82 Minnesota Plant Diseases. 



parts. This stunting is shown in a few cases in the leaves of 

 plants. It may be accompanied by a stimulation of certain parts 

 of the leaf. For instance, a very simple little fungus may attack 

 dandelion leaves and produce tiny galls which appear as rough- 

 nesses on the surface of the leaf, while the leaf as a whole is con- 

 siderably smaller than an unattacked one. 



More frequently one meets with the stunting or total sup- 

 pression of flowers. Some plants, for instance, which may sup- 

 port a parasitic fungus for many years, never produce flowers. 

 Again, curiously deformed flowers are produced in which one 

 or the other kind of floral leaves are missing. Sometimes the 

 floral parts are present but strangely unlike the normal struc- 

 ture ; petals may be green and like simple foliage leaves or like 

 sepals. Fruits may be stunted in their growth. In cherries or 

 plums when the fruit is attacked by certain sac fungi "pockets" 

 are produced. These fruits, though much enlarged over the 

 normal fruits, never produce natural seeds and the stone is also 

 undeveloped. Not only may stunting affect the form and size 

 of the host, but the life of parts may be shortened. The 

 witches'-broom often furnishes examples of such age shorten- 

 ing. Here the leaves may fall long before the normal time. 



Stimulation of host. More conspicuous and more common 

 is the occurrence of stimulation of the host by the fungus para- 

 site. This stimulation may result in one or more of four effects, 

 viz. : (a) an increase in size ; (b) an age stimulation ; (c) the de- 

 velopment of normally undeveloped organs ; and (d) the forma- 

 tion of new organs. 



(a) Many cases of increase in size of organs are met with 

 as a result of fungus parasitism. The fungus galls mentioned 

 above are the simplest cases of such enlargements. The 

 branches of witches'-brooms are usually enlarged not only in 

 size but in numbers. The plum and cherry "pockets" are like- 

 wise enlargements. On the leaves of Labrador tea and other 

 heath plants may be formed large solid galls which are covered 

 with the spores of the parasite. Rusts produce enlargements of 

 the stems of various pines, forming huge spherical, burl-like 

 sw r ellings. Roots of the rushes are enlarged by the attack of 

 a smut fungus. Moreover, floral parts are often enlarged. 

 Petals, sepals, stamens or pistils may be stimulated by fungus 

 parasites to extraordinary growth. 



