APPLE POWDERY MILDEW AND ITS CONTROL. 25 



ting out the mildewed twigs on which the infected buds are borne 

 is the only successful method of dealing with this phase of the 

 problem. Fortunately, this work can be done during the dormant 

 season, for the grayish mildew covering remains on the diseased 

 twigs, making them readily distinguishable from the healthy ones 

 even after the leaves have fallen. The cutting out of the mildewed 

 twigs should therefore be made an important part of the regular 

 pruning operations. 



Vigorous foliage is less susceptible to mildew attack than that 

 which is puny and slow growing. One method of obtaining vigor- 

 ous foliage is by winter pruning ; hence, because of the value of vig- 

 orous foliage in the problem of mildew control, as well as for the 

 betterment of the general condition of the trees, a more thorough 

 system of pruning should be practiced in the Pajaro Valley. An in- 

 creased vigor of foliage growth is also obtained by the spraying 

 methods recommended in this bulletin. 



Precipitated sulphur or sulphur in other extremely finely divided 

 forms is the most satisfactory fungicide to use in foliage spraying 

 for the control of apple powdery mildew. 



. In the Pajaro Valley several different effects may be obtained from 

 spraying with such a finely divided form of sulphur, as follows : 



(1) If the diseased areas are thoroughly covered with the spray mixture, the 

 mildew will be killed out. A deposit of spray on the healthy foliage will pre- 

 vent the establishment of new infections. As has been noted on a previous 

 page, it is practically impossible, by spraying, to clean up satisfactorily those 

 virulent cases of leaf and twig infection in which both the upper and lower sur- 

 faces of all the leaves, as well as the bark surface itself, become covered with 

 mildew. The solution of this phase of the problem is in pruning out these 

 mildewed twigs during the dormant season. 



(2) Sulphur acts as a stimulant and induces the production of a vigorous 

 growth of new foliage. 



(3) Spraying with strong mixtures or allowing too long an interval to elapse 

 between sprayings may, under Pajaro Valley conditions, result in serious foliage 

 and fruit dropping, though in the East no such damage from sulphur spraying 

 has thus far been reported. 



(4) Spraying at frequent intervals with weak mixtures causes the tree to 

 develop what the writers have termed an immunity to the damaging sulphur 

 effects noted in the preceding paragraph. 



(5) Finely divided sulphur in the spray mixture acts as a restrainer in re-' 

 ducing the tendency of zinc arsenite or acid arsenate of lead to produce the 

 arsenical burning of foliage. 



Winter spraying with crude-oil emulsion, as practiced in the 

 Pajaro Valley at the present time, has been found effective in stimu- 

 lating a vigorous growth of early foliage the following spring. Be- 

 cause of this stimulation, the use of crude-oil emulsion as a dormant 

 spray offers valuable assistance in the general program of mildew 

 control, for, as has been stated, vigorous foliage is less affected by 

 the disease. 



