REPORT OF THE SOCIETY 5 1 



by some other means, the fungus obtains a start on the new season's crop. It is 

 interesting to note at this point that it apparently makes little or no difference 

 how prevalent the fungus may have been the previous season. Successive seasons 

 when no late blight can be found may be followed by severe epiphytotics provided 

 the necessary predisposing weather conditions prevail during that season. As to 

 the successive stages by which the blight spreads in the field during June and July 

 there is little known. It is difficult to find any evidence of its presence at this 

 period, although so far as one can judge the same weather conditions prevail 

 which are accompanied by severe epiphytotics in August. The fungus must be 

 passing through successive cycles, however, or it would be difficult to account 

 for its almost uniform abundance and quickly destructive attack over large 

 areas in August. The important correlation between weather conditions and 

 blight epiphytotics occurs after the blooming period and usually after the first of 

 August. For a primary epiphytotic several days of high humidity, accompanied 

 by intermittent rains and cloudy weather, together with a low temperature 

 (52-73. Fahr.) are necessary.^ The intensity of the attack will vary with the 

 degree of humidity, range of temperature and the duration of these conditions. 

 If this period is then followed by shorter or longer periods of the same conditions, 

 progressive development often results in total destruction of the tops. Direct 

 losses thus result from low yield and more or less rot of the tubers. 



After this brief review of the factors at work in the potato field for the control 

 of which spraying is necessary, we may now consider some of the factors which 

 determine the effectiveness of spraying. A little attention paid to the proper 

 methods of making and applying bordeaux mixture will insure the maximum 

 return for the investment of labor and material. 



Bordeaux mixture is by far the most satisfactory substance to use on potatoes. 

 This mixture consists of colloidal plates or membranes inclosing lime particles, 

 lime particles in suspension and all contained in a strong solution of calcium 

 hydroxide and calcium sulfate. The composition of the colloidal membranes is 

 doubtful, but they are probably a complex mixture of basic copper sulfates. The 

 important fungicidal properties of bordeaux mixture are due to the number and 

 size of the colloidal membranes and to the lime particles. When the mixture 

 dries on the leaves the colloidal membranes and lime particles adhere tenaciously 

 to the surface of the leaf. In this condition the copper and lime remain effective 

 for a long time. After once dried the particles will not wash off.^ 



The strength of the mixture is a matter of choice to a certain extent. It is 

 doubtful if a mixture stronger than 4-4-50 or 5-5-50 is required. The strength 

 5-5-50 has been found to contain at least twice the number of colloidal membranes 

 that a 2^-2^-50 mixture contains. Stronger mixtures are, however, propor- 

 tionately wasteful of materials since the membranes are much thicker and do not 



