DANGEROUS INTERNATION\L FOREST TREE DISEASES 51 



Literature : 



Waterman, Alma M., and Cash, Edith K. Leaf blotch of poplar 

 caused by a new species of Septotinia. Mycologia 42 : 374-384. 

 1950. 



Ito, K. Parasitic diseases of poplars in Japan. Forestry Agency 

 of Japan, 22 pp. 1959. 



Violet Root Rot 



Helicobasidium mom-pa Tanaka. Slender roots attacked by the 

 fungus become yellowish-brown or blackish-brown in color. They 

 are softened, rotted, and finally disappear. In the case of severe dam- 

 age, the cambial portion of large roots is heavily destroyed, only the 

 woody tissue and the bark with no trace of small roots remain. On 

 heavily affected roots, a great number of sclerotia buried in the cork 

 layer are seen very frequently. Purple rhizomorphs creep up the 

 surface of the roots and the trunk, increasing in diameter towards the 

 ends. "When the roots are severely injured it is not rare for numerous 

 small roots to develop. Diseased plants may thus escape death at 

 least temporarily. 



Soon after the aggregation of rhizomorphs reach the basal portion 

 of the host, they develop into a mycelial mat or a sporophore. The 

 sporophore formed on the basal part of the tree spreads upwards as 

 well as sidewards covering the surface of the trunk. During May to 

 July, the formation of the hymenial layer causes the surface of the 

 sporophore to become powdery white. Many of the diseased trees at 

 first show no remarkable changes in appearance above the ground, but, 

 by and by, some of them are noticeably weakened and finally killed. 

 In heavily affected trees, the leaves become more or less smaller, yel- 

 lowish, and fall earlier than in the healthy ones. In extreme cases 

 the entire subterranean portion of the diseased plant is almost com- 

 pletely destroyed by secondary invasion of wood-rotting fungi. 



Sporophores develop around the basal portion of the trunk up to a 

 height of 10 cm. or more, sometimes leaving here and there narrow 

 parts uncovered. They are sessile, resupinate, often irregularly lobed, 

 velvety, and membranaceous. Surface of the fruit-body, which was 

 deep purplish brown in early spring, becomes whitish or light pink 

 in color during the later part of the same season. In spore-bearing 

 sporophores, four or five layers are anatomically distinguished. Ba- 

 sidia do not arise from preformed resting cells, probasidia. Young 

 basidia are hyaline, smooth, erect, club-shaped; mature basidia are 

 curved, generally 3-septate, 30-50 X 5-10^, the 4 sterigmata are elon- 

 gated, narrowed towards end, 10-15/* long, tetraspored. Basidio- 

 spores are hyaline, ovoid, slightly curved, 12-25 X 5-9^ in size. 



This is one of the most important soil-borne diseases in Japan. Al- 

 most all tree species are susceptible. Spread of this disease is mainly 

 by translocation of infected seedlings or stocks. Many different con- 

 trol practices have been tried and recommended at various times, but 

 most of them have proved either ineffective or only partially success- 

 ful. 



Distribution: Japan, Korea, and Formosa. 



Hosts: Over 100 species, both ligneous and herbaceous, belonging 

 to various genera are known as the hosts of the fungus. Some 

 woody hosts containing introduced species are listed as follows: 



