and Magazine 0/ the Ceylon Agricultural Society* 



89 



IMMUNITY TO DISEASE AMONG 

 PLANTS. 



The diseases of plants and the immunity to 

 disease of certain varieties is a subject which 

 should engage the interest of tropical planters, 

 as on their plantations fungus and other 

 diseases are very numerous. The following is 

 abridged from a lecture delivered in Manches- 

 ter by Professsor Weiss. 



The question of immunity to disease has been 

 so closely studied and so frequently discussed 

 in connection with the diseases of man that it 

 seemed to me that it might be of interest to 

 bring together 6omo of the facts now known to 

 us about the incidence of disease among plants 

 and the theories which have been advanced as 

 to the cause of the immunity which some species 

 and varieties exhibit to various diseases. 



The late Prof. Marshall Ward has shown that 

 Puccinia dispersa, the brown rust of grasses, 

 seems to exist in several "biologic forms, ,; each 

 of which attacks only one group of nearly related 

 species of Bromus, and the same condition ob- 

 tains in the Erisiphese, or mildews, according to 

 Salmon. How is it that these fungi are incap- 

 able of infecting such nearly related host plants 

 as are represented by the speaies within a single 

 genus ? The suggestion was originally made 

 that differences in the thickness of the cell walls, 

 fewer or smaller stomata, longer hairs, &c, were 

 the obstacles which repelled the fungi and ren- 

 dered certain species and genera of plants im- 

 mune to the attacks of particular fungi. Work- 

 ing with the different species of Brome, Marshall 

 vVard was, however, able to show that there was 

 no relationship between the stomata, hairs, and 

 so forth, and the infectibility of the species. 

 Immunity did not in any way depond upon the 

 anatomical characters of the host plant, but 

 entirely on physiological reactions of the proto- 

 plasm of the fungus and of the cells of the host. 

 In other words, 



INFECTION AND RESISTANCE TO INFECTION 



depend on the power of the fungus protoplasm 

 to overcome the resistance of the colls of the 

 host by means of tnzymes or toxins, and recip- 

 rocally on the protoplasm of the cells of the 

 host to form anti bodies which destroy such 

 enzymes or toxins, just as is the case with re- 

 sistance of animal organisms to their bacterial 

 foes. Salmon has shown in his experiments that 

 susceptibility in a leaf normally immune to the 

 attacks of the biologic form of a particular mil- 

 dew may be induced by various mechanical 

 means, such as cutting tho leaf or searing it with 



a re.I-hot point of a knife, or by exposing the 

 leaf to ether or alcohol vapours, or by exposing 

 it to heat. The resistant vitality is thereby im- 

 paired, and the fungus gains the upper hand. 

 Plants, if not immune to a particular disease, 

 may be rendered so to a certain extent by simi- 

 lar methods to those employed in the case of 

 animals. More or less successful injection ex- 

 periments have been made in the case of fruit 

 trees suffering from chlorosis, and as a result 

 animal parasites have been got rid of as well. 

 Undoubtedly if the general vitality of the tree 

 can be raised some diseases can be thrown off. 



Marchal has stated, 1902, that young plants of 

 the lettuce could be rendered immune against 

 Bremia latucae by feeding the plants with a 

 solution of copper sulphate (1 in 30,000). This 

 view has received support from Laurent and 

 Massee, but Salmon has not been able to con- 

 firm these results. It will be seen that the 

 views are still somewhat conflicting, and too 

 much must not be expected from such methods 

 of treatment. 



THE HOPE OF THE AGRICULTURIST 



lies in another direction. Plants, like animals, 

 are subject, as Darwin has shown, to a consider- 

 able amount of variation, and all characters, 

 whether anatomical or physiological, are subject 

 to change or mutation. Immunity to disease, 

 dependent as it is on certain physiological pecu- 

 liarities, the secretion of anti-toxins, rather 

 than on anatomical structure, is similarly a sub- 

 ject of variation. We see this readily illustrated 

 when passing through a field exposed to some 

 epidemic disease, where here and there plants 

 are found which have been either only slightly 

 damaged or not attacked at all. These should 

 bo selected for breeding purposes, and thus 

 hardier varieties can be produced. Anothet 

 method which has shown itself useful for 



PRODUCING RESISTANT FORMS IS BY HYBRIDISING. 



It is a well-known fact that hybrids, while par- 

 taking of the nature of one or both of the parents 

 in most characters, generally exceed both in 

 vegetative vigour — a characteristic to which the 

 sterility of some hybrids is attributed. But 

 vegetative vigour, as we have seen above, is 

 generally associated with immunity to disease, 

 and hence hybrids are often found to be more 

 resistant. This is not always the case, for in 

 this respect hybrids vary too, but tho French 

 horticulturists, MM. Bouttes and Guillon, have 

 been successful in producing hybrid vines which 

 are more resistant to the mildew than either of 

 the parents. 



