lo Sept., 1909.] TIic Victorian Potato Industry. 595 



Conditions Favorable to Disease. 



Before dealing more specifically with the conditions necessary to cope 

 with the pre.sent outbreak, it ma\ be mentioned that rotation of crops is 

 one of the most important measures that can be taken to combat all kinds 

 of vegetable diseases. The causes of these diseases may be classified in 

 two divisions, namely, exciting and contributing. The exciting cause is 

 in most cases one of the lower forms of plant or animal life — a mould 

 or fungus, as in the Irish Blight, or the eel worm, as in potato blister. 

 In each case the individual parasitic plant or animal is extremely minute, 

 perhaps invisible to the naked eye, while its spore or egg is of course 

 very much smaller still. In all cases the rapidity with which multiplica- 

 tion takes place soon gives rise to changes in the tissues of the potato, 

 easily visible to the naked eye, and the multitude of spores or eggs that 

 are produced renders the diffusion of the disease a very easy matter. 

 Contributing causes include such conditions as a weakened constitution 

 of the seed potato, unfavorable weather for the development and vital 

 activity of the foliage, and favorable weather for the growth and develop- 

 ment of the fungus. The popular idea, therefore, that most of the dis- 

 asters which befall crops of any kind are due to weather conditions has 

 a considerable amount of truth in it, although, of course, if the essential 

 or exciting cause were not present the weather alone could not produce 

 the disaster. To put the matter in a nutshell, the contributing causes 

 render the crop prone to disease, and make its invasion a comparatively 

 easy matter for the micro-organisms which cause the disease ; but however 

 favorable the contributing causes may be for the onset of the disease, this 

 cannot develop unless the specific exciting cause is also present. 



It will therefore be seen that where land has been de\'oted for a 

 number of \ears to the one crop it is highly probable that a sufiicient 

 amount of the special nutriment required by this crop will have been ex- 

 hausted from the soil so that the vigour of the crop receives a check ; or 

 it may be that by growing the crop for several years in succession a 

 slight attack of disease occurring the previous year ma\ have infested 

 the soil and rendered things favorable for a larger outbreak. Perhaps 

 what most frequently happens is that successive crops have paved the 

 way by weakening the plant, w'hile the exciting cause is introduced by the 

 seed Avhich is planted in the paddock, so that a combination of the two 

 causes is responsible for most epidemics. 



When once a disea.se is established, its eradication becomes a matter 

 of great difficulty. The soil may become infected, the remains of one 

 season's crop may carry on the infection to the next year ; the micro-organ- 

 ism may betake itself to allied plants, and it may be transferred from one 

 district to another by all kinds of unsuspected ways. Knowledge of the 

 life history of the exciting cause may place us on the track of the right 

 means to prevent it, but the conditions of life of both the disease and the 

 plant it attacks are so complicated that caution has to be exercised in 

 dra\\ing conclusions from any set of exp>eriments. A farmer \ery often 

 talks as if slight differences in the soil of one part of his farm, or a few 

 weeks extra dry weather, were solely responsible for this or that result. 

 As a matter of fact no kind of experimental work is so uncertain as 

 that which deals with plants and animals. The difference existing be- 

 tween living and dead things is so profound that insignificant variations 

 in any respect may make unexpectedly great differences in the final result. 

 The moral of which is that no farmer can afford to neglect the smallest 

 detail when he is dealing with any question of disease. 



