Fireblight, a Battle for the Boyne Raspberry
by Dr. Evans
In a follow-up of the excellent fireblight article by fellow Canadian H. Fred Jansen
(Pomona, Vol. XXX, No. 2) I’d like to add my experience with fireblight in
raspberries.
Traditionally we’re referred to fireblight as a destructive disease of rose family
species such as apple, pear, mountain ash and saskatoon. No one seems to mention
raspberries. Periodically fireblight is extremely destructive on raspberries in Alberta,
and I’m sure across much of North America. The damage to the raspberry crop is not so
much to the canes but in the production of berries.
All cultivars of raspberry appear to be susceptible to this bacterial disease. Damage
is most obvious at fruit maturity when whole clusters of berries are dead and dried, and
up to 10% of the season new canes turn black at the top 15 to 30 cm. These blackened canes
are often obscured by healthy green canes in the raspberry canopy. Infected canes have a
milky to clear ooze that can sometimes easily be seen on the blackened stems.
The destructive nature of fireblight on raspberries first came to my attention in the
early eighties when I grew side by side plantings of Boyne and Honey Queen. In one year
(1984) I didn’t get more than a pint of Boyne raspberries from a 6 metre row whereas
an adjacent 6 metre row of Honey Queen produced gallons of fruit. In a 1985 survey of many
raspberry growing sites in central Alberta, fireblight was of common occurrence on all
raspberry cultivars examined but it was particularly destructive on Boyne. At that time it
was understood in the literature that the strain or ecotype of the fireblight bacterium
that attached raspberry and related plants such as blackberry was not infectious to apple,
pear, mountain ash, saskatoon and other rose family species.
The high susceptibility of Boyne to fireblight was not formally solved until 1995. In a
garden experiment at Edmonton, Alberta, I was able to prove conclusively that Boyne
raspberry is unusual in the fact that this cultivar is susceptible to the apple strain of
fireblight whereas Honey Queen and other raspberry cultivars appear to be immune. I was
able to take fireblight innoculum (bacterial ooze) from Westland apples and infect Boyne
raspberry shoots, Honey Queen shoots were unaffected by this source of fireblight
bacteria. The ooze from the (Westland apple) infected Boyne shoots innoculated back to
apple produced typical fireblight symptoms on apple seedlings and apple shoots. This
research was first presented to the International Workshop on Fireblight, held at Brock
University Ontario, in August of 1995 and later published in Acta Horticulturae 411 in
1996 (Fireblight pp. 69-72).
The extreme susceptibility of Boyne now becomes apparent since raspberries are often
grown along with apples, as was the case in my garden. Apples normally bloom from around
mid-May into early June, raspberries bloom from mid-to late June although this may vary
from
year to year. The build-up of fireblight ooze on the apple trees on the shoots and
spurs, particularly on susceptible cultivars provide a high inoculum (disease) potential
for bees to spread these bacteria to the susceptible Boyne raspberry flowers, Honey Queen
and other raspberry cultivars are immune to this apple source of fireblight
Next time you are around a raspberry crop in early July examine the green berries -
check for dead flowers or blackened immature fruit - signs of fireblight. On raspberry
cultivars other than Boyne it will likely be the raspberry strain of fireblight. If its
Boyne then damage could be from either the apple or raspberry strain. Confirmation of this
research came from an Agriculture Agri-Food Canada research station in New Brunswick where
very many raspberry cultivars were under cultivation and evaluation. The Boyne planting
along with a seedling selection from Boyne were wiped out in 1996 whereas other raspberry
cultivars in the same trial were unaffected by fireblight. It is likely that the cause,
was an "apple" strain of fireblight.
In conclusion Boyne raspberry should be grown well away from apple, pear, crabapples or
other rose family members particularly if any of these have shown fireblight symptoms in
past years. If you must have raspberries alongside tree fruit then make sure that Boyne is
not the cultivar that you grow or the battle will be lost.
Dr. Ieuan R. Evans
127 - 27019 Township Road 514
Spruce Grove, Alberta
T7Y 1G5
(403) 987-4398
|
