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Discerning Shoot Blight from Cicada Shoot Injury on Apples

In the last 4 weeks or so we have witnessed the occurrence of fire blight symptoms on apples (Fig. 1A, B; Fig. 2A, B). This is a disease caused by the bacterium Erwinia amylovora and its symptom occurrence overlapped with the emergence and we can fairly say "invasion" of periodical cicada (Magicicada) in landscapes and orchards. For two weeks now we can see the signs of shoot damage from laying eggs which ensues after the cicada adults mate. These notches seen in Figure 3A injure branches and impact shoots. Due to the vascular tissues in the branches being compromised by the egg laying notches made by the ovipositor of the cicada females the shoots soon start wilting and die (Fig. 3B). These symptoms on a first glance very much resemble fire blight but are not caused by this bacterium, instead these are caused by the periodical cicada. If you have not timely protected your orchards with an insecticide from this insect pest I repeat the recommendation from Dr. Chris Bergh that the most effective insecticides for cicada control are Asana (esfenvalerate) and Danitol (fenpropathrin) based on his efficacy trial data from 2004. The differing sign between the shoots blighted by the fire blight bacterium and the shoot die-off from the cicada are the droplets of white, ytellow or orange ooze emerging from flowers, shoots, and bark infected with the bacterium. Cicada do not cause the formation of ooze, i.e. wilted shoots are dry along the stem. In pick-your own operations if you are worried for the esthetics of the trees affected by the cicada make sure your insecticide program is improved in tyhe next cover spray and you can remove these shoots injured by the cicada by pruning. In case you have fire blight symptoms instead, please read my recent short blog outlining specific instructions on how to proceed with fire blight symptom removal: Fire Blight Visible in Central and North Virginia - Blight Removal Needed; Watch for SB&FS Accumulated Wetting Threshold in NEWA Model

Figure 1A. Blossom blight caused by the fire blight bacterium Erwinia amylovora (Photo by Aćimović S. G., 2016)

Figure 1B. Shoot blight caused by the fire blight bacterium Erwinia amylovora
(Photo by Aćimović S. G., 2007)


 
Figure 2A. Orange bacterial ooze a telltale sign of fire blight disease caused by the fire blight bacterium Erwinia amylovora (Photo by Aćimović S. G., 2016).

Figure 2B. Orange bacterial ooze a telltale sign of fire blight disease caused by the fire blight bacterium Erwinia amylovora (Photo by Aćimović S. G., 2021).

Fig. 3A. Notches caused by periodical cicada causing shoot wilt and die-off (Photo by Aćimović S. G., 2021). 

Fig. 3B. Shoot die-off caused by the periodical cicada resembles fire blight but is not caused by this bacterial disease (Photo by Aćimović S. G., 2021). 



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