara cupressivora) and other conifer aphids, for example, can cause significant damage on conifers such as Cupressus and × Cuprocyparis
- Weather or soil conditions, e.g. drought, waterlogging, wind damage
- Other forms of physical damage, e.g. from pruning or hedge trimming
- Root disease problems such as honey fungus or Phytophthora root rot
- Stress caused by poor establishment
Once it is active within the damaged tissues, the fungus can use these as a foothold from which to progress into adjacent healthy parts, extending the dieback.
The black spore tendrils, which ooze from the fruiting bodies under wet conditions, contain huge numbers of microscopic spores. These are splashed around by rain droplets, and can germinate to cause new infections if the leaf or twig surfaces stay wet.
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