MILDEWS. 49 



monest weeds, are, so to speak, own cousins of valuable garden 

 plants, and many cases might be cited where the same parasite 

 attacks both. Another case within our present group is that of 

 the cabbage mildew, which is not, however, very destructive to 

 that crop, but attacks far more frequently and more fatally the 

 shepherd's purse, peppergrass, and other weeds of the mustard 

 family to which also the cabbage belongs. The life-history and 

 effect on the host-plant of this mildew agree in general with those 

 of others of the group, and further detail is unnecessary. 



Another parasite of the group we are considering has caused 

 extensive damage in the "West, during the past few years, to Fox- 

 tail grass and to Hungarian grass or Millet. The leaves and 

 flower-spikes become distorted and of a dark gray color, and winter 

 spores are formed very abundantly in the tissues. The summer 

 spores, on the contrary, are more sparingly or less conspicuously 

 developed than in most of the mildews, so that the characteristic 

 felted appearance is not so noticeable. 



Several other species of mildew-fungi attack cultivated plants 

 in Europe and may be expected in this country. Of these the 

 onion-rot or mildew has been observed in the United States, but 

 has not to my knowledge become epidemic, at least in our own 

 state. Others which, so far as I know, have not yet been found 

 with us, are the mildews of spinach, of beets, of poppies, and of 

 rose leaves, though there is no theoretical reason why the}' may 

 not be introduced at any time. 



We come now to the practical problem of combatting these 

 diseases, and are in a position intelligently to attempt its solution. 

 Against the species which produce winter spores in the leaves, an 

 obvious precaution is to thoroughly collect, remove, and destroy 

 all the fallen leaves which may contain these spores. In the case of 

 the potato rot, whose threads winter over in the tubers, great care 

 should be taken to plant only sound tubers. And here it may be 

 remai-ked that slightly infected tubers often show almost no trace 

 of their infection and would be regarded, on ordinary inspection, 

 as sound. On the other hand, in a season unfavorable to the 

 development of the parasite, badly infected tubers may fail to com- 

 municate the disease to the plants grown from them. But our 

 chief object must be to prevent the development and dissemina- 

 tion of the summer spores ; or, failing in that, to prevent their 

 germination and penetration into the tissues of healthy plants. 

 4 



