Introduction
Witches' broom is a relatively minor disease of lowbush
blueberry in Nova Scotia. The rust fungus Pucciniastrum
goeppertianum (Kuhn) Kleb. causes the disease. The
most recent survey of 12 fields in Nova Scotia indicated
2.2 percent of the plants were infected. This low level
probably results in negligible yield losses. Balsam fir
serves as an alternate host for the fungus in Nova Scotia.
Symptoms
Diseased blueberry plants have broomlike masses of swollen,
spongy shoots with few or no leaves. The brooms usually
begin to develop during the year following infection and
then persist for many years, producing new infected growth
each spring. Young shoots on the brooms are yellow-red
at first, but later they become dark tan coloured and
shiny. Eventually the epidermis becomes dull, dry and
cracked. Infected plants usually do not produce fruit.
On balsam fir, the disease causes yellowing and premature
shedding of current season needles.
Infected
blueberry plant
Click picture to enlarge
Life
Cycle
Rust spores (teliospores), produced on swollen infected
blueberry shoots, germinate on these shoots to produce
another spore type (basidiospores) from mid May to late
June. These spores are carried by wind to balsam fir trees
where the current season growth becomes infected. The
peak discharge of basidiospores on blueberry occurs for
a 3-week period in late May to mid-June. Following the
infection of fir, a new spore type (aeciospores) are formed
on the needles and these are then wind blown to blueberry
where they germinate on leaves and stems. This occurs
during mid to late summer.
Needle
rust on balsam caused by fungus
Click picture to enlarge
The fungus eventually becomes systemic and perennial in
the blueberry, stimulating the production of excessive
lateral buds that develop into the characteristic broom-like
masses of swollen shoots. It takes about one year after
infection for symptoms to become obvious. Finally, teliospores
are formed on the swollen shoots and overwinter there.
Because the brooms are perennial and produce new growth,
they may continue to serve as sources of the fungus for
many years.
Infected blueberry
plants have been reported in areas that are remote from
fir, suggesting that the spores (basidiospores) produced
on blueberry may be able to infect blueberry as well as
fir. This has not been confirmed, however.
Control
Strategy
Since witches' broom is systemic in blueberry crowns and
rhizomes, burning or flail mowing does not eliminate the
disease. Removal of the alternate host (balsam fir) within
400 to 500 yards of blueberry fields would probably be
effective. However, this may not be practical in most
areas of the province due to the abundance of balsam fir.
Infected blueberry
plants should be grubbed out or killed with a herbicide.
Acknowledgements
The original photograph of the second image was provided
by the Canadian Forestry Service.
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