As part of the experimental agroforestry farm that I want to start, I want to work on the development of chestnut rootstocks resistant to the main diseases present in Corsica.Below I give you an explanation. Two major ecological issues threaten the future of Corsican chestnut groves::
● Chestnut ink disease
● climate change
Ink disease is caused by different Phytophthora oomycetes (P. cambivora and cinnamomi in particular) and causes widespread death of chestnut trees across the island.. There is no effective treatment, so prevention is essential.
.Climate change is mainly manifested by an increase in droughts and water stress which threatens Corsican chestnut trees. Higher temperatures and longer periods without rain lead to higher evapotranspiration, which causes water stress.. Although chestnut trees are used to moderate droughts, they start to suffer when it hasn't rained for more than 2 months (which is becoming more and more frequent in Corsica, as in 2021). Additionally, there is evidence that climate change could make ink disease worse.
In my opinion, one of the main problems underlying the vulnerability of Corsican chestnut trees is the lack of genetic improvement. In nature, each seed produced by a tree, for example by a wild chestnut, is genetically unique. Some of these seeds have genes that make them less susceptible to certain diseases, such as ink disease. Others have genes that make them highly susceptible to a particular disease.
In natural ecosystems, trees are continually changing. Natural selection causes the "weak seedlings" - those not adapted to the local environment - to die, while the few "lucky" seedlings which, due to their genetics, are well adapted to the local environment, survive..This is how nature has always worked. Problems arise when we humans interfere with this process. With fruit production, this is exactly what happened.. Whether chestnut, apple or olive, humans have been grafting certain cultivars onto rootstocks for centuries rather than propagating new trees by seed. Grafting is a means of vegetative (i.e. asexual) propagation and is literally a form of cloning. Why did mankind do this? Because we want to preserve a certain delicious cultivar once it is developed or discovered.
The same problem applies to Corsican chestnut trees. Many Corsican cultivars are at least 300 years old and because they have been grafted for centuries, they have not evolved genetically. These chestnut trees therefore remained genetically identical for several centuries...
Why is this problematic? Because the environment of these trees has radically changed! The climate has changed, three different exotic pests/diseases have been introduced to Corsica (gall wasp, ink canker disease) among others.Traditional Corsican cultivars have never been used to deal with these new stressors and because of this they are often not suited to deal with them at all.
Moreover, the rootstocks onto which these cultivars are grafted have never been developed to resist ink disease or drought whereas the rootstocks determine much of the resistance to disease and drought, not the graft (hence the cultivar).
A relatively simple solution exists, however, to make Corsican chestnut trees more resistant to modern threats by following nature's example and selecting new resistant rootstocks and cultivars via sexual propagation. Let me explain what I want to do in detail.
I want to work on two key challenges, ink disease and drought resistance and mimic nature's selection process which I call "natural mass selection"..
The idea is the following.
Thousands of Corsican chestnuts are harvested and planted very close to each other (30 cm from each other). Then the following steps are followed (see Image 1) for an illustration of this process):
(1) Deliberately exposing them to a certain environmental stressor (such as ink disease and drought) in their second year
(2) Observe which trees resist best
(3) Propagate these trees vegetatively (cloning)
(4) Expose them again to this stressor to check their resistance
(5) Cross trees that seem resistant to get a 'pure line' - that is, trees in which all of their seeds (chestnuts) have the desired characteristic (strain resistance).
By choosing only Corsican chestnuts to look for more resistant rootstocks, I work with the chestnut tree best suited to the Corsican context. In many other European regions hybrids (crosses between European chestnut and Japanese/Chinese) are used but the problem is that they often require a lot of irrigation and fertilization because they are not suitable for our climate.
Image 1. Illustration of the genetic improvement program for Corsican rootstocks resistant to ink disease.
I want to find ink disease resistant rootstocks first, then drought resistant rootstocks. Ultimately, the goal is to combine “ink disease resistance” and “drought tolerance” characteristics. The final objective: rootstocks resistant to ink diseases AND to drought that can be used for future chestnut groves.
Finding rootstocks resistant to chestnut ink disease.
I want to find live chestnut trees that are surrounded by dead trees downed by ink disease as this might indicate that the surviving trees have some resistance to the disease. Then I want to take their chestnuts and also root cuttings to clone the rootstock, and use these chestnuts to test the ink disease resistance.In a controlled environment (same soil, same precipitation etc.) I will plant the chestnut trees about 30 cm from each other and after a year I will expose them to Phytophthora. Then I expect most young trees to die or have severe lesions but I also expect to find individuals that seem to survive. These individuals are then isolated, multiplied and studied in more detail (possibly in the laboratory) to check their resistance to Phytophthora.
To be successful, a large number of chestnut trees are needed to ensure that it is statistically possible to have resistant trees. If this process is successful, resistant trees can be propagated and used as rootstock for future plantings. This will greatly increase the chances that your chestnut tree will not die of ink disease when you plant it.
Finding rootstocks that tolerate drought well thanks to a highly developed root system.
For drought tolerance, I want to focus on young chestnut trees because for new plantations it is essential to use rootstocks that allocate a large part of their energy to their root system from an early age. This allocation is a genetic characteristic and selection may therefore find individuals who invest very little in above-ground growth in the early years as this means the trees do not have enough energy available to stimulate deep rooting behavior.
Again, I want to take thousands of Corsican chestnuts and expose them to varying levels of water stress. The trees that survive the highest stress will be dug up to measure their root development and I hope to find individuals that can be used as future rootstocks to resist drought.
Evidence from experimental work by world-renowned agroforestry farmer Mark Shepard, based in the United States, shows that this method works perfectly. Shepard has created chestnut trees that are resistant to chestnut blight (another important disease), extreme cold, and high yielding using this method and details his experiences in his book “Restoration Agriculture” (Shepard, 2013). He used the same method (planting seeds) to create new apple cultivars with good taste and great disease resistance and his work is an inspiration for the work I want to do here in Corsica.
II Create a beneficial environment.
Genes are not fate. Genes can load the gun, but the environment pulls the trigger. Like people with a certain predisposition to a disease, having the "bad genes" does not mean that you will actually develop the disease. Why? Because environmental factors often outweigh genetic factors.
In the case of ink disease, research has shown that different interventions can significantly reduce a chestnut tree's risk of developing the disease.
I Test different types of manure and compost
Antagonistic microorganisms are effective in reducing the number of ink disease oomycetes in soil although evidence is scarce (Aryantha et al., 2000; Choupina et al., 2014). Antagonistic microorganisms can be introduced into the soil with additions of manure or compost. Several types of composts and manures have been tested to better understand their potentially antagonistic function. One study found that adding compost and composted chicken manure was effective in reducing the survival of Phytophthora species (Aryantha et al., 2000). Chicken manure and compost contain antagonistic beneficial microorganisms such as certain fungal species and fluorescent pseudomonads, and these microorganisms exerted a strong negative effect on the development of P. cinnamomi (Aryantha et al., 2000).
Another reason for the effectiveness of compost and manure is their beneficial effect on the soil;organic matter helps improve soil structure, reducing the likelihood that Phytophthora oomycetes can thrive.
I would like to test a large number of manures (cow, hen, horse etc.) and compost (worm, vegetable etc.) and straw to see how the addition of organic matter can protect young chestnut trees against ink. How do I want to do this? I want to plant a large number of chestnuts in rows and each row is fertilized with a different type of compost/manure. Then Phytophthora oomycetes are added and mortality is recorded. What supply of organic matter best prevents the development of oomycetes?
Furthermore, I want to test the impact of different composts and fertilizers on the water retention capacity and therefore on the drought resistance of the trees. The objective of this research would be to be able to recommend to Corsican chestnut farmers what management reduces the risk of losing trees to ink disease
Image 2. Illustration of the experiment.
II Testing the inoculation of young chestnut plants with beneficial ectomycorrhizal organisms
Inoculation of chestnut seedlings with ectomycorrhizal fungi has also been shown to be effective in preventing ink disease (Branzati et al., 1999). The ectomycorrhizal fungi 'Laccaria laccata', 'Hebeloma crustiliniforme', 'Hebeloma sinapizans' and 'Paxillus involutus' have been found to protect the roots of C. sativa seedlings against infection by P. cinnamomi and P. cambivora (Branzatiet al., 1999). When young chestnut plants (seven months old) were inoculated with these four species of ectomycorrhizal fungi and with a Phytophthora fungus (while a control group was not inoculated with mycorrhizal fungi), the inoculated plants showed no signs of disease (Branzati et al., 1999). The inoculated mycorrhizal plants had better rooting and shoot development than the control group. The protective mechanism was the fungal mantle which functioned as a mechanical barrier against Phytophthora infection; chestnut roots with a fungal coat showed no infection by either Phytophthora species, while non-mycorrhizal roots of control seedlings were heavily infected with pathogens (Branzanti et al., 1999). This suggests that certain species of ectomycorrhizal species are promising candidates for increasing chestnut resistance to Phytophthora.
It can be concluded that there are suggestions that certain species of ectomycorrhizal fungi protect chestnut trees against ink disease in C. sativa, but more research needs to be conducted to find out which species are the most promising and whether this can be made into a product (eg an inoculum of certain species) which can be applied when planting young chestnut seedlings.
I would like to test several ectomycorrhizal products available on the market to see which products best protect Corsican chestnut trees. To do this, I plan to plant a large quantity of Corsican chestnut seedlings still in rows and each row corresponds to another treatment (another tested product). All plants are therefore treated with an ectomycorrhizal product as well as Phytophthora oomycetes and, as in the example above, mortality is recorded.
In addition, I want to test the impact of different ectomycorrhizal products on the water retention capacity and therefore on the drought resistance of trees. The objective is therefore very concrete; I do not want to research which particular species of ectomycorrhizal works best, but which product already available on the market is the most effective in order to be able to advise chestnut growers what treatments to do when planting young chestnut plants. I would like to collaborate with the Regional Group of Producers and Processors of Chestnuts and Chestnuts of Corsica, the Chamber of Agriculture of Corsica and the University of Corte because this project requires long-term collaboration with local partners to make it a success..But the most important thing is to find land for sale or long-term rental to be able to start my experimental farm and this research project. If you have any leads, don't hesitate to contact me!
Bibliography
Aryantha, I. P., Cross, R. and Guest, D. I. (2000) 'Suppression of Phytophthora cinnamomi in potting mixes amended with uncomposted and composted animal manures', Phytopathology, 90(7), pp.775-782.doi: https://doi.org/10.1094/PHYTO.2000.90.7.775
Branzanti, M. B., Rocca, E. and Pisi, A. (1999) 'Effect of ectomycorrhizal fungi on chestnut inkdisease', Mycorrhiza, 9(2), pp.103-109.doi: https://doi.org/10.1007/s005720050007.
Choupina, A. B., Estevinho, L. and Martins, I. M. (2014) 'Scientifically advanced solutions for chestnut ink disease', Applied microbiology and biotechnology, 98(9), pp.3905-3909.must:https://doi.org/10.1007/s00253-014-5654-2
Shepard, M. (2013) Herstellende voeding: complete voeding uit meerjarige landbouw.Herstellend landbouw.Utrecht: Jan van Arkel
.Pepels, N (2020) ‘The revitalization of Corsican chestnut groves’, dissertation, ISARA & Wageningen University.Available at: www.lejardinuniek.com/recherche