The interaction between host and pathogen, for example, between apple
and scab, is a two-way street. Heavy planting of any one cultivar
puts tremendous selection pressure on the pathogen, particularly if
the only way it can survive is to infect a host. We don't really
understand all the ins and outs of resistance and pathogenicity
between apples and Venturia inaequalis. However,f fungi and other
microorganisms can change rapidly under selection pressure. For ex.,
we have resistance to a number of fungicides, and in some cases it
can develop in a matter of a few growing seasons. MacHardy's point,
and I agree, is that strains of Venturia develop which are adapted to
different cultivars.
So sure, people screw up when they graft and propagate apples. But
host resistance is only half the story. Pathogens can and do change
quickly when conditions demand it of them. Considering that the vast
majority of resistance used in apple breeding has been a single gene,
Vf, I'd bet that widespread use of those cultivars would soon lead to
a strain of scab that could overcome the resistance.
The resistance present in some of the old cultivars is much more
interesting. Chances are that there are more genes involved, and that
therefore it would be more stable.
However, if the problem with heirloom or modern scab resistant
cultivars is "consumer acceptance", it suggests that consumers don't
like them. Maybe that's why the heirlooms became heirlooms. On the
other hand, people's tastes are becoming more varied and
sophisticated, so maybe some aggressive marketing would do the trick.
Dan
________________________________________________________
Daniel R. Cooley 413-577-3803
Dept. of Plant, Soil & Insect Sci. [EMAIL PROTECTED]
Fernald Hall 103
University of Massachusetts
Amherst, MA 01003 FAX 413-545-2115
http://people.umass.edu/dcooley/
