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, vzz., sweet corn and cabbage, it is notori- 
ous that they are propagated for seed largelyin 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 oft 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. ~ ; 
