310 Minnesota Plant Diseases. 



that, like the cup of the leaf-spot fungus of alfalfa, it can only 

 be seen clearly with the aid of a hand lens. It appears on 

 a leaf in small spots where the mycelium establishes itself. 

 These spots become thickened and from the center of each 

 the cup uncovers by the splitting out of the upper part so that 

 a star-shaped opening is produced and the layer of sacs is ex- 

 posed. Each sac contains eight spores. The spots may be- 

 come so numerous that the whole leaf or even the whole 

 plant is destroyed. The fungus may spread with great rapid- 

 ity. Burning of the fields in fall has been recommended to 

 prevent the reappearance of the fungus in the following year. 

 Frequent cuttings also tend to prevent the spread of the dis- 

 ease. 



Wheat scab (Fusarium culmorum W. G. Sm.\ The fungus 

 of wheat scab is an imperfect fungus. It attacks the grains of 

 the wheat just before ripening and causes the heads to ripen 

 prematurely. The heads may be entirely or only partially de- 

 stroyed. Affected parts turn whitish or are bleached. The 

 effects often travel from above downward in the head. The 

 mycelium runs over and through the spikelets and glues them 

 together. A gelatinous material is formed by the fungus 

 threads and this causes the glueing together of the spikelets. 

 The heads turn pinkish in color and the grains shrink. The 

 losses from this disease have at times been very serious. 



Poor drainage is said to increase the amount of scab. It 

 has also been reported that strong plants will resist the scab 

 more successfully than weak ones. No remedy for scab is at 

 present known. 



Flax wilt (Fusarium lini Bolley). Flax all over the world 

 is subject to a disease known as wilt. Whenever flax is raised 

 continuously on the same ground for a number of years it sick- 

 ens, and it soon becomes impossible to raise the plant success- 

 fully. This fact has been known for a long time in Europe arid 

 rotation of crops has long been practiced there to prevent the 

 disease. The wilting is due to a fungus parasite which attacks 

 the roots and stems of the flax plants in all stages. So virulent 

 does the disease become that after six years of continuous cul- 

 ture of flax on one plat of ground it has been found impossible 

 to raise one plant longer than three weeks. The following ac- 



