Nancy,
The problem is that it takes several years to produce a broom of a size
that results in culling, and hence the current culls are almost
certainly a legacy from before the time of chick weed control. The
broom in your picture is at least five years old.
So, even if chickweed control is effective, it will take several more
years before the damage disappears.
Infection by basidiospores occurs in the spring from telia on dead
foliage of the previous year, and the first symptom is a slight swelling
of the infected twig a year or more later. Two years after infection
the first systemically infected shoots appear. Assuming that chickweed
control became effective in 2007, then 2007 was the last possible year
of infection (from telia on 2006 chickweed foliage). The first
systemically infected shoots on these infections would have been
produced in 2009. They would be nearly impossible to find this late in
the year, but they should be visible early summer 2010 when the aecia on
their new shoots are in their full glory.
But there is a further consideration. Rusts of this sort typically have
wave years of infection. I would expect that the large majority of
brooms in a single plantation belong to a single or at most a few age
cohorts. This means that you may have to wait several years before you
can begin to be confident that the chickweed control has worked (or that
other hosts are involved).
Bart van der Kamp
UBC
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