10 Nov.. 1917.] Notes on Downy Mildeu'. 691 



Mildew visitations are much less intense than in the moistcr centre and 

 north. In France the warm south winds cross the Mediterranean, and 

 are saturated with moisture, causing heavy dews at night, which, to 

 an Australian, seem very curious in hot weather. It is the cold north 

 wind which is dry. This is known in the south as the " Mistral " ; when 

 it blows Mildew is not feared. 



In Victoria we have no moi.st wind ; our sea breezes are too cold 

 to hold much moisture. On the nther hand, our dry north winds are 

 our greatest safeguards against fungus pests of all kinds. The almost 

 complete absence last season of this characteristic feature of our climate 

 is one of the main factors which rendered last year's outbreak possible. 



Outward Appearance of the Disease. 



All green or herbaceous parts of the vine are liable to attack — stems, 

 leaves, tendrils, or bunches — but it is usually first observed on the leaves, 

 on which it manifests itself in the shape of spots at first of a lighter 

 or yellowish green, more or less circulai-, and varying in size from that 

 of a sixpence to a shilling, and even larger. These spots become 

 yellowish, then brown, and finally die out altogether. In the early stages 

 they are more easily distinguished if the leaves are held up to the light, 

 owing to their being more transparent than normal leaf tissue ; hence 

 the French term of "oil spots," which aptly describes them. After a 

 while, on the under side of the spots, provided climatic conditions are 

 suitable, a milk-white efflorescence, or down, makes its appearance — ■ 

 vrhence the name of Downy Mildew, as distinguished from Powdery 

 Mildew, the name given to Oidium (Uncinula spiralis) in America. 

 This white down is very characteristic, and readily distinguishes this 

 from any other vine fungus. It is close, dense, and glistening, and 

 reminds one of fine fable salt or sugar. Except in very rare cases, it is 

 only visible on the under surface of the spots already mentioned, which 

 are thus yellowish-green or brown above and white beneath. 



In dry weather the characteristic down does not develop, the spots 

 dry up, and assume a dead-leaf colour, without sending out the spore- 

 bearing filaments, which constitute the down. In moist warm weather 

 the down is very apparent, being often similar to a dense while felt, 

 but in dry weather it may be entirely absent or very scanty. If sus- 

 pected leaves are placed for a day or two in a warm dark place wrapped 

 in a moist towel, the white down will make its ap])earance. 



Mildew is quite different from Oidium; the latter appears equally 

 on the upjjer and under surface, and never causes brown spots. The 

 mottled appearance sometimes presented by leaves attacked by Oidium 

 is quite different to the well demarcated spots of Mildew. The white 

 downy efflorescence only appears on the imder side ; it is much denser 

 and whiter than Oidium, which develops on upper and under surface 

 alike. 



Mildew is also very distinct from Eriiiose, though at first sight there 

 might appear to be some similarity; the spots of the latter are always 

 convex above — they resemble hollow warts; Mildew spots are always 

 quite flat. In the case of Erinose, the felt inside the hollow warts, 

 though white at first, soon becomes i-ust colour ; Mildew down is always 

 white. 



When other organs, such as canes or bunches, are attacked they also 

 show yellowish-green patches,, the white down appearing later. If 



