2 86 



Minnesota Plant Diseases. 



Fig. 142.— Spores of the common "black rust" (Puccinia 

 graminis) of wheat. 1. Cluster-cup spores from the 

 barberry plant. 2. Summer spores from the wheat 

 plant. 3. Winter spores from the wheat plant. Highly 

 magnified. After Arthur and Holway. 



aggerated because the smaller losses due to the presence of the 

 rust in very slight and therefore unheeded quantities may never 

 be computed. These 

 are, nevertheless, a' 

 certain loss. There 

 is only a difference 

 in degree. The en- 

 tire eHmination of /^^^^^ 

 rust would therefore ' 

 increase the value ^~^ 

 of crops throughout 

 the country by an 

 enormous per cent. 



At present there 

 is no known method 

 for successfully com- 

 bating wheat rust. 

 Numerous attempts 

 to fight the disease 

 by spraying with 

 bordeaux and other mixtures have always proved unsuccessful. 

 Where stem rust is the principal form and barberry bushes are 

 common, it has been found that the removal of barberry bushes 

 will diminish the rust considerably. In Minnesota, however, 

 very few barberry bushes are grown, and of these many are often 

 not infected with grass-rust cluster-cups, while the host plants 

 upon which the crown rust and orange leaf rust grow are very 

 common wild plants. The cluster cup of the orange leaf rust 

 forms on plants of the borage family, but no cluster cups have 

 yet been found on these plants in this state, so that either our 

 orange leaf rust differs from the European form or else it can 

 here dispense with the cluster-cup stage altogether. Little can 

 therefore-be hoped from the removal of the cluster-cup host 

 plants. There is still another factor which would help to de- 

 feat such a method. In at least one of these rusts, the my- 

 celium which produces the summer spores may live through 

 the winter and produce summer spores again in the following 

 summer, thus dispensing with the necessity of the cluster-cup 

 stage. It is also possible that even in some forms, where the 



