W. \. llliro.N. - TIIK llKVIil.OI'MKNT (IK DISKASK liKSISTAM VAIIIKTIKS llK l'LA.NÏS. W.) 



(î. Hediictioii ol' loxiii rnrni;ilion ov injurv lo llic luisl,. Tlic anioiinl of 

 iniurv iilîords au insiglil iiilu llie detfree ot' pei-l'cctioti of parisitisni, wlien con- 

 siderod in connection w illi thc incans of reprodiiclion of the parasile, llic course 

 of évolution bcing in llic direction of jessened injnry. 



7. The ahilily lo infect an uninjured liosl and tlie existence of spécial adap- 

 lalions for infection. The mère tact of pénétration does not prove the degree 

 of parasilism, since certain saprophytic forms may be sUmuIaled by chemo- 

 taxis, for exaniple, lliru llie présence of sugars, to enter plant tissues, and some 

 parasitic forms penetrate the epidermis of plants they cannot infect'. 



It will therefore be seen that disease résistance is not to be spoken of in 

 gênerai terms, nor is it to be confused with hardiness or with gênerai vigor of 

 growth, but that it is spécifie for each disease, and is associated willi a definite 

 reaction on the part of the plant cell or its contents, against the invading para- 

 site. We shall show laler that plants may resist one disease and be suscep- 

 tible to another. Whatever the fundamental différence may be which confers 

 this résistance, it may be viewed as a character developed in the évolution of 

 the species and as an adaptation lo existence in association with the parasite. 



Failure lo consider thèse points lessens the value of records on disease 

 résistance made by agrononiisis and horticulturists in their varietal sludies-. 

 The too common statement thaï a variety is " disease résistant " possesseslittle 

 significance iinlcss one knows what disease was prévalent al the lime of llie 

 oltservation. For example, a variety of potato lias been advertisedas " immune 

 to blighl ", lliis being bascd on ils liealth and vigor in a dry season, when tip- 

 biirn and carly l>liglil, .{llrniiirin ^nlnni were prévalent. Subsequently, a wel 

 season showed this sori lo lie one of llie most susccplible to laie Ijjighl, l'Iii/- 

 lop/il/mrn infcsian», in llic ciilire collection. 



Fnrlher confusion may l'csull llirii failure In uiaintain tlic distiiiclinn 

 lielween aclual disease résistance and <}ii^eiixf {■scaji/in/ aiid (//sivy.sc ciiih/niiii-i'. 



Plants often escape disease wilhoiil possessing any real résistance, for 

 example, very early maluring potatoes ui'Ien ripen before Ihc onset of Plnjlopli- 

 t/iiirii infi'filaiix. Certain .lapanese cowpeas mature early and prodiicc a crop 

 in Soiilli Cnrolina licforc the roots beconie galled by llelcrtulnd rKilicirn/ii, 

 whereas lalcr \,iriclics die licfore fruiling. (^.créais somelimes escape smul 

 infections bcc.uisc of Inw IciiipcMatiires dui'ing the period of germination. 



/J/s(v/,s(' rinh/rinifr is iisiiallv (\\\r lu (il incr icaf tissues. ^\llich rcsisi dessic- 

 <'.'iliiin but do nul {H'nlccl IVoin iiircclidii. 



l']\i>i:iiiMi;.\i's i\ l!iii:ijii>(, loii l)isi;\si: Hicsistanck. 



Excellent sulijecis for llic study of disease résistance hâve been all'orded by 

 maladies of cotton, c()\\|ica. ^\alermelon, potalo and other crops, which hâve 

 been imder investigation iii Ihc United Slales Department of Agriculture during 

 iccciil y<'ais"'. Thc niosl notable instances are presented by a group of will 

 (liscaM's. caused by closcly rclalcd fiingi of Ihe genus Fusarium. Thèse are 

 Ihc <'()ll(iii will {Fiiao ri/nu riisiiifrrhnn. Atk.). Ihc cow pca will (F. Inirliriiiliiliiim 



1. SvMhiN K.-S. . Snr flnilulniii.-l, W . pp. ■il7--i'J'J. Hlli:.. 



'2. llliT.iN (W.-A. . /';■(»■. .SV.'r. Iloit. Sririirr, HUIT, |i. 'IS : pp. :,[-",. 



7>. Willi llie oxcoplions nicrilif I in lli.' Icxl llic «ml, c-iI.mI Ii.is hcii i-inicd un li\ Un' wiilcr 



iliii-iii^ llie period IVoiii .lime, IMillMn llie pieseiil >r.ii-. Sin.v l'.MIi. .\lr \\.-\\. i;ill„.|| h;, s l.ei'ii M-\i\rl\ 

 nssoci.ileil willi hiiii in llic liieediiiK wni k. 



