HYGIENE OF PLANTS. 189 



these microscopic pests. He should write them down in his list of "worst weeds, " and try 

 to keep his fields free from them. There ought also to be some legal means of redress when 

 a nurseryman or seedsman has taken hard-earned money and in return has infected a man's 

 land and rendered his business unprofitable, but in most cases there is not. The ounce of 

 prevention, therefore, is the thing to be thought of, and more and more the farmer must 

 consider the advisability of suitably disinfecting trees and seeds before planting them, 

 unless he knows their source to be a safe one. Many dealers who own propagating farms 

 also buy large quantities of stock from other growers, so that the farmer seldom knows 

 from what part of the country his plants have come. He may think he is buying from an 

 uninfected region while in reality his trees may have come from diseased localities. In case 

 of two kinds of seed sold extensively by the trade, viz., sweet corn and cabbage, it is notori- 

 ous that they are propagated for seed largely in districts where no such seed should be grown, 

 because the plantations often reek with disease, and the germs of these diseases (Stewart's 

 disease of corn and the black-rot of cabbage) are liable to be distributed on the seeds. 



Excess of water undoubtedly renders many plants more susceptible to bacterial dis- 

 eases. The evidence here is very good in a number of cases. It is now a well-known fact 

 first observed, I think, by L. R. Jones, working in the writer's laboratory, that the soft-rot 

 organisms need tissues filled with water in order to make rapid progress. In pear-blight 

 slow growth is favorable to freedom from the disease and excessive moisture leading to 

 rapid growth renders the plant much more susceptible to disease : This has been observed 

 over and over again in many localities. Russell obtained black-rot of cabbage more readily 

 on well watered plots. Halsted observed the same thing in beans. The writer has seen 

 the same thing in sweet corn and in tomatoes. It is, therefore, desirable that moist soil 

 should be properly underdrained and that irrigation should be managed with great care. 

 Lack of subsoil drainage is very favorable to the development of root rots of all sorts espe- 

 cially in rainy seasons. Not infrequently an entire field of potatoes rots within a week from 

 this cause. Excess of water on the foliage also leads to numerous infections as in the case 

 of black spot of the plum, bean-spot, begonia-spot, etc. Where the plants are out of doors 

 this can not be avoided, altogether, but under glass it can be modified by care in throwing 

 water and by so arranging the houses that all parts shall have proper ventilation. Dis- 

 eases often begin in ill-ventilated parts of the hothouse. 



Overcrowding may also be a cause of disease in some instances, especially if it leads 

 to imperfect ventilation and the persistence of water upon the foliage. 



In one instance the writer observed a bacterial rot to be favored by excess of shade, 

 namely the iris rot. Under his observation this was very serious one year in heavily shaded 

 parts of a garden, and not at all present on the same grounds in clumps of the same iris 

 exposed fully to the sun. Slow evaporation of water was probably the predisposing cause. 



Excess of manures, especially of those containing nitrogen, renders the plant more 

 susceptible to cold and probably also to disease by throwing it out of physiological balance 

 but I can cite no specific instances of bacterial diseases particularly favored by this in any 

 way other than by the formation of rapidly-growing juicy tissues. Infected manures 

 are to be avoided carefully. By this I mean barnyard manures with which diseased 

 plants have been mixed. The writer saw a field of cabbage in Michigan, a small portion 

 of which was much worse diseased by the black-rot than the remainder. This portion 

 contained perhaps an acre or two, and the only difference between it and the remainder of 

 the field appeared to be that it had received as a manure the refuse from a neighboring cab- 

 bage storehouse, in which heads affected by the black-rot had been placed the preceding fall. 

 The potato-rots are diseases likely to be transmitted in this way since diseased potatoes are 

 often fed to stock or thrown out on manure piles. Stewart's disease of sweet corn is another 

 disease likely to be distributed in this way since the farmer often feeds the stalks to cattle, 

 and these are swarming with the organism which causes the disease. The bacterial spot of 

 beans is another. 



