July 23. 191 7 Black Rootrot of Apple 171 



slight foliage symptoms of rootrot are found to have the infection of the 

 root system so well established that the removal of infected parts from 

 the tree and soil seems a practical impossibility. Control measures 

 must therefore be directed toward the prevention of replant infection 

 and further spread of the infection to healthy trees. Experiments with 

 soil disinfection on a small scale offered but little promise. Replants 

 set after soil treatment with some of the common disinfectants died 

 as rapidly as those on untreated soil. Enough work on this line has not 

 been carried on, however, to warrant general conclusions. Tests of the 

 susceptibility of different rootstocks is suggested as a promising line of 

 experiment. 



It has been the practice among some orchardists to lime the soil from 

 which trees affected with rootrot were removed before replanting. We 

 have not secured suflEicient data to judge of the value of this treatment, 

 but it seems probable from laboratory studies that an alkaline soil 

 is less favorable for the growth of species of Xylaria than an acid soil. 

 The radius of colonies of X. hypoxylon after three weeks' growth on 

 starch-agar media, which ranged in reaction between — 20° and +10° 

 Fuller's scale was as follows: — 20°= 2.6 cm. ; — 10° = 3.0 cm. ; neutral = 

 3.0 cm.; +io° = 4.5 cm. Similarly the addition of hydrated lime 

 (Ca(0H)3) to the medium produced a check in the growth of colonies of X. 

 hypoxylon, the radius of the colony on the untreated medium at 17 days 

 being 4.0 cm., while that on the medium receiving hydrated lime was 

 2.5 cm. 



Field observations indicate that infection, is commonly distributed 

 through the orchard in cultivation. The establishment of quarantined 

 areas, where the disease is confined to limited blocks of trees, and the 

 withholding of cultivation within these areas, thus preventing the 

 carrying over of infective material to the noninfected areas, should 

 prove of value in some cases. The removal of borers without disinfection 

 of the knife may readily transmit infection from diseased to healthy trees. 

 It further seems quite probable that the infection of healthy trees may 

 result from the intermingling of their roots with those of diseased trees, 

 but infection would probably progress slowly in this manner. 



The Xylarias are commonly present on stumps of forest trees, and 

 rootrot is consequently most severe on newly cleared land. It seems 

 that the use of cultivated land only for orchard sites may be of consider- 

 able practical importance as a preventive measure. 



SUMMARY 



Black rootrot of the apple is an infectious disease of very considerable 

 economic importance which has become a serious menace in the chief 

 orchard sections of Virginia. 



Foliage symptoms of the disease are not markedly different from those 

 produced by injuries due to other agencies, but the black encrustations 

 98975°— 17 2 



