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Disease Update: Nectria Twig Blight or Fire Blight?

Disease Update: Nectria Twig Blight or Fire Blight?
By Kari A. Peter
 
This weak fungus will girdle shoots causing shoot blight. Prompt removal from trees will limit further infection.
 
The 2019 season has been a relatively calm fire blight season: few infection periods and temperatures remained cool throughout the bloom and petal fall period. Reports have been coming in from all over Pennsylvania about “fire blight” showing up in apple orchards. Based on our unfavorable conditions during bloom for fire blight, chances are very slim this is fire blight.
 
When I have been able to observe affected orchards, all instances of “fire blight” are Nectria twig blight, which is caused by the fungus Nectria cinnabarina. Wet weather favors this pathogen and it not aggressive. When examining closely, the fungus has girdled the twig where shoots begin to grow, most often at the base of the previous season’s cluster bud. Consequently, the girdling resembles shoot blight.
 
Nectria twig blight or shoot blight caused by Erwinia amylvora?
 
If shoot blight is suddenly occurring in your orchard, look at the affected shoots very closely by examining the base of the shoot. If you see small orange pustules or small faint orange spots on the bark, this is Nectria twig blight (see figure below). Shoot blight caused by the bacteria E. amylovara will not have these types of symptoms.
 
A = Faint orange spots on bark. B = Orange pustules at the base of the shoot. Be sure to examine shoot blight in your orchard to determine the causal organism: Nectria will produce orange spores at the base of blighted shoots.
 
Management of Nectria twig blight
 
The best management for Nectria twig blight is to prune out affected shoots and remove from the orchard. Removal from the orchard will limit the number of available spores to cause further infection. Leaving cuttings in the orchard will allow the fungus to persist.
 
For commercial fruit growers, please note: When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one’s own orchard. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label.