James G. Horsfall

Injuriousness of bordeaux mixture

SummaryAlthough Bordeaux mixture was known to reduce potato yields in the absence of pests, it has been demonstrated that the yields are also reduced where pests are serious. In such cases the dwarfing effect is masked by pest control.A method is described for separating the two mutually exclusive effects when operating on the same plots. It involves determining the graphic relation of pest attack to yield, and determining by interpolation on parallel curves, the yield of treated and check at equal levels of pest attack.The yield reduction is approximately 12 per cent on Cobblers, and 11 per cent on Green Mountains at median levels of pest attack.It does not follow that Bordeaux should be discarded. That would be catastrophic in war time because no adequate substitutes are in sight. We must search for ways of reducing the injury. These are as follows: begin spraying as late in the season as possible; spray as seldom as possible rather than as often; use lots of water in the fewer sprays with less copper per tank full, rather than less water and more concentrated; reduce lime to half the weight of the blue stone; and use dolomitic lime.

Organic fungicides for late blight in Connecticut

SummaryThis paper briefly reports three years research on new fungicides especially dithiocarbamates on potatoes in Connecticut. Dithane, first tested here in 1941, and reported in 1943 has long been under test for late blight of potatoes. It has succeeded in areas like Florida where flea beetles and leafhoppers are of minor consequence. Until the advent of DDT to control these two pests, dithiocarbamates could not succeed elsewhere. The data reported herein suggest that dithiocarbamates, particularly Dithane, are not much if any better than Bordeaux mixture in the control of late blight, but they do permit the potatoes to set a larger yield.It is suggested that this is due to a lower level of phytoxicity which, in turn, is due to low lime content of the Dithane.

The strategy of finding fungicides.

An old English proverb says that, “You get what you ask for.” The fungicides we have are those we have asked for. Unless we change the form of asking occasionally, we shall run out of candidates. This is strategy.

Copper spray substitutes

SummaryFrom the above discussion it is evident that there are a number of materials which are useful as copper substitutes both as seed protectants and as foliage sprays. At the present time three materials are in production and have the abilty to control disease. These are tetrachlorquinone (Spergon), tetramethylthiuram disulfide (Thiosan and Dubay 1205 FF), and ferric dimethyldithiocarbamate (Fermate, Dubay 870, and IN-870). All of these have shown excellent disease controlling power under field conditions on a number of crops. Doubtless there are other materials of equal promise. We have, in fact, tested a number of other organic compounds and some copper salts of organic compounds which have made excellent showing under field conditions, but these compounds are not on the market or else have not been tested sufficiently widely to be safe to recommend for large-scale use at the present time. The three mentioned are merely the beginning; the possibilities remain legion in the field of copper substitutes.Shortcomings of the copper substitutes are their high cost, their limited production, their high specificity, and low tenacity. Methods of surmounting these difficulties are discussed.