November, 1910.] 



419 



Plant Sanitation. 



bark has been attacked for some 

 time. There are two ways, at least, 

 in which such exudations can happen. 

 When bark is attacked by "pink disease," 

 it dries up, cracks, and splits away from 

 the wood. These cracks may extend 

 into the surrounding healthy tissue and 

 the latex then exudes from the latter. 

 When the pink fungus spreads further, 

 it involves the bark from which the 

 latex issued, and therefore the strands 

 of rubber are found on the diseased bark. 

 But the latex issued from this bark be- 

 fore it was attacked. The other way in 

 which the phenomenon can occur re- 

 quires the assistance of boring beetles. 

 In the early stages of "pink disease" and 

 "canker," the disease may affect only 

 the outer half of the bark, the part next 

 the cambium being still unaffected. If 

 this diseased bark is bored by beetles, 

 the latter may draw latex from the inner 

 half. This especially occurs if they bore 

 into the bark when the latex is stagnant: 

 they then penetrate through the diseased 

 bark right into the sound bark without 

 drawing latex at the time, but the latex 

 exudes from the hole "when the tree has 

 absorbed more moisture. 



After nearly six years' investigation of 

 Hevea diseases, it appears to me an axiom 

 that only healthy bark can yield latex. 



The question is frequently asked whe- 

 ther the pressure of the latex inside the 

 tree can burst the bark and in this way 

 produce streaks of rubber on the stem. 

 So far as the stems and branches which 

 have reached the "secondary" stage are 

 concerned the answer must be in the 

 negative ; all the supposed cases of this 

 phenomenon are capable of explanation 

 in other ways. Whether it can occur on 

 green stems is doubtful. In Hevea, it 

 most probably cannot; but the behav- 

 iour of Manihot dichotoma on some 

 occasions alter heavy rains suggests that 

 it can happen in that species. Given the 

 necessary apparatus, the question is one 

 which can be easily solved. 



It may be noted, as a matter of history 

 that, whatever credit attaches to the 

 proclamation of the elementary botani- 

 cal fact that a tree should not be tapped 

 all round at the same time, is due to 

 Parkin, who, in June, 1899, wrote "on 

 theoretical grounds it seems hardly desir- 

 able to tap all round the trunk of a tree 

 at one time, but better to tap one half 

 of the surface, leaving the other half 

 intact for another year or tapping season ; 

 thus there will be no risk of ringing the 

 tree. The food materials formed by the 

 leaves will have an uninterrupted path 

 on one side of the tree, whereby to pass 

 for the nourishment of the roots." 



SOME CONSIDERATIONS IN THE 

 TREATMENT OF PLANT 

 DISEASES. 



(Prom the Agricultural News, Vol. IX, 

 No. 215, July 23, 1910.) 

 One of the most important consider- 

 ations which enter into the treatment of 

 plant diseases is the relation between 

 the expense it involves and the return 

 in additional profit which it is likely to 

 yield. This relation is often dependent 

 on several factors of a purely local nature, 

 so that the correct solution of the pro- 

 blem, in any given instance, depends on 

 useful co-operation between the planter 

 and his advisers, Jt is with the object of 

 indicating in what ways this co-operation 

 is specially important that the following 

 matters are brought under discussion. 



When any crop is attacked by disease 

 of a fungoid origin, the nature of such 

 attack may be of two kinds : either it 

 may be epidemic and destroy, or threaten 

 to destroy, the whole crop in the course 

 of a short space of time, or it may cause 

 the steady loss of a certain percentage of 

 the produce during a long period; that 

 is, it may be eudemic. In the seaond 

 case, there is always to be taken into con- 

 sideration the additional danger of such 

 diseases becoming suddenly epidemic. 



The general nature of advice given by 

 a plant pathologist or mycologist in 

 dealing with disease falls under three 

 heads. Firstly, total destruction of the 

 diseased plants ; secondly, the application 

 of remedial and preventive measures to 

 diseased plants or to plants likely to 

 become diseased ; thirdly, permitting 

 the disease to take its course unchecked. 

 The first two kinds of advice may be 

 given in dealing with either epidemic or 

 endemic diseases. The third is only 

 applicable in the case of such endemic 

 diseases as do so little harm that the 

 expense involved in checking them is not 

 compensated for by the additional profits 

 obtainable from the crop. 



In the case of the sudden outbreak of 

 an epidemic on an estate, the best advice 

 that can be given is frequently for the 

 total destruction of the infected plants, 

 as, although it necessarily involves a 

 certain amount of loss, yet if the trouble 

 is treated at an early stage, this loss is 

 not serious in proportion to that which 

 would be sustained if the whole was 

 destroyed. In this case, the sacrifice of 

 a portion of the crop is not only justi- 

 fied, but often absolutely necessary, in 

 order to prevent the loss of the whole. 

 Remedial, followed by preventive mea- 

 sures, or remedial measures alone, can 

 only be recommended in such case, whew 



