128 J.E. FAUST, J.M. DOLE, AND R.G. LOPEZ
next generation of stock plants referred to as the “increase block”. Cut-
tings from the increase block are harvested to create the third and final
generation of stock plants comprising the “general production” area,
from which the commercial cuttings are harvested, shipped, and sold to
propagation customers. The stock-plant process requires approximately
1 year from starting a small block of elite stock plants to commercial cut-
ting harvest from the general production area. There are exceptions to
these procedures, such as with petunia, where tissue culture plantlets
are sometimes established directly as production stock plants.
A. Clean Stock Programs
Plant production of vegetatively propagated species begins with the
selection of true-to-type plants that accurately represent the character-
istics for which individual cultivars are known. These selections are
exposed to an extensive process for removing pathogens, that is, bac-
teria, fungi, or viruses (Smith and Oglevee-O’Donovan 1977; Oglevee-
O’Donovan 1993). This processing is necessary for stock-plant growers
to offer “clean,” disease-free cuttings to their customers. The first step in
the cleaning process begins with virus testing on many plants, followed
by plating individual cuttings from clean tissue onto Petri dishes con-
taining broth for the growth of known bacterial and fungal pathogens.
If pathogens are observed growing on the dishes, those cuttings are
destroyed, while disease-free cuttings move on to meristem culture for
viral screening. First, the meristems are exposed to thermotherapy at
35–38◦C for 3 weeks to reduce the concentration of spherical viruses
(Oglevee-O’Donovan 1986). Meristems are then grown out in tissue cul-
ture labs to produce the first set of “clean” cuttings. Viral screening is
performed for the emerging tissue in order to confirm that the cuttings
are free of known pathogens.
B. Elite Stock
The tissue cultures are then shipped to stock-plant-production facili-
ties to become the “elite” stock, also known as the “nucleus block.”
Elite stock are grown under high sanitation protocols to prevent disease
infection because the small initial number of cuttings will eventually
produce many thousands, and possibly millions, of cuttings through
their subsequent generations of cuttings and stock plants (Williams and
Ruis 1995). Protocols may include that workers not be tobacco users
in order to minimize contact with highly infectious pathogens, such
as tobacco mosaic virus, and urine testing may be required to verify