298 FUNGOUS DISEASES OF PLANTS 



affected areas and then water-soaked, dark and dead. Conspicuous 

 lesions occur at the sheath nodes and upon the stems. When the 

 disease appears at or above the topmost stem node, it is generally 

 most serious. The maturing heads droop or fall to the ground. 

 Leaves affected at the tip of the sheath also hang downward. Old 

 leaves may develop spots with ash-colored centers and bright 

 brown borders. 



The fungus. Conidiophores and conidia of the fungus may be 

 found abundantly upon the affected parts in moist weather. The 

 former emerge from the stomata, generally in clusters of two or 

 three. They are ordinarily simple, fuliginous in color, septate, 

 and they bear in succession several conidia, each from a tip 

 which is for the time terminal. The spores are ovate, two-septate, 

 and measure 24-29 x 10-12 /A. Careful inoculation experiments 

 have shown that the fungus is able to induce the disease in unin- 

 jured, growing plants of various ages. The fungus on rice was 

 described as Piricularia Oryzce Briosi & Cavara, but the evidence 

 available indicates that the fungus concerned is identical with 

 Piricularia grisea, as above given. The latter is the name applied 

 to the fungus occurring in many regions upon the crab grass 

 (Panicum sanguinale LI). 



Control. It is unquestionably important in rice culture to pro- 

 vide the most favorable conditions for a vigorous growth of the 

 rice plant, and at present other direct preventive measures seem 

 impracticable. It would seem that varietal resistance will in time 

 offer the safest means of control. 



X. POLYTHRINCIUM 



Polythrincium Trifolii Kzc. Sooty spot of clover. This fungus 

 is very generally distributed upon certain species of clover, notably 

 red clover (Trifolium pratense\ in many parts of the world. The 

 wavy or spiral character of the conidiophores and the sooty or 

 fuliginous color of conidia and conidiophores are characteristic. 

 This species is the only one which has been described in the genus. 

 On account of the characteristics and habits of the mycelium and 

 of the stroma sometimes produced, it has been assumed that the 

 perfect stage would be a species of Phyllachora, and the plant 

 actually bears also the name Phyllachora Trifolii (Pers.) Fckl. 



