Nebraska's Forest Health Report April 2024

Nebraska's Forest Health Report April 2024
Nebraska's Forest Health Report April 2024
Drought Continues in Eastern Nebraska

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.
What are Galls?
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.

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. 
Photo courtesy of Laurie Stepanek
Bark shredding from woodpeckers feeding on Callirhytis flavipes larva.
Fire Blight
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.  
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.
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
Compiled by Jennifer Morris, Forest Health Specialist
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