96 The Resistance to Fungicides 



Further, some evidence has been collected showing that the age of 

 the mildew even when in the "powdery" conidial stage is a factor of 

 importance. The older conidial patches have less power of resistance 

 to the soluble sulphide spray than the young patches. 



The explanation of the difference in resistance to fungicides shown 

 by the earliest and the later stages of development cannot be given until 

 further data are available. It would appear that to some extent the 

 age and condition of the host-leaf are concerned; the mildew has less 

 resistance in its conidial stage when on an old hop-leaf than when on 

 a vigorous young leaf. But that this is not the whole explanation is 

 shown by the fact, mentioned above, that on one and the same leaf 

 a sulphide solution will kill the mildew where present in its well-developed 

 conidial stage and fail to kill it in its youngest stages of development. 

 It is possible that the earliest stage of development is more resistant 

 because the first-formed haustorium from the appressorium of the germ- 

 tube has more vigorous powers of resistance than the later-formed 

 haustoria, or because the appressorium offers powers of resistance ; on 

 the other hand it may be that the vigorously growing conidial stage is 

 less resistant through the formation of thinner- walled hyphae, or that 

 the great number of haustoria present allows the fungicide to reach more 

 easily the epidermal cells of the host- plant and cut off the food-supply. 



On the old hypothesis that the sulphide solution acts fungicidally by 

 virtue of the sulphur deposited, it could be held that the "powdery" 

 conidial patches are killed because a sufficient amount of the sulphide 

 solution is collected round them to give the requisite deposition of 

 sulphur, while this would not take place in the case of the earliest stage 

 of development when there are few mycelial byphae and no conidio- 

 phores. We are unable to accept this explanation, however, since our 

 work of last year has led us to the conclusion that with regard to this 

 class of fungicides the sulphides contained in solution act as such and 

 not by virtue of the sulphur deposited. 



Whether it will be more economic to attempt to kill a "powdery 

 mildew" by spraying at a strength lethal to its earliest stage of develop- 

 ment, or to wait until the mildew has developed into the less resistant 

 conidial stajje, must be decided by future experiments in the laboratory 

 and field. It is clear, howe\^er, that in the treatment of "powdery 

 mildews" generally this difference of susceptibility at different stages 

 of development must be kept in mind as a new factor of importance. 



