Minnesota Plant Diseases. 



59 



the rest of the plant. Then arise the carbuncle-like swellings 

 of the leaves. If a kernel of the cob is attacked it increases 

 perhaps tenfold in size. During this increase of size the fungus 

 is also gaining strength and keeping pace with its partner 

 plant-part, and when the proper moment has arrived for the 

 formation of its spores it proceeds rapidly and utilizes all the 

 extra food stored up by the swollen host plant-parts and de- 

 stroys the latter rapidly. Such a parasite stimulates its host 

 to unusual activity for a long time and at the same time pre- 

 pares to use to best advantage all of the nutrient material laid 



up by the host, delaying its destruc- 

 tive effects until the most advanta- 

 geous moment. Sometimes, as in oat 

 smuts, the presence of the fungus is 

 not determinable until harvest-time, 

 when the fungus forms its smutty 

 powder of spores in place of the 

 grain. This is a very efficient meth- 

 od of parasitism but, in some respects 

 at least, the fungi producing grain 

 rusts are even more capable. The 

 smut produces but one kind of spore 

 on its host plant and that is a resting 

 spore for tiding the fungus plant over 

 the winter season. The rust fungus 

 can produce spores from early spring 

 to autumn and is able to do this by 

 forming different kinds of spores at 

 different seasons. Such a rust will 

 produce a spring spore, a summer 

 spore, and in autumn a so-called win- 

 ter spore, the latter having the same 

 function as the spore of the smuts. 

 This continuous production of spores 

 is of course a very efficient device. 

 In addition to the multiplicity of 

 spores the rust fungi often possess the stimulating powers al- 

 ready mentioned for smuts. Such have also been described in 

 witches'-brooms of red cedar and balsam fir. In the simpler 



FIG. 27. Oat smut an accom- 

 plished parasite. After G. P. 

 Clinton. 



