American Tar Spot

(Rhytisma americanum)

Conservation Status
American Tar Spot
 
  IUCN Red List

not listed

 
  NatureServe

not listed

 
  Minnesota

not listed

 
           
           
           
 
Description
 
 

American Tar Spot is a parasitic fungus infecting several species of maple (Acer). In Minnesota it infects only red maple, silver maple, and possibly sugar maple.

In the spring the infection appears as a yellow spot on the leaf. These turn first brownish-black with a yellow border, then, in late summer, black with a yellow border. The black spots resemble tar. They are raised and solid, not a cluster of small spots. They are usually 3 16 to ½ in diameter but can coalesce into spots up to 1½ in diameter. On close examination a serpentine pattern of alternating ridges and depressions can be seen.

 
     
 

Similar Species

 
 

Speckled Tar Spot (Rhytisma punctatum) causes dense clusters of very small black spots.

Tar Spot (Rhytisma acerinum) in Minnesota infects only Norway maple.

 
     
 
Habitat and Hosts
 
 

Red maple, silver maple, and possibly sugar maple

 
     
 
Biology
 
 

Season

 
 

Spring to fall

 
     
 
Distribution
 
 

Distribution Map

 

Sources

4, 7, 26, 29, 30, 77.

Until 1998 this species was thought to be the European species Rhytisma acerinum. Some or all of the records in North America reported as Rhytisma acerinum may be Rhytisma americanum. The map at left does not include those records.

 
  6/6/2022      
         
 

Occurrence

 
 

Widespread and very common

 
         
 
Taxonomy
 
  Kingdom Fungi (fungi)  
  Subkingdom Dikarya  
  Phylum Ascomycota (sac fungi)  
  Subphylum Pezizomycotina  
  Class Leotiomycetes  
 

Order

Rhytismatales  
 

Family

Rhytismataceae  
 

Genus

Rhytisma (tar spot fungi)  
       
 

This fungus was formerly thought to be the same as the European species and went by the name Rhytisma acerinum. A study published in 1998 (Hudler and Banik) showed differences in host specificity, tar spot diameter, and spore-producing stages. The authors proposed a new name for the North American species, Rhytisma americanum.

 
       
 

Synonyms

 
 

Xyloma americanum

 
       
 

Common Names

 
 

American Tar Spot

Sycamore Tarspot

Tar Spot

 

 

 

 

 

 

 

 
 
Visitor Photos
 
           
 

Share your photo of this fungus.

 
  This button not working for you?
Simply email us at info@MinnesotaSeasons.com.
Attach one or more photos and, if you like, a caption.
 
 

 

 
     
 

 

 
           
           
 
MinnesotaSeasons.com Photos
 
    American Tar Spot   American Tar Spot  
           
    American Tar Spot      

 

Camera

     
 
Slideshows
 
 
     
     

 

slideshow

       
 
Visitor Videos
 
       
 

Share your video of this fungus.

 
  This button not working for you?
Simply email us at info@MinnesotaSeasons.com.
Attach a video, a YouTube link, or a cloud storage link.
 
 

 

 
     
     
       
       
 
Other Videos
 
     
     
     

 

Camcorder

 
 
Visitor Sightings
 
           
 

Report a sighting of this fungus.

 
  This button not working for you?
Simply email us at info@MinnesotaSeasons.com.
Be sure to include a location.
 
  Maureen Branton
10/24/2015

Location: Southern Michigan

I have had this problem for three years on my tree. I am hoping to find a solution so I can save my tree. As the years have passed it is getting worse.

 
           
 
MinnesotaSeasons.com Sightings
 
   

 

 

Binoculars


Last Updated:

About Us | Privacy Policy | Contact Us | © MinnesotaSeasons.com. All rights reserved.