18 ''Reversion" and Resistance to '' Bbj BmV 



much smaller (Fig. 10). The growing point has had time to change into 

 a flower rudiment but the scale leaves are deformed, showing the charac- 

 teristic semi-spherical shape of big buds. There is however no mite 

 present though sometimes the flower rudiment shows brown areas where 

 it has presumably been injured by mite. In extreme cases of this stage 

 the flower rudiment has been killed and one of the accessory growing 

 points has changed into a very weak flower rudiment. 



The last stage is the "big bud." Here the attack is so mild or the 

 growing point so strong that the latter is not killed but is stimulated 

 to irregular growth. In this resistant variety they occur almost without 

 exception low down on the yearly growth. 



The terminal bud may be either a " blind" one or a "killed" one but 

 is only in the most rare cases "big." 



These killed buds cause an interesting pathological effect on the pith. 

 Ever since the disease had first come under the author's notice certain 

 curious brown marks in the pith have been observed. They occur as 

 smaller or larger streaks in the pith of the main stems but appear to start 

 as spots just below a lateral bud. 



I am indebted to my colleague Mr 0. Grove, Oenologist at Long 

 Ashton, for his investigation of it. He failed to find any organism 

 microscopically and also failed to obtain any living culture from such 

 spots. It is improbable therefore that they are caused by any organism. 

 Microscopically they appear as brown gummy masses and strongly 

 suggest degenerative changes. 



When examining this mited material from Messrs Seabrook it was 

 found that such brown spots were always connected with killed or blank 

 buds. In a few cases the very beginning of the process could be seen 

 consisting of a small spot only, immediately under the killed growing 

 point. Not in every case of a killed bud could the spots be seen but 

 wherever there was a brown spot a killed bud could be found close by 

 from which the browning had started. It would appear probable therefore 

 that such brown spots are due to degenerative changes in the pith cells 

 consequent on the sap flow to the killed growing point being stopped. 

 The phenomenon is an effect of the disease rather than a cause. 



The presence of these "blind" and "killed" buds on shoots of "Sea- 

 brook's Black" immediately suggests the reasons of its resistance. 

 Strictly speaking it is not resistant but very susceptible, so much so that 

 the mite kills the growing point in an attacked bud and as a consecjuence 

 cuts oft" its own supplies and thus perishes. A few buds however become 

 " big " and serve as a small source of infection the following year. 



