— 2— 



rather of insects in their transformations than of plants. Each stage 

 has its own kind of spores. Frequently different stages occur on dif- 

 ferent host plants, as in wheat rust. This produces on the wheat itself 

 yellow summer spores and blackish spores which live over winter. The 

 summer spores retain their vitality only a short time; only the blackish 

 spores can carry the fungus through the winter. In spring they may 

 germinate on a different plant, the barberry, and produce a third stage 

 there; the spores from this stage, in their turn, produce yellow rust on 

 the wheat. 



It has been claimed that where fungi grow on plants, the plants 

 first become diseased and weakened and the fungus comes afterwards, 

 following and not causing the disease. This may be true in some cases, 

 but in all ordinary cases of this kind the fungus is the true cause of the 

 disease. Certain conditions of weather are favorable to its development, 

 just as certain conditions are favorable to the development of any oth- 

 er plant; but the conditions which favor the fungus may be unfavor- 

 able to the plant it grows upon and so give the former a double advan- 

 tage. Moreover plants have a greater power ana resist disease when/ 

 abundantly supplied with food materials, when they are not exhausted 

 by fruiting, when every condition is favorable to robust growth; but 

 the fungus may grow and produce the disease in spite of all this. A 

 man in robust health is less likely to succumb to a contagious disease, 

 as small pox, but still he is likely to take it. 



There are several ways in which a fungus may produce injury to the 

 plant. One way, common to all, is by taking away the food of the 

 plant for its own growth. On green parts, they destroy part of the 

 leaf-green and so reduce the power of the plant to supply itself with 

 food; in many cases the leaves fall, as from premature ripeness. Fre- 

 quently the fungus causes an abnormal development of plant tissues 

 as in the black knot of the plum tree and the curl of the peach leaves; 

 or they arrest development, as in the orange rust of blackberry, so 

 that rusted leaves are smaller than healthy ones. In many cases the 

 flower or fruit alone is destroyed, as iu the smut of wheat and oats, 

 "double blossom" of blackberries and the swelling of young plums. 



ORANGE RUST. J^l~^_Z-£-~, 



The most striking and most destructive fungus disease of swell fruits 

 is the orange rust, (Casoma nitens, Schw.) which occurs on raspberry 

 and blackberry leaves, and is especially destructive on the latter. This 

 has been most thoroughly studied by Professor T. J. Burrill, of the 

 University of Illinois, and many of the following facts are from his 



