Conservation of Endangered Plant Species in the South-West (continued)

by Ros Smith

As the saying goes, ‘a lot of water has flowed under the bridge’ since previous research was reported in the 2006 CGT Journal (pp.21-3). For new members a brief explanation of this earlier work is necessary: Cornwall’s old gardens contain a wealth of ancient and historically important plant introductions and hybrids. Conservation of these important plants is difficult by conventional means. Woody plants such as rhododendrons become increasingly difficult to propagate from cuttings because as they age their rooting ability decreases. Micropropagation overrides any age inhibitions and can produce a number of rooted plantlets which are clones of the parent. The process takes place under sterile laboratory conditions and involves decontaminating the surface of small pieces of plant tissue and growing/multiplying them up aseptically in a special nutrient jelly.

Work had begun to conserve the historical collection of rhododendrons from Heligan gardens, but the young vegetative shoots were proving too difficult to decontaminate. There were just too many cracks and crevices for fungi, bacteria or other contaminants to lurk in and survive the decontamination processes. The smallest fungal spore remaining could and did set up a thriving colony of mould which overwhelmed the precious plant material. We had started to research the use of more protected plant parts to try to overcome this problem and had reported that the use of floral tissues seemed very promising. This technique has, indeed, enabled over 250 rhodo-dendron species and cultivars to be processed with a far higher success rate.

The new technique uses the tightly closed flower buds. These are surface sterilised, and then the protective scale leaves are removed under sterile conditions. Individual florets complete with flower stalk are placed into the nutrient jelly and after 8-12 weeks in culture, small shoots begin to appear from the flower stalk (see Fig.1).

sod1sod2sod3

In time these become large enough to remove and grow on and will eventually develop into well rooted plants (see Fig. 2). The rapid response of these floral tissues appears to be dependent on the stage of development of the floral bud: taken too early in the season and the flowers are not sufficiently developed; too late in the season and the scale leaves are not so tightly closed and contaminants may creep in. Nothing is straightforward! Research continues in this area. It was found that well developed vegetative buds could be treated similarly and where complete decontamination has been achieved, multiple shoots have been produced at the cut surface (see Fig. 3).

There are other historically important plant genera which are under threat by virtue of their age, climatic changes and disease. Two genera which play a large part in the spring gardens’ display are Magnolia and Camellia. In theory any plant can be micropropagated, but in practice this is not always easy. The latter applies to the magnolias and camellias that we are working on. It is just a matter of finding the correct recipe of ingredients that will enable them to thrive and multiply under our laboratory conditions. There are a number of magnolia pieces growing and multiplying in jelly, but as yet they refuse to produce any signs of roots. Research is underway to try and root these as micrografts, that is assuming that our magnolia seeds can be successfully germinated to become the rootstock!

Camellias, too, have their problems. Young shoots start to develop nicely but suddenly decide that the formulation is not to their liking and turn brown and die! It is very frustrating to follow protocols published by researchers from abroad and find that their results cannot be duplicated.

Disease, as mentioned earlier, can also be a threat to old historic plants. The recent identification in 2004 of Phytophthora ramorum and then P. kernoviae on rhododendrons in the South-West was the beginning of serious micropropagation to conserve the rare and historic plants from Cornwall’s old gardens. It has proved difficult to identify the important rhododendrons from some historic gardens as planting plans have been lost through the passage of time or the younger generations may not be certain of names or unfortunately not interested in their horticultural heritage. However there are rhododendrons, both named and unidentified, in micropropagation from 21 of Cornwall’s gardens. The micropropagation laboratory at Duchy College, Rosewarne, is extremely grateful for the financial support from the CGT which has enabled this Cornish plant conservation programme to continue. Sudden Oak Death diseases, as they are commonly named, are still advancing across the country particularly in the milder and moist areas warmed by the Gulf Stream, and have reached Scotland. The laboratory is also helping to conserve important rhododendron collections from further afield.

Identification of Phytophthora ramorum (Sudden Oak Death)
Two websites give photographs and detailed information: www.defra.gov.uk/planth/ph.htm and www.forestry.gov.uk

The wild rhododendron (R. ponticum) appears most susceptible to P. ramorum and unfortunately in larger gardens this was often initially planted as a windbreak. Larger estates used it as undercover to deciduous woodland where P. kernoviae has become a threat to beeches and other trees. Disease symptoms in rhododendrons vary from the rapid wilting of whole shoots (as in the name ‘Sudden Oak Death’) to the browning of leaf tips. If other rhododendron species and hybrids are grown in close proximity to diseased plants then they, too, are likely to succumb to disease within a season. Infected viburnum stems can also show this rapid wilting and their leaves, as with pieris and camellia, show similar browning. There is a range of disease signs for many woody garden plants on the websites, but similar browning of the leaf extremities can be signs of other problems too. Strong winds can dry out leaf margins and other root problems can prevent moisture reaching leaf tips causing them to die back. What appears common to all infected plants is the rapid development of the disease.

Disease precautions
There are certain precautions to take which may help to prevent the disease infecting susceptible plants. Good garden hygiene is important. Remove any dead plant material and burn, do not put on the compost heap. It is also prudent to remove all leaf debris from underneath susceptible plants as the disease organism can survive on this. Burn the debris to destroy. Mulch well with a proprietary growing medium (unless you are certain that your heap of garden waste has truly composted at temperature); this should ensure that plants are not stressed through shortages of nutrition or moisture.

The fungal spores which infect plant material need moist or humid conditions to ‘germinate’ and attack. Removing the lower branches from susceptible plants to open up the environment will prevent the accumulation of static humid air which encourages fungal activity and will create good air flow. This may be very difficult to achieve in some larger gardens where plants have become overgrown and dense and brambles have invaded; unfortunately the ideal situation has been created for this disease to get a hold.

Recent mild winters and wet summers have provided good conditions for fungal growth, but hopefully this winter’s cold weather will have checked its spread.

Pruning tools should be cleaned with methylated spirit before moving on to the next plant, and footwear should be carefully washed to remove any leaf litter and cleaned of all mud in the treads, especially if these same shoes/boots are used for garden visits and rambles elsewhere! It would seem likely that Sudden Oak Death will not go away, so it is worth taking as many of these precautionary measures as possible. The disease will inevitably lead to the death of the infected plant and potentially spread to many surrounding plants.

 

If you think you have a problem in your garden with this disease, contact Defra at Polwhele, near Truro, on 01872 275063 or 222673.