December i, 1882.! THE TROPICAL AGRICULTURIST. 



46J 



paniments of leaf-disease, and has attracted the at- 

 teution of most iilanters. The prevalerce of gruh in 

 the youngir districts has been generally assigned as 

 the cause, but it was unaccnnntable in the older dis- 

 tricts where grub does not exist to any cousideralile 

 extent, unless it were in eoine mysterious way con- 

 nected with leaf-disease, the re-action after a bad 

 attack, or the unhealthy condition of the trees caused 

 by hemileia. But none" of them appear to me to 

 satisfactorily clear up the mystery. I have been much 

 interested in the letters of your correspondent "W.," 

 and w hile agreeing w ith him in the conclusion he 

 arrives at in tlie last" paragraph of his letter No. 7, 

 wherein he attributes the falling-off of the bearing 

 of our trees to a cycle of unfertile years, I must j.jin 

 issue with him in his attempt to dr.w a wide dis- 

 tinction between the nature of the coffee leaf fungus 

 and that of the potato " peronospera." He says the 

 difference cannot be over-estimated. I would, however, 

 draw his attention to a pamphlet bj' J. L. Jensen, 

 Director of the Bureau Ceres, Copenhanen, the name 

 of which is " How to overcome the potato disease," 

 published by Menzies & Co., Edinburgh, (an extract 

 from wliicli I herewith enclose) a careful perusal of 

 which, will 1 tliiuk, on the contrary shew that the 

 simUarity cannot be over-estimated, and mayiiap throw 

 an important light on the question of the dearth of 

 feeding rootlets. You wdl see that the exhaustive 

 experiments of Mons. Jensen leave no room to iloubl 

 that if the spores of the peronospera which fall from 

 the foliage, can be prevented from lieing carried by 

 rain water into direct contact with the tuber, the 

 potato does not liecome infected, even although the 

 foliage may be entirely destroyed. It is now some 

 forty yeare since the potato disease was first dis- 

 covered, and hitherto it haa been understood that 

 either the foliage was first attacked and the disease 

 conveyed by tlie stem to the tuber, or vice versa ; 

 this too, notwithstandmg tuat the science of every 

 civilized nation has been brought to bear on the 

 "subject, studj'iug its life history and devising remedies, 

 as we kuow, to little or no purpose. It has been 

 reserved for the seicutitic and practical Mons. Jensen 

 to discover the real working of the pest and a simple 

 and economical remedy. In the face of the fore- 

 going, then, may not Marshall Ward's "Life His- 

 tory of Hemileia V;istatrix " be to some extent in- 

 accurate ? Is it not possible, nay I think extremely 

 probable, that the spores of the fungus may be 

 carried to the young and tender rootlets, entirely 

 destroying those they come in contact with ? Mar- 

 shall Ward found no traces in the stem or roots, 

 which may be easily enough explained by tlie faoi 

 of the germinal tube of the spore being unable to 

 penetrate the more tough epidermis of the stem and 

 root. It must be remembered that the spores do 

 germinate on the giouuil under favoutable conditions, 

 and the very tender rootlet may bi come their temporary 

 host, the destruction of which would be a matter 

 of very short time. If the potato fungus finda an 

 equally genial host in the foli.age and tuber of the 

 potato, why should not hemileia devastate both leaf 

 and rootlet. I have always held that our coffee 

 suffers more from want of ronilets than from the fall 

 of leaf. Should it therefore be found that there is 

 truth in my conclusions the combatting of our com- 

 mon enemy will be much simplified. It has occ\irred 

 to me further that the extraordinary increase of the 

 grub pest may be entirely owing to thi rotting rootlets 

 being a f-vorite source of food for the grub. I can 

 from actual observation voucli for 'he fact that 

 grub feeds greedily on decaying rootlets. A' all 

 events, the whole subject is well worthy of further 

 investigation by planteis, as well as by local experts. — 

 Yours faitlifnlly, J. S. 



[We have already given Beveral extracts bearing on 



Mr. Jensen's method in the Tropical AyricuUuriM. The 

 followingis theesseulial portion of his pap'-j-. — Kd] 



The vital point iu the system, as it will be noticed is 

 the " Protective Moulding." — In order to understand the 

 effect of this, it will be necessary here to add a few words 

 about the cause of the disease. 



The disease is solely due to the attack of a parasitic 

 fungus, Peronospera (Phytophtora) iufestans. When the 

 summer has somewhat advanced this fungus produces the 

 well-kuown dark-brown spots on the foliage, oi the potato, 

 where it develops its " seeds," the so-called spores. The 

 fuugus-seeds are often so numerous that a .single plant, ac- 

 cording to countings and computations, successively can bear 

 20 to 30 millions of spores. Falliug to the ground, these 

 spores are carried down with rain-water to the tubers, 

 upon the surface of which they germinate. The sprout- 

 fibres penetrate the skin of the potato-tubers, and develop 

 under the skin a dense tissue, the so-called myeeiium. As 

 a consequence of this, the potato is, "sick," i.e., covered 

 with brown spots (and at last become* smuttish-brown upon 

 the entire surface), has a bad taste, and is in process of 

 rotting. 



Luckily the soil has the property of impeding to a great 

 extent the progress of the .sijores, operatiug as a filter. 

 The object to be sought, then, is to throw up upon the 

 tubers a covering of e.nrth ot sufficient thickness to pre- 

 vent the spores from filtering, or only allowhig an insig- 

 nificant minority to filter through to reach the tubers. Bv 

 means of systematic experiments in the open field, and 

 several series of corresponding investigations in the labor- 

 atory, I hiive shown, that, when a 5 inch layer of earth 

 is hoed up upon the uppermost tubers, only very few will 

 become diseased, be the attack ever so violent. Such a 

 layer of earth is, therefore, the principal thing iu the pro- 

 tective moulding. The drawing over of the potato tops to 

 one side of the drills, the second point iu the perfect pro- 

 tective moulding, is, in comparison with the thick cover- 

 iug of earth, of a subordinary, but, nevertheless, by no means 

 un-essential importance. The object of the bending over 

 of the tops is to prevent the rainwater from trickling down 

 the vine into the ground, whereby they would find a less 

 obstructive w.ay to the tubers. When furthermore, the 

 tops hang out over the adjoining furrow, fewer spores will 

 fall upon the ground directly above the tubers than if the 

 stalks stood erect. 



'W. D. B." ON "W.'S" LETTERS 



AND ON THE COMBINED INFLUENCE OF LEAF-DISEA.SE 

 AND METEOROLOGICAL CONDITIOXS ON COFFEE CROPS. 



October 24th, 1882. 



Dear Sir, — "W.'s" letters are apparently written 

 to prove that neither leaf-disease nor unfavourable 

 seasons are the cause of our present infertility. Few 

 people are there who lay the blame on either one 

 without the other, but to most the combination of 

 the two is suiBcient to account for the present year 

 of disaster. In attributing the present infertility 

 mainly to season, I am not one of those who would 

 leave out of account the " fatal fungus," but, as in 

 spite of it we still see estates bearing comparatively 

 well when favoured by season, I have laid the chief 

 weight upon "climate influences." I maintain that 

 " W.'s" arguments are in favour of my theory, as in 

 the letter of your issue of 19th instant, where he 

 says that the hot parching weather usually experi- 

 enced in January and February, gives the trees a 

 check in growth. This check is necessary to the pro- 

 duction of crop, a« 1 will endeavour to show. 



I am aware that my theory runs counter to that 

 of the new school of botanists, who luok upon all 

 flowers as aborted leaves, and which it appears to mo. 

 therefore, reduce the question of reproduction to a 

 combination of apparently accidental conditions, or 

 conditions which they are unable to reduce to law. 

 As a strong upholder of the necessity of maintain- 

 ing the highest conditions of fertility iu the soil and 

 plant for the production of the best crops, I cauaot 



