Peak Fire Blight Season
Southwest Yard & Garden
by Dr. Marisa Thompson
The drooping branch with dead leaves on this pear tree is exhibiting classic shepherd’s crook symptoms and was diagnosed with fire blight by the NMSU Plant Diagnostic Clinic.
This
week I’ve selected a column on fire blight from July 2018 to reprint with a few
updates. The NMSU Plant Diagnostic Clinic analyzes plant material for plant pathogens and
environmental stresses and suggests appropriate control measures when
available. Insect and weed identification services are also offered. The Plant
Diagnostic Clinic is accepting physical samples that are submitted and shipped
from your county’s NMSU Cooperative Extension office.
Question: I think my Asian pear might have fire blight. Can you
tell from this branch? [Sample submitted with wilted, blackened leaves at tips
of new growth.]
-
Jade W., Albuquerque, NM
Answer: I couldn’t tell for sure from the sample, so I sent it to the NMSU Plant Diagnostic Clinic (PDC) in Las Cruces. Former NMSU Extension Plant Pathology Specialist Jason French explained how samples are analyzed in the lab when a bacterial infection is suspected. First, they excise a small amount of tissue at the margin of the infection (the intersection of healthy and symptomatic tissue). Then they macerate and streak the solution onto a specialized media in a petri dish. All of the work is done under sterile conditions. Within 24 to 48 hours, any bacterial growth colonies on the plate are tested further with a metabolite panel.
The final PDC report for this Asian pear branch included the NMSU Extension Guide A-230, “Fire Blight,” for more information. “Young, succulent shoots infected with fire blight often develop a characteristic ‘shepherd's-crook’ symptom-the bending of the shoot tip—the bending of the shoot tip to approximately 180°.”
Again, fire blight only affects plants in the rose family. There are a number of steps you can take to fight it.
In late winter, prune out branches with cankers (sunken, discolored areas that ooze when the bacterium is active—ewww). Be sure to clean pruning tools between cuts with a common household disinfectant like Lysol (I avoid bleach because it’s been proven to damage and corrode metal pruning equipment, which leaves tiny pockmarks where bacteria can hide). Don’t compost those clippings.
In the growing season, active infections (like the branch you sampled) should be “removed at least 12–15 inches below the margin of the visible infection because the bacterium moves inside the plant ahead of visible symptom development.” Cleaning your shears or pruning saw blade between every single cut reduces the risk of spreading the disease from tree to tree or even within the same canopy.
Because this bacterium is more likely to infect new growth, remove suckers that sometimes sprout out at the trunk base. That new growth is more susceptible because it’s fast-growing, succulent tissue. “Therefore, avoid heavy applications of nitrogen fertilizers.”
Side note: Other pests, like aphids, also take advantage of tender new growth on plants. By fertilizing your roses less, you may be able to avoid midseason aphid infestations and reduce the need for insecticides.
Bactericides
can be used to reduce infection spread, but proper timing is crucial. For
example, treating symptomatic tissue with copper compounds at budbreak in late
winter or during bloom time in spring can be effective, but less so later in
the season, so routine spraying in the summer is not recommended. Additionally,
as fruit are developing, copper compound sprays can cause russetting in fruit,
which lowers market value. Russetting doesn’t affect fruit flavor, but the skin
gets rough and shelf life can be shortened. After a hailstorm, however,
bactericide application can reduce the spread of fire blight on an already
infected tree. Always read the product label before use, and remember that just
because a product is labeled as “natural” or “organic” doesn’t mean it’s
necessarily safe to use without precautions like gloves or a face mask.
Plant tissues infected with fire blight. Photo credits NMSU Plant Diagnostic Clinic.
Fire
blight symptoms can be difficult to discern depending on what part of the tree
is infected, especially on older plants, and also depending on both the plant
and bacterial growth stages. The blackened, drooping leaves and small branches look
a lot like tissue hit by a cold snap, but if you see those signs mid-summer, it
might be a good idea to grab a sample and submit it to the NMSU PlantDiagnostic Clinic via your NMSU County Extension Office. Sample analysis AND shipping are FREE! Go ahead and get several photos to document the
symptoms while you’re at it.
Send gardening questions to Southwest Yard and Garden - Attn:
Dr. Marisa Thompson at desertblooms@nmsu.edu, or at
the NM Desert
Blooms Facebook page (@NMDesertBlooms)
Please
copy your County
Extension Agent (http://aces.nmsu.edu/county/) and
indicate your county of residence when you submit your question!
For more gardening information, visit
the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture
Publications page at http://aces.nmsu.edu/pubs/_h/.
Marisa Y. Thompson, PhD, is the Extension
Horticulture Specialist, in the Department of Extension Plant Sciences at the
New Mexico State University Los Lunas Agricultural Science Center, office: 505-
865-7340, ext. 113.
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