19 



infected by means of spores blown from the rosette of first 

 formed diseased leaves or flowers, and from them additional 

 spores are produced which provide for further infection. 



The appearance of diseased trees later in the year varies 

 with the season and the locality, but more especially with the 

 variety of apple. In favourable seasons and in localities which 

 do not suffer badly, the mildew, except for crippling a few 

 shoots, does not cause extensive damage, and those varieties 

 which in other districts are usually badly injured may almost 

 entirely escape. In other localities, especially in parts of 

 Cambridgeshire and Worcestershire, the mildew is always 

 troublesome, and in the case of susceptible varieties (see list 

 below) may so injure the leaves that by July a considerable 

 amount of defoliation may take place. Such trees present a 

 very characteristic appearance, the young shoots being devoid 

 of leaves, except near the growing tips, and, if the attack has 

 been very severe, altogether leafless (Fig. 2). The affected 

 shoots are much weakened and most of them die back very 

 considerably during the winter. If the number attacked in this 

 way be not too great the mildew can, with judicious pruning, 

 be controlled and the trees saved, but if such measures are not 

 resorted to whole trees will in course of time become so 

 seriously injured as to be almost worthless. 



Though the flower buds are often affected the fungus in 

 later stages is confined as a rule to the shoots and leaves, 

 but in a few varieties, notably in Lane's Prince Albert, it 

 may attack and injure the young fruit. Bad cracking of the 

 fruit of both apples and pears by the mildew has been reported 

 from the Continent. 



Method of Overwintering. As is well known, the mildew 

 fungi possess two kinds of spores, conidia or summei 

 spores, and resting or winter spores. The former are pro- 

 duced very abundantly in spring and summer, their function 

 being to distribute and reproduce the fungus during the 

 -growing season, the latter, which occur in minute spore 

 cases termed perithecia, are formed in autumn and normally 

 remain dormant till spring, when they germinate and 

 infect the new foliage. The apple mildew is remarkable 

 in possessing an additional method of hibernation. Though 

 its perithecia are now known to be produced more 

 frequently than was formerly thought, they are, except 

 on certain varieties, always scarce and often entirely 

 absent. When they occur they are produced onthe " wood,' 1 

 usually at the base of the current year's growth. It 

 is very doubtful, however, if the resting spores contained 

 in these perithecia play any part in the propagation of the 

 disease, as it is quite certain that the first outbreaks of the 

 mildew in spring are to be found, not scattered! here nnd 

 -there on a few leaves as in cases resulting from, spore- 



