162 MANUAL OF TREE DISEASES 



Caiise. 



The gray mold twig-blight of conifers is caused by the fungus 

 Botrytis dnerea. The name first given to this fungus on conifers 

 was Botrytis Douglasii Tubeuf. In some publications this 

 fungus is erroneously called Sclerotinia Fuckeliana (De Bary) 

 Fuckel, due to a suspected connection of the Botrytis with this 

 ascomycete. Spores are borne in abundance on the gray mold- 

 like growth of myceliiun over the affected parts. These spores 

 are wind-disseminated and serve to distribute the fungus dur- 

 ing the summer. The small black, more or less globose bodies 

 formed on the twigs and needles are called sclerotia. They are 

 resting stri^ctures composed of mycelium which is rich in re- 

 serve food material and covered by a black rind-tissue of 

 mycelium. The fungus over-winters in this way and many 

 upright branches bearing spores are sent out from the sclerotia 

 in the spring. 



The mycelium within the needles and twigs causes at first 

 an enlargement of some of the tissues and later their death. 

 Abundant atmospheric moisture is required for the general and 

 destructive distribution of the fungus in a given region. Fogs 

 are conducive to epiphytotics. This is due to atmospheric 

 moisture stimulating an abundant growth of superficial my- 

 celium which is necessary for the formation of the spores. Also 

 these same conditions insure that a larger percentage of the 

 spores can germinate and cause infection. 



Control. 



Since the severity of attack is largely dependent on a con- 

 tinuously humid atmosphere, any measures which will lead to 

 a greater circulation of the air will serve in a measure to control 

 this disease. Damp soil and close planting should be avoided 

 for the more susceptible trees. In the nursery and in young 

 plantations weeds and any plants or artificial structures which 

 shade the trees should be removed. 



