EBTS UK Report on ‘Scientific symposium on boxwood pests’
Held on 16-17th October 2018 in Tours, France
The conference was held to discuss the knowledge and progress in dealing the various pests and diseases that affect Buxus. Around 130 people attended the event organised by Vegephyl and held in the Auditorium Theleme of the University of Tours. During the two full days of presentations there were over 30 different speakers from France, Belgium, Canada, Germany, Hungry & Switzerland.
The headline on blight was the announcement by Didier Hermans of Herplants, that after 9 years, they had successfully created 4 hybrid boxwood plants that are resistant to blight and will be sold commercially from Spring 2019. Unfortunately for most, due to the very limited number of growers, currently only Herplants (who own the commercial rights to the hybrid), the UK is not likely to be able to purchase stock for some time. The Château de Villandry will be the first place to benefit from the new plants as part of its Buxus rejuvenation programme.
Blight Resistant Hybrids
Didier and his team have worked for nearly 10 years to get to the point where his company can commercially grow blight resistant Buxus plants. This achievement and dedication was given due prominence by being one of the first presentations after an explanation by Hubert Puzenat (landscape architect & technical advisor to one of the 3 public Buxus collections in France, Château du Grand Jardin de Joinville) on the genus of Buxus, its main species and variations. This explanation helped put in context the work done by the team in breeding the hybrids.
Didier then explained how they set about breeding the blight resistant hybrids which involved assessing data on growth shape, vigour, frost resistance, leaf shape, winter colour, disease resistance, botanical characteristics, ploidy levels (number of sets of chromosomes), genome size and genetic relationship. Herplants has over 200 different Buxus species in its collection and in 2008 it cross pollinated the 100 most resistant species to create 10,000 hybrid plants which were later reviewed on a number of criteria. In 2013 3,000 were inoculated with Cylindrocladium buxicola to see how they coped. By 2014 this number was reduced to 150 as the selection to take forward to field tests before just 4 were chosen in 2015 as the final selection.
These four represent 4 distinct habits that Herplants hope will cover most designers needs and therefore be commercially appealing. They are low, medium and tall growing for hedging with the fourth described as special as it has a slightly less vertical habit. All 4 hybrids are from different genetic backgrounds which it is hoped will help with possible future disease issues.
EBTS UK spoke to Didier at the event and invited him to come and talk about how this has been achieved and explain the properties of the 4 different hybrids.
Katrijm Van Larer from the Flemish Institute of Agriculture, Fisheries and Food Research in Belgium worked with Herplants carrying out the laboratory growing trials. This research was challenging as it is a long process to cross pollinate all 100 species with each other, then collect the resulting seeds (very small), germinate the seeds, grow the plants, inoculate them and assess the results. Rather than testing on full grown plants, the initial tests were done on seedlings to speed up the process.
The inoculated plants were kept in mini poly tunnels to give a controlled environment. 1 week after inoculation the seedling plants were assessed for lesion size. Between 2008 and 2014 the process of weeding out the least resistant plants took place. Further trials included tests on the hybrids based on plant separation to show susceptibility to infection based on proximity to infected plants. To make sure they didn’t miss a hybrid that might not have liked the lab conditions, they put all the plants into the field after the lab tests to see how they fared. The outcome of the field tests didn’t through up any surprise candidates and fully backed up the lab results.
In the end the 4 most resistant hybrids were selected, theses showed an order of magnitude more resistance to blight than the best-known variety to date Buxus macrophylia ‘Faulkner’.
Symptoms & Diagnostics
During the two days of the conference, Laurent Jacob of ASTREDHOR the French Technical Institute of Horticulture gave some very technical information on Buxus diseases, how to spot them and the route of some infections. He also pointed out that some issues that appeared to be disease symptoms weren’t, for example salt & urine can cause yellowing of leaves, too much or too little water will cause leaves to change colour, spraying weed killer on grass near hedges can cause browning of leaves or reduced growth where the wind has blown the spray. All these can display what look like symptoms of disease when disease isn’t the problem.
Laurent stressed the importance of assessing the time frame for the appearance of the symptoms and the context in which they appeared. For example, had the amount of water given or around the plant changed or had there been a change in wind direction/temperature since the appearance of the issue, all these things are important to getting the right diagnosis of the problem. Another key point is that although a plant might show an effect of a problem this is secondary effect of the primary issue which may well be elsewhere in the plant such as in the roots. Laurent did say the only way to fully diagnose a problem is to dig up the plant, so you can inspect the roots, but that’s a bit drastic, so you need to check soil conditions around the roots as well as just the plant above ground.
Orangey or reddened leaves on potted box
Shortage of food – repot every couple of years and feed
Orangey leaves on one side of plant
Browning of some leaves
Lack of organic matter
Yellowing around edges of leaves
Can indicate a root problem causing the blocking of magnesium & calcium
Brown circular bumps on underside of leaf and underside of leaf eaten
Box tree midges
Spooning of leaves
Tom Hsiang of the Environmental Sciences department of the University of Guelph, Ontario, Canada talked about the asexual nature of Volutella blight whose fungal spores can still be viable on dry leaves after 6 months. The disease cycle is quite quick with the spore gemination and penetration of wounds with just 18 hours of wetness. These spores infect younger tissue or susceptible parts of the plant which go on to produce sporulation (the creation of more spores) causing greater infection if not treated. With Volutella it is best to treat pre-emptively if possible and it is most prevalent during spring and autumn.
Salmon-pink spore clusters on the leaves and stems formed during periods of wet weather + cankers on shoots and stems resulting in dieback of the shoot ends and leaf death.
Control of Box Blight
Bruno Gobin & Laurent Jacob discussed the genetics of Cylindrocladium which has been spreading rapidly since 1995 and challenging scientists to find a cure or control for the fungal spores. They said there are two genotypes G1 Calonectria pseudonaviculata and G2 Calonectria henricotiae, with G1 being the predominant type in Europe.
Bruno explained that research has found that both genotypes are as aggressive and have the same pathogenicity. Key factors for the spores to infect a plant are wetness and temperature (note: spores do not need a wound to enter the leaves). They also looked at pathways of spreading the infection trying different groupings and distances apart from plants that had been inoculated with the spores.
A ‘Host Resistance summary’ table was displayed showing a ‘weighted disease aggressiveness score’ i.e. most susceptible to least susceptible species/cultivars.
They also ran some efficacy trials on inoculated plants to test 14 different fungicides to look at both preventative and curative qualities.
Real Life Experience of Blight
A couple of experiences of blight were presented, one by Paulo Fornara of the Parks Service and Domains of the city of Lausanne, Switzerland which will be covered in a future piece detailing different regimes and the following by Laurent Chabane, Head Gardner at Manoir d’Erygnac.
Laurent gave a timeline for how things had happened.
|2009||Cylindrocladium wasn’t considered a problem for the garden|
|2011||Some spot chemical curative treatments were used|
|2013||Bad attack mainly in areas where lawns were watered – the watering systems were changed to avoid wetting the hedges|
|2015||Started using 4 difference organic products in rotation to fertilise their plants (including a 20% urine feed, plant extracts and algae)|
|2016||Continued the treatment monthly from March through to October|
|2017||This wasn’t a good year as treatments weren’t possible in June due to rainfall|
In addition to changing the watering system and using of organic products to enhance the soil, hedges were reduced in height & depth and selectively pruned to decrease the density. Laurent said that all their Suffruticosa was affected, but it has all recovered given the feeding, spraying and pruning/clipping regime they were using. He also mentioned the use of Bordeaux Mix (use no longer allowed due to high copper content) and Black Soap (sprayed to reduce a range of bugs).
Box Tree Moth & Caterpillar
Information was presented on a number of different aspects of the box tree moth/caterpillar and video of the life cycle of the caterpillar and moth was shown (this will be available on the EBTS UK YouTube Channel). Daniel Sauvard of INRA talked about the way the moth had spread across all regions of France in just eight years after its discovery in 2008. INRA therefore did lab research to look at the potential distance a moth can fly with an aim to helping chart where the next localised issues might be. The current information had shown that moths flew up to 10km in their lifetime. In the lab however, it was found that when you measured the distance flown by a moth attached to an arm on a central pivot, such that when the moth flapped it’s wings the arm rotated thus allowing measurement of distance, moths flew between 6.6 to 45km. The reason for the difference from the 10km figure is that moths don’t fly in a straight line in the wild.
- 125 female and 84 male moths were tested
- Cumulative flying abilities for females 0-153km – median 9.9km
- Cumulative flying abilities for males 0-68km – median 3.3km
- Flight capacities decreased during the year with the first flight being 2/3rds greater than the second flight which was 4 times the distance of the third flight
- The maximum recorded length of a single flight was 13km
- Length of flights shorten as the moth gets older
- Male moths only live half as long as the females
The movement around the county was also mapped based on the genetic makeup of the moths and this proved that ‘active’ dispersal was taking place i.e. man made not just natural flight.
Cyrill Kruczkowski of Fredon CVL (a network of independent experts in the service of plant health, the environment and mankind) discussed the financial impact of moths and caterpillars on parks and gardens (public & private). Their aim was to give guidance on when the best time to spray was, but their monitoring network suffered from inconsistent reporting due to funding and lack of standardisation of tools used (traps and pheromone types).
Monitoring was also the theme of Maxime Guerin’s talk which introduced the EPIPHYTII database which is collecting data on pest frequency/intensity and also looks at growing conditions and weather. There was more on monitoring from Jean-Claude Martin of INRA who gave information about a smart phone app called AGIIR they had written to allow citizen collection of sightings of invasive pests (available from the Google Play & Apple App Store).
Box Tree Moth & Caterpillar Predators & Parasitoids?
A number of speakers gave information on using native species to try to control infestations. To date, about twenty natural enemies has been identified: 6 species of predators including 4 in Europe, 12 species of parasitoids including 4 in Europe or Russia. There is one species of entomopathogenic fungus – Beauveria bassiana which was found in Iran (though so far in tests it has insufficient efficacy).
Maxime Guerin of Plant & Cite presented a summary of current position based on 330 international publications:
- Parasitoid insects worked in Europe and Russia in the laboratory since 2009, with some promising leads to intervene on eggs
- Predatory insects: worked in Europe under controlled conditions since 2015, with some opportunities to intervene on eggs or caterpillars
- Entomopathogenic fungi: worked in Asia since 1991 and in Europe. The strains tested have insufficient efficacy
- Entomopathogenic bacteria (Bacillus thuringiensis): worked in Asia since 2004 and in Europe. This is the most promising and already widely used to manage box tree caterpillars
- The bacterial extracts: worked in Europe and Russia since 2015, some interesting ideas but work must continue before considering marketing
- The insect viruses: worked in Europe and Russia since 2013, some interesting ideas but work must continue before considering marketing
- Entomopathogenic nematodes worked in Asia since 1991 and in Europe. The species tested have insufficient effectiveness to be used alone
- The plant and animal extracts: worked in Asia since 2004 and in Europe, with varying efficiencies. Some extracts insecticide and / or repellent are sufficiently promising and should continue to be processed
In addition to this in France sparrows, chickadees, blue tits, coal tits have all been observed to eat the caterpillar but currently have only a minimal impact. The original concerns that the toxicity of the caterpillar was the reason for birds not eating them might not be the case. In one test bird boxes were placed in an area of infestation. The birds that nested caught caterpillars and fed their young with them – no young birds were found dead in the nests at the end of the season, so toxicity, certainly in the short term, doesn’t appear to be an issue and it maybe that the birds just need to acquire the taste for the new source of food.
Annette Herz of the Julius Kuhn-Institute (JKI) Germany summarised the research carried out on the effectiveness of Entomopathogenic nematodes (EPN – nematodes that cause death by disease in insects) and how best to apply them.
- carpocapsae caused 100% mortality
- Effective on larval stages L2 to L4 but no effect on pupae
- Watering before application has a positive effect
- Pressure sprayers better than blower sprayer
- Can persist on foliage for up to 7 days
- Effective rates in the field between 16-72%
It was also noted that although nematodes can be used to help kill off unwanted pests, others can also be the cause of problems like root knot.
There are around 200 species worldwide with 60 species in Europe. These wasp parasitoids lay their eggs in the eggs of their hosts preventing hatching of the pest larva and thus reduce feeding damage on the host plant.
4 native and 4 non-native species were tested by JKI in Germany and although the lab trials were successful, in the field they failed to get the same positive results with near zero effect.
- Most effective between 20-25c
- Die if temperature drops below 10c or goes above 35c
- 1st generation are the most effective as sex ratio of offspring is often not good with significantly more of one sex than another
- Extreme care needs to be taken introducing a non-native trichogramma to avoid damage to non-target species
Larval Frass Volatiles
Peter Bela Molnar of the Plant Protection Institute at the Centre for Agricultural Sciences in Hungry gave a talk on the use of a synthetic blend of larval frass volatiles to deter egg laying by female moths. This is the same research that has been used by a German company that EBTS has been trialing at Ham House in South West London.
During a lunch break I was able to talk with Peter who revealed that his research was made much easier due to so many caterpillars and moths being available at the time of his research due to the levels of infestation in Hungry. However, it wouldn’t be so easy now as there is almost no boxwood left in the country.
Biological Insecticide - Bt
There were many mentions of and discussions about Bacillus thuringiensis (Bt) throughout the conference and it was regarded as the best current form of defence against caterpillars in garden and park situations. However, the number of times it can be used per year varies from country to country. In the UK it is 8 treatments per year but in Germany, only 2 are allowed which given there are three life cycles in the southern part of the country it can’t be used effectively. There was also concern about the build-up of resistance to Bt, though no evidence was presented to show this to be the case. Some speakers recommended using different strains of Bt in rotation (some are more effective depending on the generation of caterpillar in the year).
One comment made at the beginning of the conference raised a smile, the idea of splicing Bt DNA into boxwood so that it naturally contained the Bt bacteria and would thus kill the caterpillar when it eats it. However, the EU has some of the toughest regulations on genetic manipulation, so it isn’t likely to happen any time soon – the US has very different rules and given box moths have been identified in Toronto it seems likely it will soon crop up elsewhere.
Natural Boxwood in forests and open spaces
A number of speakers gave information about how the destruction of natural areas of box was affecting the ecosystems. The consistent message was not good, the consensuses was that not enough has been done to save box in the wild and that it is unlikely to happen because funding is not available. Treatments that had been tried using drones, aerial spraying etc had not worked as because coverage with either Bt or Trichogramma was not high enough to control the pest.
A map of France showed the areas of natural box where it was either dominant or codominant, these ranged from sea level up to 1600m, were of importance for species to shelter in and had a near neutral pH. The forest of Buxberg dans le Sundgau had 300-year-old boxwood forming the under story of 4 hectares of broad-leaved woodland when an infestation was reported in 2013.
The box has since been fully defoliated and has not recovered even after there are no more moths/caterpillars. This is because when it attempted to regrow, it dries out and dies over the summer because of the lack of shade from other boxwood plants. In another area, Auvergne-Rhone-Alpes, where a similar situation occurred the understory has also not recovered, now oak trees are appearing which assuming the survive will give a very different ecology for the forest being a deciduous tree.
In addition the moth feeds on the same nectar as other insects so depletes the available source of food further disrupting the ecology of an area.
Other issues caused by the defoliation and death of the boxwood:
- Fire – were the box has been eaten and dried out, there is a significant increase in the risk of forest fires, this lasts whilst there are dead leaves still on the bushes but reduces when they fall off.
- Rock falls – if the plants were on hillsides then there can be increased erosion and without the plants to bind the soil together
- Pollution of water – as the soil is eroded this can lead to poisoning of water courses as the natural filtering done by the soil is removed
During the round table discussion at the end of the conference the following points were made about the use and care of boxwood in parks and gardens across Europe.
Don’t give up on boxwood trees, wherever possible effort should be made to support them.
There isn’t a one size fits all approach and it should be used appropriately
We need to protect what we already have and be ready for the moth by making ourselves aware
Box is part of our artistic heritage and should be saved
Some elements of garden aesthetics could disappear with out buxus
Buxus will become something for specialists again
Mark Jones – Buis de Beausséré
Historic gardens are still keen on boxwood and want to keep it
Increase the variety of plants used and improve the health of the gardens
Boxwood will never be what it was in our forests
Report by Chris Poole