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THIRTY-FOURTH FRUIT-GROWERS ' COX\^XTION. 



its product most desired for commercial ends, may not be synonymous 

 at all with the normal, natural development which we ordinarily regard 

 as health. On the other hand, an unproductive condition or the'develop- 

 ment of undesirable qualities, may be most disastrous commercially, 

 and yet not indicate a condition of disease or anything abnormal 

 in the natural development of the plant. In other words, the perfection 

 of desirable commercial qualities is by no means the same as perfect, 

 normal, natural development or natural health. The fact is, that 

 the work of the plant pathologist in California, particularly in the 

 case of citrus fruits, is not limited to the study of actual* disease, 

 and the means of avoiding it. He is rather called upon to advance 

 commercial perfection, and learn how to overcome conditions which 

 are undesirable from this .standpoint, whether they be ones of actual 

 disease or not. 



Disease in the ordinary sense is the effect of some cause or condition 

 which interferes seriously with the normal functions of organic life. 

 We commonly understand such a condition as one of a somewhat more 

 complicated nature than that resulting from a simple injury. The 

 causes of plant disease are many. Most important, ordinarily, or at 

 any rate most easily accounted for. are those which are caused by the 

 action of what we call parasites. Parasites are living organisms, either 

 of an animal or vegetable nature, which live at the expense of other 

 individuals, usually growing closely associated with their tissues and 

 drawing nourishment from them. The plant pathologist does not 

 ordinarily undertake to investigate cases of parasitic injury caused 

 by insects, as the subject of entomology has become a large and im- 

 portant one. Plant pathology is, in the usual sense, the study of all 

 diseases or troubles with plants other than those caused by insects. 

 The parasites with which the plant pathologist has to deal are mostl.v 

 of a vegetable nature, and are largely of the class known as fungi. 

 The fungi are real plants of a low order of development, many of which 

 are parasites to a greater or less extent upon other plants. A great 

 majority of our best known plant diseases are caused by fungi, such 

 as the rusts, smuts, mildews, and many blights, rots. etc.. from which 

 very few of our commercial crops are free. A number of other plant 

 diseases are caused by bacteria, organisms of a still lower class, forms 

 of which are responsible for most of the serious animal diseases, but 

 of which only a comparative few affect plants. Some of the m( st 

 distinct bacterial plant diseases are the pear blight and walnut blight, 

 both well known in California. 



Aside from the troubles caused by parasites it is well known that 

 plants are affected in most various manners by influences resultina: 

 from the conditions under which they grow. Vegetable life is 

 dependent upon certain indispensable factors, which, acting in the 

 proper degree, contribute to the normal development of plants. These 

 factors may also vary to a considerable extent without seriously affect- 

 ing the life or condition of the plant, but beyond certain limits their 

 influence results in injury, or may go still farther and bring about a 

 condition which may be classed as a disease. The factors to which 

 we refer are those upon which all plant life depends, namely, moisture, 

 temperature, light, chemical food elements, and freedom from other 

 influences which might cause injury. In regard to these different 



