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Ever wonder what came first, the strawberry plant or the strawberry fruit?
Well, for all intents and purposes, in commercial fruit production systems the plants very definitely come first.
Strawberries are propagated vegetatively, clones from their mothers. The strawberry fruit is actually part of the mother plant, not a product of cross-fertilization. Botanically the achenes which we tend to call ‘seeds’ are actually the fruit that contains the seed, and the berry is a swollen flower base. That’s why commercial strawberry varieties are propagated clonally rather than by seed.
The down-side of clonal propagation is if the mother plant has a virus, then every single daughter plant does as well. One infected mother plant after two years of propagation would make 40,000 infected daughters. Clearly, clonal propagation is vulnerable to accumulating pests and pathogens, unless managed very carefully.
That’s why a High Health production system for strawberry plants is critical.
In New Zealand, the NZ Berryfruit Propagators who import the University of California strawberry varieties manage a High Health Production System for strawberries. It starts with a Foundation Stock Unit which is situated in Lincoln, far away from commercial fruit production.
When plants of a new variety get released from MPI quarantine (newly tested and assured to be free of pathogens and pests), they come to the Foundation Stock Unit where they enter the production cycle that supplies Elite mother plants to outdoor runner plant propagators, and renews tissue culture material regularly for the plug plant producers.
The Foundation Stock Unit is a special greenhouse unit. It’s carefully screened and accessed through double doors to keep out insects, as whiteflies and aphids are the main virus vectors for strawberries. Access to the greenhouse is restricted, and care is taken to not track in soil or pests from the outside.
Several steps are taken preventatively for pest management, even when pests aren’t seen. Mother plants get a hot water bath before planting, a treatment that would kill cyclamen mite and parasitic nematodes. Growing runners don’t touch soil or water shared with the motherplants, creating a clean break from any soil-borne pathogens such as phytophthora. And most importantly, everyone entering the greenhouse and handling the plants follows strict protocols to avoid tracking in “baddies.”
Several pathogen screens are completed annually. Twice a year soil and root samples go to the lab to be screened for phytophthora. Two methods of virus screening are used. Molecular tests done by MPI’s lab use genetic markers to look for pathogens. The grafting method uses indicator species (Fragaria vesca, or “wild strawberries”) who receive a piece of test material leaf into their vascular system by graft. Any virus, bacteria, phytoplasma or liberibacter (bacteria-like organisms that spread like viruses) will transfer over the graft union and show symptoms rapidly in the indicator plants. These steps assure that the processes in place are successful to keep out pathogens, and that runners used to renew the tissue culture are indeed pathogen-free.
The clean daughters produced through this system are the Foundation Plant start of the commercial outdoor runner grower propagation cycle. Outdoor propagators plant Foundation Plants into an “Elite bed” which is carefully maintained to assure trueness-to-type and high health. The plants produced in the Elite bed are checked for quality and trueness-to-type, and they become the mother plants in the “Runner” beds, which produce the plants growers use for fruiting.
“Trueness-to-type” is critical. It means that when fruit growers buy a plant labelled “Monterey,” it actually performs like a Monterey instead of, say, Ventana. NZBP’s variety trials check Elite plants every year to see that they act the way they are supposed to. Plant mix-ups have been discovered before they have continued on to the market.
Plug growers also depend on the Foundation Stock Unit as the tissue culture material they use as mother plants needs to be renewed on a regular basis from the clean foundation stock. If TC isn’t renewed regularly it can accumulate genetic mutations (lose trueness-to-type).
It is the whole system that is important:
Although at present the only plant material in the Foundation Stock Unit is managed by NZBP, the unit is open to strawberry varieties managed by other entities in NZ.