370 J.L. BECKERMAN AND G.W. SUNDIN
on fresh fruit at harvest, and overwintering inoculum that increased
disease pressures the following year. The window of opportunity for
improved scab control provided by the development of “second gen-
eration” SBI fungicides like fenbuconazole and difenoconazole, which
have a higher intrinsic activity towardV. inaequalis, may also be rela-
tively short-lived (Villani et al. 2015). Newer succinate dehydrogenase
inhibitors (SDHI) like penthiopyrad, fluopyram, and fluxapyroxad pro-
vide new tools to reduce the pressure on the SBI fungicides, but do not
provide the levels of control, and post-infection activity in particular,
that growers came to expect with the SBIs. Growers also recognize the
inevitable problems of fungicide resistance that they face using these,
or any single-site mode of action fungicide, and question the long-term
sustainability of apple production east of the Mississippi.
More problematic than the sequential development of fungicide resis-
tance to every class of systemic fungicide used in apple scab man-
agement, is the identification of isolates ofV. inaequaliswith resis-
tance to multiple classes of fungicides in orchards in some states in the
United States (Chapman et al. 2011). Fungicide resistance is an inher-
ited trait that can sometimes be associated with a fitness penalty that
negatively affects fungal growth, reproduction, or pathogenicity (Brent
and Hollomon 2007). As such, multiple resistances would be expected
to impose a greater penalty. For example, a fitness penalty was found
to result from resistance to pyraclostrobin inBotrytis cinereain lab-
oratory studies (Markoglou et al. 2006). However, studies ofV. inae-
qualisand some other plant pathogens have shown that little to no fit-
ness penalty was associated with fungicide resistance (Koenraadt et al.
1992; Raposo et al. 2000; Chapman et al. 2011). In annual agricultural
systems, pathogens have to contend with selection by cultivar deploy-
ment employing new sources of genetic resistance in addition to fungi-
cides, but in the perennial, effectively monocultural system of apple
orchards, cultivars do not change year to year, and the scab pathogen has
already tightly co-evolved pathogenicity or virulence factors matching
the host. In this scenario, the only selection pressure on the pathogen
is from fungicides, with most new fungicides possessing a single-site
mode of action. In other words, in a well-established orchard with older
cultivars of apples, the scab pathogen is only being challenged by one
selective pressure at a time, which may be a factor leading to the fairly
rapid evolution of fungicide resistance. Therefore, fungicides with a
single-site mode of action will not provide the long-term management
necessary for sustainable apple production. Furthermore, when fungal
strains are resistant to multiple chemicals, techniques such as mixing
fungicides with different modes of action or chemical rotations may not
be effective disease management strategies. Finally, when resistance is