62 MASS. EXPERIMENT STATION BULLETIN 202. 



If growers would raise or lower the night temperature of the snapdragon 

 house to 52° F. or 48° F., the rust would decrease in amount about 50 

 per cent. This is indicated in the constant curve showing the relation 

 between temperature and germination. It must be remembered that 

 this temperature change prevents infections and prevents the spread of 

 the disease, but it does not kill the spores. So if the temperature ap- 

 proaches the germination optimum even for a few hours, the disease may 

 break out again. Rise of temperature as a control is further considered 

 under treatment with sulfur. Growers may object to raising the tem- 

 perature very much above 50° F. because of the danger of shortening 

 the blossom spikes, but a rise of even two or three degrees will check 

 the rust, and is not likely to diminish the value of the blossom spikes. 



Selection of Resistant Varieties. 

 Forty-six varieties of snapdragon have been observed by the writer, 

 and their relative resistance to P. antirrhini has been determined. The 

 most susceptible varieties are Half Dwarf, Rose Queen, Fiery Belt, Crim- 

 son Queen Victoria, Ruby, Carter's Pink, Delicate Rose, Dwarf Golden 

 Queen, Sulphur Yellow, Venus, Carter's Gold Crest and Chamois. It 

 is recommended that the above varieties be not grown at all. The most 

 resistant varieties are Queen of the North, Pure White, Rose Dore, 

 Giant White, Crimson, Giant Blood Red, Giant Yellow, Striped Varieties, 

 Hephsetos, Phelp's Wliite, White Queen Victoria, Firebrand and Mont 

 Blanc. It is recommended that outdoor gardeners confine themselves 

 principally to these varieties. These varieties, while not absolutely 

 resistant, are the nearest approach to it among snapdragons. Florists 

 grow only a few varieties, as a rule, notably. Keystone, Silver Pink, 

 Buxton's Pink, Phelp's White and Nelrose. None of these varieties is 

 really resistant. Florists can control this disease less by the selection 

 of resistant varieties than can outdoor gardeners, but the florist can 

 propagate from resistant individuals if any appear, and meanwhile safe- 

 guard his crop by the sulfur treatment. 



Regulation of Moisture. 

 It has been shown that although temperature does not kill the ure- 

 diniospores of P. Antirrhini, six weeks of drying does kill them. The 

 teliospore may be eliminated as a factor; and as the urediniospores cannot 

 germinate after six weeks of drying, there is no danger of the disease 

 being transmitted on dry seed; also it is evidently impossible for ure- 

 diniospores to live from season to season in a greenhouse if the snapdragons 

 are removed and the house deprived of water for a period of at least 

 eight weeks. A case of this kind has recently come to the attention of 

 the writer. A house of snapdragons was severely attacked by rust last 

 year. This year mignonette is being grown in the space occupied by last 

 year's rusted snapdragons. Among the mignonette plants are many 

 seedling snapdragons, the descendants of the rusted plants, but these 



