539 



ANNUAL ADDRESS. 



Honey-Dew Smut and Photosynthesis. 



]!V DAVID L. CRAWFORD. 



^lost liomopterous sap-feeding- insects excrete a consider- 

 able (luantity of honey-dew which spreads over the foHage and 

 stems and supports the growth of certain epiphytic fungi. 

 The mycelium of these fungi is. black and usually forms a crust 

 adhering to the surface of leaves, stems and fruit. 



Sugar cane leaves are commonly blackened by this "smut" 

 where the Pcrkiiisiclla leaf-hopper occurs in cjuantity. Avocado 

 foliage is frequently badly crusted by the presence of mealy-bugs 

 (Pscudococnis nipac). Orange leaves and fruits as well are 

 commonly blackened by the mold. In California this is due 

 primarily to the black scale fSaissetia oleae) and is considered 

 as a very serious detriment to successful citriculture. In Flor- 

 ida a similar smutting on citrus leaves is caused by Aley- 

 rodidae. 



Plants which become encrusted with honey-dew smut are 

 more or less dwarfed or stunted or otherwise injured. Sugar 

 cane plants are commonly stunted and citrus foliage when 

 smutted is usually dwarfed. Fruit trees often fail to set fruit 

 when the foliage is badly covered with snuit. 



\'ery obviously a part, at least, of this injury is due to the 

 removal of sap by the insects, a state of starvation resulting. 

 It is commonlv believed, however, that the presence of a black 

 crust on the leaves is an additional injury, cutting off part or 

 all of the light energy which is needed for photosynthesis, and 

 thus reducing the rate of food manufacture in the leaves and 

 bringing about a condition of starvation. 



This appears to be a very logical conclusion, for plant 

 physiology teaches that luminous energy is one of the indis- 

 pensable factors in photos}nthesis, and when a screen is inter- 

 posed between a green leaf surface and the light source in such 

 a way as to exclude all light, jihotosynthesis ceases. The 

 honev-dew smut is black and often thick enough to exclude 



Proe. Haw. Eiit. Soc, IV, No. 3, Sciptciiihcr, 1921. 



