372 J.L. BECKERMAN AND G.W. SUNDIN
moved to New Jersey and the program involved three universities: Pur-
due University, Rutgers University, and the University of Illinois and
was known as the PRI program. The early studies developed screen-
ing methods, and identified gene pools from many sources that showed
field immunity but the major program was based on a gene identified as
VffromM. floribundabecause an early selection from Crandall’s popu-
lation had the largest size and best quality (Crosby et al. 1992).
The breeding strategy consisted of a modified backcross program with
the entire spectrum of susceptible cultivars serving as recurrent par-
ents, and later, scab-resistant selections were intercrossed. The genetic
selections were shared worldwide and breeding programs using theVf
gene proliferated. About 50 cultivars derived from PRI germplasm have
been released by breeders worldwide.Vf, demonstrated to be a cluster of
resistance genes, was used in approximately 90% of the scab-resistant
apple cultivars released. Up to the present, none has achieved large-
scale commercial success in world markets, although some, such as
“GoldRush,” “CrimsonCrisp,” “Pixie Crunch,” and “Juliet,” are widely
used in the organic apple industry. This may soon change as a recent
Washington release named “CosmicCrisp” derived from a cross of
“Enterprise” and “Honeycrisp” is being widely planted in Washington
State.
Another approach to obtain genetic resistance is through genetic
transformation of high quality but susceptible cultivars with theVfgene
or other resistance genes (Gessler and Patocchi 2007). Belfanti et al.
(2004) successfully transformed the susceptible cultivar “Gala” with
theHcrVf2gene cluster resulting in four independent transformed lines
resistant to apple scab, demonstrating thatHcrVf2cluster is sufficient
to confer scab resistance to a susceptible cultivar.
Despite the excitement created by scab-resistant apples in both the
breeding and biotechnology programs, the breakdown of resistance to
theVfgene is a real threat to the durability of genetic resistance. Over
a million trees of “‘Judeline,” a cider apple released by INRA contain-
ingVfcluster were planted in France, trees but succumbed to a new
race ofV. inaequalis. Scab was observed on resistant cultivars pos-
sessing theVfcluster in 1984 in Germany, and later, in other parts of
Europe and New Zealand (Parisi et al. 1993), and the United States.
(Beckerman et al. 2009). This throws into question the durability of
theVfcluster in apple breeding and biotechnology programs (Gessler
et al. 2006). Numerous other scab-resistance genes have been identi-
fied and mapped: theVmgene fromM. atrosanguinea804, theVrgene
from a Russian apple seedling from the Caucasus Mountains,Vmfrom
M. micromalus; and several candidates with polygenic resistance. The