That war lasted for seven decades, cost tens of millions of
dollars, and was fought by thousands of foot soldiers like Prior,
who were charged with taking out one of the pathogen’s hosts
– plants in the genus Ribes, which includes gooseberries and
currants. Like many wars against undesirable plants, it didn’t end
in victory. The rust was never completely vanquished. And now,
more than a century after it was discovered in North America, a
new mutated version of the rust is on the loose in New England
and eastern Canada. Forest pathologists are worried that it could
pose a renewed threat to one of the Northeast’s iconic (and most
valuable) trees. Especially as a new generation of homeowners
and berry producers are once again enthusiastically growing
Barbara Schultz, the forest health program manager for
Vermont’s Department of Forests, Parks, and Recreation, said
she’s very concerned about the recently detected strain of rust
that’s attacking newer cultivars of currants thought to be resistant, and what it may mean for the future as the region’s mature
pines are cut and younger ones, which are more vulnerable to
blister rust, grow in.
“I’m also concerned that people are
developing a taste for local currants, but
might not be aware there’s a downside. I
want to get the word out, so growers and
consumers understand that the risk to
white pines is going to increase wherever
currants are planted nearby,” she said.
An All-Out Assault
White pine blister rust (Cronartium ribicola) has been called one of the most
destructive diseases of five-needle pines
in North America. It attacks whitebark,
sugar, and limber pines in the West, and
white pines here in the East.
Its life cycle is complex. Pines are
infected by spores – called basidiospores
– that are produced on Ribes plants in
late summer and fall. The spores ride the
winds to nearby pine trees and alight on
the needles, where they enter through the
stomates, the tree’s pores. Masses of slimy
spores grow under the bark, rupturing it.
The disease spreads along a branch to the
trunk, where it slowly strangles the tree.
A different type of spore – called an
aeciospore – is produced on infected
pines and then windblown and spread
back to Ribes plants, where the process
starts all over again.
But the rust faces several challenges:
While spores from one Ribes plant can
infect other Ribes plants, a pine cannot
transmit the rust to another pine. And
although the spores that spread the infec-
tion from pines to Ribes plants can travel hundreds of miles
on the wind, the basidiospores that carry the infection from
Ribes to pines can only travel a couple of miles. Plus, the spores
that infect pines are somewhat fragile and require cool, wet
conditions to thrive. And the rust can’t survive the winter on
Ribes plants (though it is perennial on pines).
You’d think, with those caveats, that white pine blister rust
would be an easy enemy to defeat. Not so.
The rust is native to Asia, so North American pines have no
resistance to it. Interestingly, it arrived here via Europe on eastern
white pine seedlings. The first European settlers were in awe of our
eastern white pines – very tall, straight grained, a plentiful source
of easily worked wood – and so they took seeds and seedlings
back to Europe with them. Two hundred years later, Paul Bunyan
and his ilk had chopped their way through much of eastern North
America’s mature pines. And in an attempt to re-establish the
eastern forest, America imported seedlings from Europe, where
highly efficient tree nurseries had been propagating eastern white
pine for a long time. It was on some of those seedlings that white
Clockwise from top left: A close-up view of the aecia of white pine blister rust (Cronartium ribicola) on a pine branch.
A look at one past research project aimed at finding white pine blister rust resistance in seedlings. Eradication efforts
lasted for decades and involved federal, state, and local agencies. What the eradication crews were after: currants.
Currant leaf showing signs of white pine blister rust. Inset: Basidiospores seen germinating in stomata, magnified 40x
with fluorescence. The basidiospores are produced on Ribes plants in late summer or fall and then travel to pines.