82 



Forest Club Annual 



Rhytisma acerinum (Pers.) Fr. 



This fungus producing the disease of the maple commonly 

 known as "tar spot" or "black spot" is very common in the state 

 during some seasons. The jet black affected areas of the leaf are 

 very conspicuous especially in the late summer when they are 

 fully developed. The first symptoms of the disease are seen in the 

 appearance of roundish, yellowish, thickened areas soon after 

 the leaves have reached their full size. Soon the areas become 

 slightly depressed on the lower side of the leaf and slightly 

 raised on the upper side. A cross section at this time shows that 

 beneath the epidermis the mycelium has become compacted into a 

 stroma-like tissue, and that myriads of unicellular curved conidio- 

 spores are being abjointed from short conidiophores or basidia 

 arising from the stroma. These spore-bearing areas are quite 

 completely covered by a membranous tissue which later becomes 

 somewhat broken. It is supposed that these spores, as usual, 

 serve to spread the disease during the growing season. The 

 conidial stage is usually referred to the form genus Melasmia 

 among the Fungi Imperfecti. 



As the season advances these spots gradually acquire a tough, 

 , . leathery consistency on 



Fig. 12, Rhytisma acerinum on maple. 



the upper side and be- 

 come black externally. 

 Internally the spots, which 

 may be a centimeter 

 across, are pure white. 

 The whole spot is in real- 

 ity made up of sclerotial 

 tissue with a rather firm 

 enclosing membrane. At 

 this time the spots are 

 much thicker than the 

 normal leaf, and they 

 continue to increase in 

 thickness until the leaf 

 falls from the tree, so 

 that at leaf fall they may 

 be three or more times as 

 thick as the uninfected 

 portions of the leaf. In 

 this condition the fungus 

 passes the winter. See 

 fig. 12. If the black spots 

 are examined with the 

 han^l lens at this time it is 



