DISEASES AND INSECTS 



DISEASES AND INSECTS 1025 



trate into the tender tissues of the blossom, causing 

 the blossom blight. From these blighted blossoms, 

 sucking insects like the aphids carry the bacteria to 

 the tips of the rapidly growing shoots when in sucking 

 sap they introduce the organisms and twig blight fol- 

 lows. The striped cucumber beetle is probably the 

 chief disseminator of Bacillus tracheiphilus, which 

 causes the cucumber-wilt. 



Ecological conditions as affecting disease. 



By ecology is meant the influence of such environ- 

 mental factors as climate, weather, soil and fertilizers, 

 on the disease, its severity, epidemic occurrence, and 

 the like. These factors may influence the severity of 

 the disease by their effect on either the pathogen or the 

 host, or both. For example, most fungous parasites re- 

 quire the presence of water on the host plant in which 

 their spores may germinate, hence severe epidemics of 

 such diseases as potato-blight, apple-scab, brown-rot of 

 stone fruits and black-rot of grapes usually appear in wet 

 seasons. Moreover, the attacking pathogen is especially 

 favored by wet weather at certain seasons or pe- 

 riods in its development, especially the infection 

 period. Continued spring rains about blossoming 

 time favor apple-scab and peach leaf-curl. 

 Late summer rains bring with them epidemics 

 of late blight of potatoes, brown-rot of 

 peaches or late infections of apple- 

 scab. Frequent or continuous rains 

 during June and July in grape re- 

 gions are usually accompanied by 

 severe attacks of the black -rot 

 pathogen. The relation of rainfall 

 to the pathogen explains why, when 

 there has been a severe epidemic the previous season, 

 the crop may escape if the following season be dry. 

 There is ever a critical period in the development of 

 the pathogen, usually when it is passing from its rest- 

 ing or winter stage to the active vegetative period of 

 the growing season. Moisture and temperature condi- 

 tions at such periods largely determine whether the 

 disease will be epidemic or not. Of course the neces- 

 sary abundance of spores to be disseminated is an 

 evident necessity. Favorable weather alone cannot 

 bring on disease as the grower too often believes. 



The absence of rains at certain stages in their develop- 

 ment is for other pathogens equally essential. The loose 

 smuts of cereals afford good examples. Their spores 

 are powdery and wind-borne and if rains fall when they 



are being dis- 



fij seminated, they 



are washed to 

 the ground and 

 perish instead 

 of finding their 

 way into the 

 open blossoms 

 of their host. 

 Thus, clear 

 sunny weather 

 during the blos- 

 soming period 

 of wheat and 

 oats one season 

 usually means a 

 more or less se- 

 vere epidemic of 

 smuts the next, 

 while rains at 

 this time, even 

 though there be 

 an abundance of 



. _. the disease, may 



1284. Disease of cucumber leaf. The , 



dying margin indicates that the trouble is 



due to some interference with the food crop the fOUOW- 



supply. (XH) ing year. 



On the other hand, weather conditions may deter- 

 mine the severity or absence of certain diseases by its 

 effect on the host. Long-continued cold rainy weather 

 in the spring, especially following a warm spell, results 

 in a slow succulent growth of the developing peach 

 leaves, rendering them especially susceptible to the 

 attacks of the leaf -curl pathogen. 



The application of cer- 

 tain fertilizers to the soil 

 is known to have a direct 

 effect, either favorable or 

 unfavorable, on different 

 pathogens. 

 The applica- 

 tion of lime 

 or of manure 

 to the soil 

 tends greatly 

 to increase 



1285. Peaches of last year's crop still hanging on the tree, attacked by monilia. 

 The branch is dead from the effects of the fungus . ( X 1 A) 



the scab of potatoes planted thereon; while, on the 

 other hand, liming the soil prevents infection of cabbage 

 and cauliflower by the club-root pathogen. Lime like- 

 wise favors the development of the root-rot of tobacco 

 and ginseng caused by Thielavia basicola, while appli- 

 cations of acid phosphate tend to prevent infection by 

 this pathogen. The effect of fertilizers on the suscep- 

 tibility of the host has also been shown to be marked 

 in certain cases. Barley, when fertilized with nitro- 

 genous manures, becomes very susceptible to attacks 

 of the mildew Erysiphe graminis. Certain varieties of 

 wheat have been observed in Denmark to suffer 

 severely from attacks of the rust Puccinia glumarum 

 only when nitrogenous manures are applied. Exces- 

 sive applications of barnyard manure to greenhouse 

 cucumbers often cause a physiological disease, the 

 symptoms of which are a curling, and dying of the 

 margins of the leaves, accompanied by marked chloro- 

 sis or yellowing. Fertilizers or late continued cultiva- 

 tion or pear trees, by lengthening the period of active 

 twig-growth, favor fire-blight, the bacteria of which 

 infect only tender actively growing tissues. 



Control of diseases. 



By the term control is meant the profitable reduc- 

 tion of the losses ordinarily sustained from a given 

 disease. The absolute prevention of many plant dis- 

 eases is either impossible or unprofitable. 



There are four fundamental principles upon which all 

 methods of plant-disease control are based, viz.: (1) 

 exclusion, (2) eradication, (3) protection and (4) im- 

 munization. Upon the first two are based those meas- 

 ures which are directed primarily against the pathogen, 

 upon the last two those which are directed merely 

 toward the protection of the host from pathogens 

 commonly present in the environment. The order in 

 which these principles are here presented represent the 

 logical, though unfortunately not the historical or 

 usual order of their development and application. We 

 will consider briefly under each some of the more 

 important methods now employed for the control of 

 plant diseases. 



