RUST OF ANTIRRHINUM*. 63 



seedlings are absolutely clean and free from rust. Here we have a case 

 of seed from infected plants producing seedlings free from disease, although 

 they are growing in the space occupied by the diseased plants the previ- 

 ous year. Apparently their only protection is the drying out of the 

 urediniospores. 



Use of Fungicides. 



The copper salts and copper mixtures, the toxicity of which to P. 

 Antirrhini was tested in the laboratorv, are copper sulfate, cuprammonium 

 sulfate (Eau celeste), Bordeaux mixture (cupric sulfate to calcium oxide 

 in the ratio of 1 to 0.3 present), and Hammond's Copper Solution. It 

 is shown that Bordeaux mixture is absolutely useless for the control 

 of this disease, for at no strength suitable for use on plants does it prevent 

 germination, and sprayed plants when inoculated develop quite as much 

 rust as plants similarly inoculated but not sprayed. The toxic constitu- 

 ent of Bordeaux mixture is copper sulfate, and this used alone has a 

 toxic effect on the spores of P. Antirrhini, but in principle does not dissolve 

 with sufficient rapidity to be efficient against either P. Antirrhini or 

 U. Canjophyllinus. 



Copper sulfate solution, 0.25 per cent copper, is toxic to the uredinio- 

 spores of P. Antirrhini, but the use of this strength of copper sulfate on 

 snapdragon is precluded because of its toxic effect on the foliage. Cup- 

 rammonium sulfate (Eau celeste) is toxic to the urediniospores of P. 

 Antirrhini at 0.0625 per cent copper, but this strength of Eau celeste is 

 liable to result in a toxic action to the foliage of snapdragon, unless the 

 foliage can dry off in less than one hour. This nearly precludes the 

 use of Eau celeste on thick crowded plants, for the bottom foliage would 

 dry off too slowly. Eau celeste can be used only when the principle 

 toxic to the foliage can be volatilized by rapid drying. Hammond's 

 Copper Solution is not toxic to the urediniospores of P. Antirrhini, and 

 is therefore of no use for the control of snapdragon rust. 



A method for the control of snapdragon rust by dusting of the plants 

 with sulfur has been described by Dr. 0. R. Butler (1917). During the 

 winter of 1916-17 the writer inspected, at intervals of two weeks, green- 

 houses of snapdragon which had been thus treated. When the treatment 

 began the plants were in very bad shape, leaves and stems were fairly 

 covered with rust pustules. The first thing done was to cut out those 

 shoots so badly infected as to be hopeless. Many of them were girdled 

 and dying. The sulfur used was obtained from the Union Sulphur Com- 

 pany and from Corona Chemical Company. It is powdered finely 

 enough to pass through a sieve having 40,000 holes to the square inch. 

 It was applied with a good bellows that filled the air of the greenhouse 

 with dust, which settled as a thin even film on the foliage. For plants 

 10 inches high, 4 ounces of sulfur were applied to 150 square feet of bench. 

 The sulfuring was repeated at intervals of two to three weeks, as necessi- 

 tated by new growth of plants. Exposed blossoms were injured, but 

 there was no injury to the leaves. For two days after sulfuring the night 



