- SCAB AND FIRE BLIGHT OF APPLE 367
and “Golden Delicious” provided a preliminary bottleneck that once
passed, allowed scab pathogen genotypes to establish and rapidly
evolve greater virulence. As said the great horticulturist Bailey, “It is
possible a good market apple spring up that Scab-proof; but when we
have learned to produce such kinds with tolerable certainty, the fungi
will have grown cunning, too—I fear” (Large 1962). This phenomenon
has repeated itself, with “Baldwin,” then with the expansion of “Golden
Delicious,” and in England with “Bramley’s Seedling,” all of which
were originally reported as fairly scab resistant but are now consid-
ered quite susceptible (MacHardy et al. 2001). This phenomenon is
commonly observed with the introduction and establishment of orna-
mental crabapple as well. For example, popular scab-resistant culti-
vars eventually succumbed over time including “Adams,” “Prairiefire,”
“Bob White,” “Red Jewel” and an unknown selection ofMalus flori-
bundawith resistance that persisted for over 50 years (Beckerman et al.
2009).
B. History of Fungicide Use for Apple Scab Management
As early as 1899, Aderhold recognized that certain apple cultivars were
more resistant than others to scab and that a key issue in apple produc-
tion would be managing an “adaptation of the fungus” (Aderhold 1899).
Growers began to develop new ways to protect their crops and it was
recognized that spraying copper sulfate and/or lime sulfur (Bordeaux
mixture) that had been used to control downy and powdery mildew
in grapes could control apple scab. Unfortunately, Bordeaux mixture
caused fruit russeting, which resulted in fruit that was unappealing to
consumers. Early extension publications recognized cultivar sensitiv-
ity to Bordeaux mixture, and rates were adjusted accordingly, going so
far as to recommend lime sulfur alone over Bordeaux for particularly
sensitive cultivars (McCue 1912).
Brooks (1912) recommended that spraying begin before leaves
appeared, particularly in badly affected orchards, followed by sprays
at tight cluster and petal fall, which he described as the most important
sprays. McCue (1912) began with similar advice, but for Delaware, he
suggested spraying continue at least every 3 weeks, through 12 August,
resulting in a minimum of 12–15 applications. Stakman and Tolaas
(1916) discussed the role of sprayer nozzles to improve coverage, but
little progress would be made, and these spray schedules would not
change for approximately 25 more years. In 1944, Mills provided grow-
ers with a model to time applications of sulfur fungicides against scab
(Mills 1944). Ferbam, introduced in 1943, ushered in the beginning of