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PLANT SANITATION. 



[June, 1909. 



MISCELLANEA : CHIEFLY PATHO- 

 LOGICAL. 



ByT, Pbtch. 



When arsenical compounds were 

 first introduced as insecticides and weed- 

 killers, doubts were expressed that the 

 continual addition of these compounds 

 to the soil would result in injuries to 

 the roots of the plants, and that the 

 soil would become incapable of grow- 

 ing further crops owing to the accu- 

 mulation of the poisons. Experiments 

 in this direction, however, indicated that 

 practically no danger was to be feared 

 since the arsenic became insoluble in 

 the soil and passed downwards to a very 

 little distance. More recently, owing 

 to extensive injuries to fruit trees in 

 Colorado, which were attributed to 

 arsenical and lead poisoning from the 

 use of calcium arsenite and lead arsen- 

 ate for the control of insect pests, the 

 subject has again been investigated by 

 W.P. Headden. of the Colorado Experi- 

 ment Station. He finds that arsenical 

 poisons have accumulated in the soil 

 of the orchards to a large extent, and 

 that, while most of the arsenic is in an 

 insoluble form, the sodium salbs of the 

 soil— e.g., sodium carbonate, sodium 

 sulphate, and sodium chloride— have 

 rendered so much of it soluble that it 

 has exceeded the limit of danger. It 

 is thought that systematic poisoning 

 may occur through the absorption of 

 this soluble arsenic by the roots of the 

 trees, since the wood in extreme cases 

 contains over twelve parts of arsenic 

 ner million, but the chief effect is attri- 

 buted to local irritant poisoning at the 

 collar. The affected trees are girdled at 

 the collar, the bark ou portions of the 

 trunk dead and sunken, and most of the 

 roots dead, their bark destroyed, and the 

 wood discoloured. The first marked 

 symptom is an early ripening of the 

 foliage, usually followed by the death 

 of the tree about midsummer m the 

 following year. Experiments with solu- 

 ble arsenical compounds showed that 

 these would produce all the effects 

 noted, and the arsenic in ai senate of 

 lime was found to be more re £ a , dll y 

 brought into solution than that ot lead 

 arsenate. While the above investiga- 

 tion refers to effects produced by pro- 

 longed application of insoluble arsen- 

 ates which are only slowly rendered 

 soluble in the soil, and the effect of a 

 single such application might be negli- 



gible, it may be pointed out that "weed- 

 killers " are soluble arsenical compounds 

 and take effect immediately. It is ex- 

 tremely doubtful, therefore, whether 

 arsenical weed-killers can be used with 

 safety in permanent cultivations. 



The cultivation of clove trees, which 

 was once an extensive industry in 

 Singapore, is said to have almost ceased 

 owing to the attacks of disease ; and 

 it is stated that the chief cause is a 

 fungus which produces red spots on the 

 leaves, On the appearance of this 

 report, the few clove trees at Pera- 

 deniya were examined to see whether 

 the same disease occurred in Ceylon. 

 All were found to be fairly vigorous, 

 and showed no signs of any serious 

 injury, except in one instance. Dark 

 red spots were fairly common on the 

 leaves, but these were purely local 

 and did not affect the general health 

 ot the trees ; they answer to the des- 

 cription of the spots on the leaves in 

 Singapore, though, until the fungus in 

 the latter case is identified, it is impos- 

 sible to determine whether the cause is 

 the same. The red spots at Peradeniya 

 are caused by a red ale'a, Cephaleuros 

 'parasitica. This species is closely re- 

 lated to Cephaleuros mycoidea, which 

 causes the *'red rust" of tea; but, 

 whereas the latter lives on the surface 

 of the leaves of tea and most tropical 

 trees without causing much damage, 

 Cephaleuros parasitica lives inside the 

 leaf, and only its fruiting branches are 

 visible externally like a tuft of minute 

 red hairs. C. parasitica is fairly com- 

 mon ou cinnamon and tea in the low 

 country; on cinnamon it causes a dark- 

 brown, stellate, raised patch, while on 

 tea it produces a spot which superfici- 

 ally resembles " Grey Blight." The red 

 spot on clove leaves is due to the death 

 of the tissues attacked by the alga., not 

 to the colour of the latter. Neither ou 

 tea, nor cinnamon, nor clove does it 

 cause any serious damage. Some of the 

 branches of the clove trees were found 

 to be killed by Loranthus cuneatus ; aud 

 in one case, where a branch had been 

 broken off, the main stem was attacked 

 by Fomes substygius, which is thus 

 proved to be a wound parasite, 



It is recognised that in the extension 

 ot green manuring in the tropics to 

 regions where Crotalaria striata aud 

 Erythrina will not grow, success is more 

 likely to be attained by the cultivation 

 of leguminous weeds of the country 



