lyo Journal of Agricultural Research voi. x. no. 4 



Trichoderma koningi, when placed on wounded living apple roots in a 

 moist chamber, grew for a time on the medium transferred, but produced 

 no penetration of the root. Sphaeropsis tnalorum, however, proved 

 pathogenic on apple roots, but the infection progressed more slowly than 

 that produced with Xylaria hypoxylon. The lesion had extended 1.5 

 cm. proximally and distally and to a depth of 5 mm. after four weeks 

 (PI. 1 5 , £) . Recovery in pure culture was obtained. 



FIELD INOCULATIONS 



Results similar to those obtained in moist chambers have been obtained 

 in inoculations of living apple roots in the field. A number of i -year-old 

 apple trees were inoculated with Xylaria hypoxylon through a small bark 

 wound near the base of the main root and then planted. A few of these 

 examined after four months all showed infection. One bore a lesion 

 1.5 cm. long which had penetrated to the center of the root. X. hypox- 

 ylon was recovered from the margin of the lesion. A number of roots 

 of a 20-year-old apple tree were cut across about 5 feet from the trunk, 

 and mycelium of X. hypoxylon was inserted beneath the bark at the cut 

 end. The wound was covered with cotton and the soil replaced. Two 

 of these roots examined after seven weeks had been killed 4 cm. from the 

 wound and bore the typical black encrustations on the surface. Another 

 series of roots were inoculated through small bark wounds. After five 

 weeks infection was well established and was advancing into living tissue. 



CONCLUSIONS FROM INOCULATION STUDIES 



Xylaria hypoxylon has proved to be an active wound parasite on apple 

 roots; Xylaria sp. from Harrisonburg is also a wound parasite, and X. 

 polymorpha appears to' be only slightly or occasiong,lly parasitic. Sphae- 

 ropsis malorum is the only other fungus obtained from apple roots that 

 has proven parasitic. The isolation of X. hypoxylon from roots of affected 

 trees from 9 of the 1 1 orchards under observation and the experimental 

 evidence of its pathogenicity, accompanied by the production of symp- 

 toms in inoculated roots typical of those found in the orchard, indicate 

 that this fungus is the common cause of the black rootrot of the apple in 

 Virginia. Confirmatory evidence must be obtained in the production of 

 typical symptoms and death in growing apple trees from pure-culture 

 inoculations. This is provided for in a series of 500 young apple trees 

 inoculated and planted during the past year (19 16) at Blacksburg. Posi- 

 tive results from these inoculations can not be expected for a year or 

 more, since the disease progresses slowly in nature. 



CONTROL OF BLACK ROOTROT 



No control measures of proved value are known. There is little 

 possibility of effecting the cure of diseased trees. Those showing but 



