RUST OF ANTIRRHINUM. 



61 



As a continuation of this experiment urediniosjiorcs were taken from 

 snapdragon plants which had been dusted with powdered sulfur at a 

 temperature not less than 21° C. These plants bore; many spore pustules, 

 and the fungus was to all appearances vigorous. But these spores failed 

 to germinate when placed under optimum conditions for germination. 



Fungine is a commercial preparation which has been used by some 

 growers in their attempts to control snapdragon rust. As stated by 

 the makers, it contains potassium polysulfide 6 per cent, and potassium 

 thiosulfate 4 per cent. The writer tested the toxicity of this preparation 

 to the spores of U . Caryophyllinus and of P. Antirrhini at various strengths 

 and at the strength recommended by the makers. Fungine proved 

 toxic to the spores of both of these fungi. 



Table 12. — Effect of Various Strengths of Fungine on the Germination of 

 the Urediniospores of P. Antirrhini and of U. Carijophyllinus. 



Fungine, though toxic to the spores of these fungi, has certain disad- 

 vantages. It is no more efficient than powdered sulfur, but it costs 

 more than the sulfur, and the sulfur dust reaches parts of the plant which 

 a liquid spray would not. Fungine when sprayed on a slide or leaf has 

 a physical character resembling a soap film, and this soapiness makes it 

 wash off the leaf too easily, which may in part account for Peltier's con- 

 clusion {loc. cit.) that snapdragon rust in the field cannot be controlled 

 by Fungine. 



Temperature Regulation. 



It has been shown that the urediniospores of P. Antirrhini cannot 

 germinate below 5° C nor above 20° C, and that they germinate best 

 at 10° C. (50° F.); 50° F. is the night temperature at which the snap- 

 dragon is usually grown under glass; it is frequently grown in the house 

 with carnations. This temperature results in a maximum amount of 

 rust on the snapdragon. The carnation, on the other hand, is grown 

 at a night temperature of 4° C. below the optimum for germination of 

 the spores of carnation rust, and this may in part explain why carnation 

 rust is so much less serious than snapdragon rust. Day temperatures 

 of the houses are not important in the study of snapdragon rust, for 

 those temperatures are usually too high for germination of the uredinio- 

 spores, so we may consider infection as taking place only in the night. 



