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 Hast 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. 
