10 Nov., 1917.] Notes on Doiriu/ Mildew. 687 



waterings, and the proximity of tlie young growth to the moist soil 

 surface. It has also been noticed in France, as will be seen from the 

 following extract : — * 



The defence of youiif; nursery vines n;»ainst Mildew presents, as is known, 

 great difficulty. Weekly, and even daily treatments, according to some, have 

 not always <;iven satisfactory results, because of (1) almndance of winter spores 

 left by previous nursery ojierations; (2) proximity to tlie soil, where dew and 

 moisture persist longer than at a certain height from it; (.3) the necessary 

 waterings; (4) the late start and late growth of the scion shoots, which cause 

 the disease to develop with a violence, and especially with a persistence very rare 

 in the open vineyard. 



It might appear at first sight, especially to those inexperienced in 

 the treatment of this particular fungus, that energetic steps in the direc- 

 tion of wholesale spraying should have been taken, with a view to 

 stamping out the disease in its early stages. Any such action would 

 have been very costly and quite futile, in view of the widespread nature 

 of the outbreak at the time when the fungus was first identified. Abso- 

 lute extermination of this, as of any other fungus, once it has become 

 fairly widespread, is, of course, altogether out of the question, and 

 in view of the very small extent to which the fungus could be found 

 in any one vineyard wholesale spraying could not logically be recom- 

 mended, much less insisted on. 



Mildew belongs essentially to the category of fungi, the dissemina- 

 tion of which is exceedingly rapid, given suitable weather conditions. 

 It differs radically from several other slow spreading diseases, such as 

 Bla.ck Spot, for example, in that, given an exceedingly limited Source 

 of infection and suitable weather conditions, wholesale contamination 

 soon results. t In other words, the number of spores present is of little 

 consequence in deciding whether a severe outbreak will or will not take 

 place ; this depends exclusively on the weather. Unless atmospheric 

 conditions prevail, altogether different from those usual in our inland 

 climate, the spread of the fungus is impossible. 



That the severity of an outbreak of Mildew depends on suitable 

 climatic conditions, and not on the number of spores which started it, 

 receives confirmation from a recent article by M. J. CapusJ dealing 

 with the behaviour of Mildew during the French summer of last year. 



" It is easy to understand that Mildew, with its extraordinary variations 

 from one year to another, should disconcert vine-growers." .... In answer 

 to a correspondent, he continues, " It is true . . . that we are far from know- 

 ing everything concerning the evolution of Mildew, nevertheless we are equally 

 far from being altogether ignorant concerning it. . . . All the factors which 

 act on fungi are probably known to us; but wliat we do not j-et know thoroughly 

 is tlie precise moment when each factor commences to act, the extent of their 

 actions, and the reciprocal influences they exert upon one another. 



The scarcity of the disease in the vineyards of France, considered as a whole, 

 following on a year of invasions as sevei-e as last year's were, shows clearly, 

 once again, that abundance of germs at the close of a season does not fatally 

 indicate that invasions mil be serious the folloicing year — (the italics are ours). 

 It is not the first time that a year free from Mildew follows a year when it was 

 redoubtable; on the other hand, the disease has frequently been known to cause 

 serious damage after having disappeared from the vineyards for one or more 

 years. It is not the number of germs that matters, it is the fate with which they 

 meet in the shape of atmospheric conditions. 



• L. Ravaz. in Progres Aliricule, 9th April. 1916. 



t " One single vine infected in spring suffices for the disease to spread immediately to a distance 

 under the action of the most gentle breeze which transports the conidia . ..." P. ViaUi. — Lu 

 Maladies de la Vyjtie. p. 100. 



J Rei'W lie Viticulture, 3rd August. 1916. 



