Authors: Markus Bünter, Gabriella Bachmann and Beatrix Buchmann, Wädenswil
Nuclear stock has nothing to do with nuclear energy, nuclear weapons or genetic manipulation. A nuclear stock is an archive or collection of healthy, virus-free varieties of fruit. It contains at least one plant of all the varieties currently certified in Switzerland, i.e. varieties that are important for Swiss fru t production, plus a number of old varieties and, naturally, different new varieties that have been bred. The nuclear stock is defined as the location at which the smallest used plant unit of a variety that is permitted for certification is kept. The term nuclear stock presumably has its origin in the Latin world “nucleus”, meaning “kernel”.
– German: Nuklearstock
– French: conservatoire
– Italian: conservatorio
– English: nuclear stock
Agroscope Changins-Wädenswil ACW has been engaged in testing fruit-trees for viruses and eliminating viruses from them since 1959. August Schmid was the pioneer here, who built up the first nuclear stock in Grabs, Canton St. Gallen, in 1976. In spring 2004, the nuclear stock of some 420 fruit varieties was moved to Wädenswil.
The requirements placed on nuclear stock are defined in the EPPO standard “Schemes for the production of healthy plants for planting”. These include safeguarding plants against insects and nematodes, and also general hygiene measures.
(EPPO = European and Mediterranean Plant Protection Organization www.eppo.org).
Inclusion in the nuclear stock
The nuclear stock currently contains 307 varieties of fruit. 151 varieties of apple (from Ariwa to Wilmuta), 56 varieties of pear, 53 varieties of cherry and 33 varieties of plum, plus 6 varieties of quince, 7 varieties of apricot and 1 variety of peach.
The Variety Group, which comprises three representatives from the Swiss Tree Nursery Association (JardinSuisse and Schweizer Obstverband) and three from Agroscope Changins-Wädenswil ACW, takes a decision each year on which varieties of fruit are to be added or removed from the nuclear stock. Requests of this nature are received from nurseries, plant breeders and other interested parties.
Before a plant is included in the nuclear stock, protracted selection work and variety appraisals have already been carried out in the field, in the laboratory and in the greenhouse and warehouse. Procedures to show up virus diseases take at least three years in the case of pip fruit and two years for stone fruit. If a plant has been attacked by a virus, the virus disease will be eliminated by subjecting the plant to heat treatment. Only after renewed testing for viruses with a successful outcome, and a true-to-type test, is the plant entitled to be planted in the nuclear stock and used as parent material for the propagation of certified fruit trees. This so-called prebasic material is supplied to P1 and P2 cutting gardens in nurseries for grafting certified fruit trees. Prebasic material from Switzerland’s nuclear stock is supplied to fruit breeding institutes, nuclear stocks and tree nurseries in fruit-growing areas throughout the world.
Why does Agroscope Changins-Wädenswil ACW keep a nuclear stock for fruit trees?
By maintaining a nuclear stock, Agroscope Changins-Wädenswil ACW provides the basis for successful fruit-growing in Switzerland, namely the production and supply of healthy, true-to-type and certified propagation material for fruit. This is the parent material for certified fruit trees.
The 334 or so varieties of fruit are kept in the insect-proof gauzehouse, which covers an area half the size of a football pitch. The frame of this gauzehouse is made up of a metal structure which is covered with a lightweight, permeable plastic fabric during the vegetation period from early spring to late autumn. This ensures a climate that is virtually identical to the outdoor climate, while preventing insects from getting inside. The double-door entry system fulfils this same function. In the winter half of the year, the fabric is removed from the roof on account of the heavy snow masses.
The soil is made up of different layers, separated by fleece, and is 0.75 metres deep. Each plant grows in its own container, which is filled with sterile earth, and stands in expanded clay (hydroponic clay pebbles). This prevents roots from penetrating the natural soil, where they could be infected with viruses by nematodes.
To check on the health of plants in the nuclear stock, all the plants are subjected to periodic laboratory diagnoses and greenhouse tests and are replaced from time to time.
Nuclear stocks of this type, moreover, are kept in fruit-growing areas throughout the world. Switzerland’s only nuclear stock for pip and stone fruit is the one held by Agroscope Wädenswil site.