Minnesota Plant Diseases. 



285 



internally a mycelium. The latter, under the most favorable con- 

 ditions in about eight days, and under less favorable conditions 

 usually within tw^o weeks, again produces the summer spores. 



The life-history of a wheat 

 rust can therefore be divided into 

 three parts; first, the stage on the 

 grass or wheat plant, producing the 

 red (summer) and black (winter) 

 spores in succession; second, the 

 germination of the winter spores 

 and the production of sporidia on 

 the ground or in the straw in the 

 springtime; third, the germination 

 of the sporidia on barberry (or bor- 

 age, buckthorn, according to the 

 particular form of wheat rust), and 

 the subsequent formation from the 

 mycelium so produced, of cluster- 

 cups and pycnidial stages. 



In seasons that are favorable 

 for the development of the rusts 

 whole crops may be completely 

 ruined ; but the danger does not end 

 here. Crops are often mot so con- 

 siderably affected and may appear 

 but slightly rusted. The latter cases 

 are often lightly passed over as of 

 no account but such is not the case. 

 If the rust is present in any notice- 

 able amount it is safe to say that 

 the parasitic fungus is levying a tax 

 of nutrition and energy upon the 

 host or crop plant which results in 

 crops correspondingly lighter than 

 should be the case if no rust were present. That is to say, the 

 nutrition which could normally be expended in the formation of 

 more and heavier grains is required to nourish the parasitic rust. 

 The almost fabulous figures recorded each year as the loss to 

 farmers by rust is more often probably underestimated than ex- 



FiG. 141. — Oat stems and leaf bases 

 with clusters of summer spores 

 of the oat rust. The spots are 

 large and not sharply defined. 

 Original. 



