Tar Spot

(Rhytisma americanum)

               
Occurrence

Widespread and very common

Tar Spot (Rhytisma americanum)
Season

Spring to fall

Habitat/Hosts

red maple (Acer rubrum var. rubrum)

silver maple (Acer saccharinum)

and possibly

sugar maple (Acer saccharum ssp. saccharum)

 

 


Identification

This 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.


Distribution Distribution Map  

Sources: 4, 7, 26, 29.

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.


Comments

Taxonomy
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.


Taxonomy

Division:

Ascomycota (ascomycetes)

  no rank:

saccharomyceta

 

Subdivision:

Pezizomycotina

 

No Rank:

leotiomyceta

 

No Rank:

sordariomyceta

 

Order:

Rhytismatales

 

Family:

Rhytismataceae

 
Synonyms

Xyloma americanum

 
Common
Names

Sycamore Tarspot

Tar Spot


 

 

 

 

 

 

       

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  Tar Spot (Rhytisma americanum)   Tar Spot (Rhytisma americanum)
       
  Tar Spot (Rhytisma americanum)    
       
       
       

 

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name withheld
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.


     
     
 

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