Studies of spray deposits. II.—The Tenacity of Copper Fungicides on Artificial and Leaf Surfaces

Some of the factors influencing the tenacity of spray deposits of cuprous oxide, cupric oxide, copper carbonate, copper oxychloride, Bordeaux and Burgundy mixtures have been investigated. Variation in the period of drying of the initial deposits, in the intensity of washing and in the temperature (up to 45°) had little effect on tenacity on artificial surfaces. Tenacity values were found to vary with the level but not with the type of distribution of the deposit. Except for Bordeaux, the tenacities of the copper fungicides increased with decreasing initial deposit and this effect is briefly discussed in terms of the relative importance of particle‐particle cohesion and particle‐surface adhesion. The tenacities of cupric oxide and copper oxychloride increased with decreasing particle size, but cuprous oxide and copper carbonate did not show this effect. The fungitoxicities of different size fractions of cupric oxide, determined against Alternaria tenuis , did not increase with decreasing particle sizes below 6 μ, suggesting that a saturation value of the surface had been reached. The physico‐chemical forces which determine the tenacity of a spray deposit, in the absence of supplements, are briefly discussed. The relative importance of wind and rain in the weathering of deposits was determined in a field experiment. Although rain was the more important weathering agent, the effect of wind was considerable and seemed to be more prolonged.