We are noticing a lot of Callery pears (some people call
them Bradford pears) with brown and black wilted foliage and stems, an
indication that they are infected with fire blight. That’s good news for those
of you who don’t like Callery pears, but it could mean trouble for edible
pears, quince and apples!
 |
| Wilted stems and foliage with scorched appearance characteristic of infection by fire blight. |
Fire blight is caused by the bacteria Erwinia amylovora and is especially common in warm, wet springs. It
attacks a variety of plants in the rose family, particularly pears and quince,
though apples, hawthorns, cotoneasters and pyracantha can also be infected.
Oddly enough, the Callery pear is supposed to be resistant to fire blight, but
it isn’t this year!
The bacteria most often enters through blossoms,
transmitted by pollinators, including honey bees, and splashing by rain. It can
also enter through wounds. The infected blooms and stems wilt and turn brown,
reducing fruit production. The bacteria can be transported through the vascular
system. If the disease becomes systemic and reaches the roots the tree can die.
Infected branches can be pruned and destroyed to prevent
the bacteria from reaching the vascular tissue in the trunk. However, pruning
can stimulate growth of tender tissues which are more susceptible to fire
blight. The addition of nitrogen fertilizers should be avoided for the same
reason.
If you do decide to prune, cut at least one foot below
where the symptoms are visible and make sure your pruning implement is cleaned
before/after each pruning cut to avoid transmitting the bacteria to new
branches. Pruned material should be removed from the area and destroyed to
reduce the risk of re-infection.
To avoid loss of
production fruit growers sometimes apply antibiotics, but this must be done
PRIOR to infections. In some locations fire blight has shown resistance to
commonly used antibiotics and they are no longer effective. Some work has shown
that innocuous bacteria or yeasts can outcompete the fire blight bacteria and
protect blossoms and stems if applied PRIOR to infection.
