- SCAB AND FIRE BLIGHT OF APPLE 371
present in a large portion of the population, it is possible that none of the
major curative fungicides labeled for apple scab will provide adequate
disease control. In fact, this is the phenomenon currently observed in
many orchards in the eastern United States.
As a result of wide-spread scab resistance to single-site fungicides,
protectant fungicides (e.g., copper, lime-sulfur, captan, and mancozeb)
that had been relegated to a back-up status because of the improved con-
trol and flexibility provided by systemic or curative fungicides, are now
being increasingly relied upon for disease management (Cooley et al.
2013; Cooley and Green 2013). These fungicides are generally applied
starting at silver tip to green tip, and then at 7-d intervals through to
fruit set to effectively manage scab, and other foliar fungal pathogens.
This requires more frequent application than had been the case with
SBIs and other systemics (approximately 7 d intervals vs. 10 d), and
higher amounts of product (e.g., mancozeb applications can be up to
six pounds per acre, compared to the five ounces per acre originally
recommended for myclobutanil when it was first released). At the same
time, this new reliance on the protectant classes of fungicides comes at
a time of their loss and potential loss due to human and environmental
health issues. For example, the label for ferbam was withdrawn from
apples in the United States in 2005, and Captan, a protectant fungicide
used for over 60 years, is currently classified by the EPA as a “probable
human carcinogen,” using 1986 guidelines for cancer risk assessment
(EPA 2005).
D. Breeding for Apple Scab Disease Resistance
Genetic resistance to apple scab has long history (Crosby et al. 1992),
beginning with “Antonovka,” which was introduced by I.V. Michurind
in Russia in the late 19th century, followed by C.F. Rudoff and M.
Schmidt in Germany. These crosses involvedM. micromalus,butthe
fruit size and quality was so low that the breeding program was discon-
tinued.
In 1943, L. Fredric Hough, a graduate student at the University of Illi-
nois, discovered that progenies related to interspecific crosses involving
M. floribunda821, made by C.S. Crandall carried out early in the 20th
century (Crandall 1926), showed 1:1 segregation for field resistance to
scab (Crosby et al. 1992). This paper initiated a collaboration between
J. Ralph Shay, a plant pathologist at Purdue University, and Hough to
develop scab-resistant apples in order to serve the needs of fruit grow-
ers. Shay had worked on the genetics of pathogenicity of the fungus as
part of his doctoral thesis at the University of Wisconsin. In 1948, Hough