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Drought Continues in Eastern Nebraska
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Drought conditions have lessened somewhat in the SE central region of Nebraska
though soil moisture is very low around the Lincoln and surrounding areas. Additional watering of
trees is advised as severe drought may persist into the summer months.
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Gall
Produced by the Poplar Vagabond Aphid on Cottonwood
Souix County
Galls come in all sorts of shapes, sizes, colors and are formed in many
different ways. But what are galls and are they harmful to trees? Forest Health receives
questions and pictures about this phenomenon several times throughout the
year.
The most common galls we see are caused by insects and mites, but a virus, bacteria, or
fungus can also cause galls. Galls are swollen tissue or abnormal growths of the host
plant caused by secretions, infection, egg laying or even feeding by the gall-making organism.
The host meristematic cells are manipulated chemically or hormonally to grow faster and differentiate
in a certain way, benefitting the gall maker. Galls can form on stems, shoots, trunks, flowers,
leaves and even roots.
As mentioned, most galls are caused by insects and mites. These types of gall makers usually
use the galls they have stimulated to protect and feed the next generation - eggs, larva or
nymphs.
In the picture above, the cottonwood has created a gall surrounding Poplar Vagabond
Aphid (Mordwilkoja vagabunda) nymphs. The adult aphid lays its eggs on
cottonwood and aspen trees in the fall. The eggs hatch during shoot expansion in the
spring. The young (nymphs) feed on the leaf petioles causing the leaf to grow around the
aphids, surrounding them in a protective covering until maturity. Each gall can have up to an
estimated 1600 nymphs. Winged adults emerge in the summer and find an alternative host for
feeding. Some speculate that the adults feed on grasses or even plants like loosestrife where
another generation is produced. This new generation then lays eggs on the trees.
While sometimes unsightly, galls generally don't pose a problem for their hosts. Shoot,
leaf and flower galls typically do not disrupt photosynthesis
enough to impact tree health. Natural enemies should help in keeping these types of gall
makers in check. Cultural controls like removing the galls or picking up leaf litter in
the fall can help. It is important to identify what type of gall maker is present to know if
any action is deemed necessary.
Stem, trunk and root galls can affect tree health depending on the type
and severity of the gall. Callirhytis flavipes, for instance, is a
small wasp that causes both leaf and stem galls on bur oaks.
Picture courtesy of Scott Digweed, University of Alberta,
ResearchGate.Net
In the summer, the female lays eggs in individual chambers underneath the
bark of stems and trunks. These stem gall chambers protect the eggs. Once hatched, the
larva feed inside the chambers until the next spring. This new generation lays an egg in the
terminal leaf bud, causing gall formation on the leaf.
In high numbers, this gall wasp larva damage can be harmful to the tree. Not only by the amount
of chambers present but by the damage caused by woodpeckers who find the larva a tasty treat.
The extensive debarking may lead to dieback of limbs and tops of trunks.
Life cycles of gall makers and structures of galls can be complex making control measures
difficult. Contact Forest Health for more
information.
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Bur Oak, Scotts Bluff County
Photo courtesy of Laurie Stepanek
Note the empty chamber cells (white arrows) where woodpeckers have
removed larva. A lucky larva (red arrow), who could be in its pupal stage, has escaped to live
another day.
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Photo courtesy of Laurie
Stepanek
Bark shredding from woodpeckers feeding on Callirhytis
flavipes larva.
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Fire blight is a native bacterial disease of many trees and shrubs in the family
Rosaceae. More commonly in Nebraska, infections occur on crabapple, hawthorn, and
edible fruit trees like apple and pear. This disease can have a significant impact in
orchards by reducing tree function and yields.
Fire blight bacteria overwinter at the margins of cankers created the previous year. As
temperatures and humidity rise in the spring, the bacterium start to multiply, creating
a bacterial exudate called "ooze". This sweet substance is picked up by insects such as
bees and flies or is dislodged a short distance from rain splash or wind. The insects
move the bacteria from flower to flower and plant to plant.
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Photo courtesy of Purdue University
Pictured above is an example of fire blight canker on an apple
branch. Notice the necrotic tissue at the base of the shoot causing the entire shoot to
die. Shoots and branches, once infected, turn brown or black, wilting downward into a "shepherd's
crook" form. Bacteria that survive the winter on the margins of the canker will ooze and spread the
following spring (pictured below left). Flowers and fruit can also have bacterial oozing once
infected (below right).
Photos by T. DuPont, WSU.
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Monitoring for sign and symptoms of fire blight in susceptible trees is crucial in successful
management of the disease. If fire blight is found, several steps can be taken to help
stop the spread.
•Cut out infections (4” past infection to healthy wood) and use a 10% beach solution,
some also add a few drops of liquid soap (1 part bleach, 9 parts water) to sanitize pruning
tools after each cut.
•Infected wood material should be destroyed, buried or burned away from susceptible
plants
•Bactericides on the market are effective especially streptomycin but beware of bacterial
resistance
•Select more resistant varieties when planting
•Limit excess nitrogen to manage plant vigor
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Compiled by Jennifer Morris, Forest Health
Specialist
jmorris12@unl.edu
402-326-7276
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