10 



\n\>\\ (iAiM)i.M.\(; 



Sil 



ver 



Leaf 



in 



Fruit Trees. 



In r«'.rnt y.-JU's fruit k'>"»^ '•■•'< •>">«• sullm*! SfVi-rc 

 I.IRS4-S fn.i'p. II disias4- of fiiiil (r.cs wliicli fur n 

 want of a ln'tt^r imnu- is known as Silvii- l..af. 

 AtT.'tt.d tn-rs apiM-ar ( • »•«• in ii.rf.rt* luallli and 

 t4> pri'Si-nt no i-xti-rnal ajipraninrr of dist-as.-. 

 «'Xrc|»t tliat inst.ad of th.- vivid ^Ti-m conmionlv 

 a.s.s«n-iat<-d with h.-altliy plants, tii.- Icavrs S4ini. - 

 tini.s >f a sinK'lr l>iMn«h onl> . at otlu r tiim-s ol 

 t!u' wliolc tr.M- hivi- a silv.Vy or liad.ii sln.ii. 

 fspciallv wli.n lo<»k«'d at. from an an^,'Ic. Manx 

 kin. Is of"tr.-.s an- liaM.- to 1.,- at.t,i.k.-d. an<l in 

 addition to Iriiit tr.-.s it has h.-.-n r.-cord-d in 

 horsf rhfstnuts. syriuiorts. laliurnums. l*ortw}.'al 

 laurels. wah>ut><. rno«k oranu'ts. an»l cVen in the 

 whit.- d.a.l n.tth'. and oth.r li.ri)a.e<.us i-lnnts. 

 Tile dis<'ase. liowever. is iiuiili more ire«iuent in 

 fruit trees than other kinds of trees, and eoiiimoner 

 in phnn trees than in apples, cherries, aprieot. 

 p«'a«lies. red currants and ;,'oosel)erries. 'i'he 

 shade of silveriness varies, and not only is the 

 colour of a silvered apjile leaf slij,ditly dideient 

 fl-oni that of a silven-d jihim leaf, hut a vaiiatiou 

 mavhe noticed in the folia>,'e of a Victoria plum 

 antt that of a ("zar, when attacked hy the disease. 



( Al>:E OF TlIK DiSE.V.SK. 

 Alth..\ii,'li this disease is now very coiuinon in 

 some parts of l-Tngland and is known on tlu- 

 Continent of Europe as well as in South Africa. 

 New Zealand and Canada, the cause is not fully 

 understood as yet. in s|>itc of the scieiitilic in- 

 vestigations into its nature which have been made 

 l)oth here and ahroud. >h>st authorities are now 

 agreed that in fruit tre( s and certain otlier 

 woody ]ilants, tyj'ical Silvt-r Leaf may be caused 

 by tile presence of the fungus, SlercHm y it rjiu renin. 

 in the wood of the affected tree, and all symiitoms 

 of the disease have })een produced by artilicially 

 inoculating a healthy plum tree with a livin;,' iiart 

 «if this fungus, and even with the sports, und-r 

 conditions which j-reclude the p« ssiliility of intec- 

 ti«in from other sources. But while we may 

 assume that, unkss there is evid- nee to the con- 

 trary. mo.st cas'S of Silver Leaf in fully grown 

 ])lun*i and ai>]ile trees and perluipjj also in other 

 fruit trees are due to this fungus, the following 

 points nmst be remembered : — 



(1.) The t-ilv.-ry appearance of the leaf is 

 chiefly caused by the presence of air spiices 

 inunediat-ly bi'neath the skin, which has a ten- 

 dencv to break awav from the cells which lie 

 beneath it. The chaAge in the colour of the leaf 

 therefore, is due to the change in the light 

 reflected fr >m the surface, and not to the |)resence 

 of the fungus in the leaf tissues. Where silvering 

 is due to Stcrcnni yurpurcum these changes in the 

 leaf are to be attributid to disturbances in the 

 normal life of the branch induced by the presence 

 of the fungus below. 



(2.) In the case of certain plants, such as the 

 dead nettle referred to above, and of certain 

 seedling i»lums. which have come under observa- 

 tion, no trace of fungus attack could be found, so 

 that it is clear that silvering may be dvxe to other 

 causes than the i)resence of the fungus. 



(3.) Leaves of many kinds of jdants are often 

 bleached by insect attacks and by other means, 

 in such a way as to resemble Silver Leaf sujier- 

 ficially. A little experience of true Silver Leaf in 



I lumS, hoWeVer, enJlbleS the observer t > ditect 



dilTereiiceS between the disease in (pi'sliiiM and 

 tlw.se Htt.ii<-kH without n.iich dlllhullN. 



rn«uiicK 



• I- rilK iMsKAsi:. 



* Leaflet 302, Board of 

 Fisheries, Whitehall Place, 

 November, 1915. 



Agriculture and 

 London, S.W., 



'I'he usual cdurse of an attack of true " Silver 

 I.eal " in the case if a plum tree is as follows : 

 ">) \ single l>ran<h of the atTe.ted tre.- 

 il. velt.ps the charictcristii- silvery 

 sliiMii, tile other branches remaining 

 normal. 

 {h) Sev.ral brandies beai' sil\. r, d bdiage. 

 oft -n marked with bmuti Hecks and 

 streaks. Sometimes the |e.i\cs die ,is 

 the siMunier advances, 

 (c) Silvering becomes gem-ral. .iinl th"- 

 liranch.s whidi h.ive been alTectrii 

 lon;.'< st die b.ick. S<imetimes a wh<de 

 br.inch will .lie su.l.I.nlv. 

 {,!) Several branches die and the <r.-e b.ars 

 •I sickly app.arance. Shouts l.iw down 

 on till- trunk and suckers. i).>th .if wiiich 

 may .ir may not b.- silver.-*!, often 

 appear. Thes.- su.k.'rs cannot be 

 suc.'.'ssfullv used for propagation. 

 ir) Th.' who!.- ti-ee di.-s. 

 Sli.irtly aft.-r the d.-atli <if a large bran. h. Wu- 

 fructilicati.ius <if Slcrmnt iiurimninn gen.-rall\ 

 appear <in th.- bark, and aft.-r the d.ath ..f th.- 

 tree the fnictili.-ations appear on tii.- trunk. 



According to Brooks, who has mad" a careful 

 study of the dis ase. m tlu- .arly stages of attack 

 the w.i.kI is tht- only part afte.t.-d. and fr.-.pi.-nt ly 

 a n.irr >w zone of the y.uingi-st Wood remains 

 uninvaded for wmie time thiugh so.mer or lat.-r 

 b.ith this and the bark become aftect:-d. The 

 lungus s; treads much moi-e rapidly lengthwiss 

 than a(-ross the wood, and in the case of the 

 larger branches and trunk of the trei- it may 

 spre.id for a long time in tin- wood without any 

 sign of injury to tlu- bark bv ing evident. The 

 wood and the bark whi.h ar.- invaded by th.- 

 fungus become dark brown in c.dour. chi.-fly 

 owing to the presence of a gum-like subst^mce in 

 tile cells. S ) abundant is the gum produced by 

 tli<- fungus in iilum trees that larg.' masses ..t it 

 s imetimes f.irce their way .>ut from th.- bark. 

 The fungus mav sjiread from tlie trunk int<i tlu- 

 ro.its .)f the tree. 



Tile amount of dis(-<doured wond. as s.-eii in tin- 

 cross section of a silvered bi-aiich. varies acc.trding 

 t^) the length of time the tis-sues have been alTected 

 by the fungus. The disease does not always 

 apiiear in that part of the w.iod immediately 

 adjoinin;,' thy affected leaves. If. h.iwever, search 

 be made furtlier down the bran.h or in the trunk 

 of the tree the lirown stain will pr.tbably be f.umd. 

 It must be remembered that SUrruni ijnrpnreinn 

 grows normally as a sainviphyte .)n dead wood, 

 and that the greater jtart .tf the w.xid of a healthy 

 tree, th.iugh enclosed by a ring of living tissu.-. 

 i-onsists. especially in the inner and older part, of 

 dead cells. It is on this jtart f)f the tree that the 

 fungus begins its devel.))'iiicnt. 



External development of the fungus takes jilace 

 as the branches of the tree die .so that fructifica- 

 tions of Stercinn purpurcinn apjiear on the bark. 

 These bodies are variable in shape, and change 

 their c.dour with age so that they are not always 

 easy U) recognise. They usually appear in one 

 or other of the foll.iwing forms : — 



(1.) Sometimes the fructifications appear as 

 incrustations .several inches long covering the 

 under surfaces of the branches or on the sides of 

 the trunk. 



(2.) At other times they appear as projections 



