CAlSA'noN ol' PLANT I ) ISIIASIOS 



271 



toes (pust iilc Ixpci. 'I'lic luiiiiMt ion ol' cliloi'o- 

 liCiiic :i('i(l ill thccclls of the IciiticcI urea \\;is 

 \hv causal i\(' lucchaiiisin (.loliiisoii and 

 Schaal, l!).VJ; Schaal and .Idliiison, I!)."),"); 

 Schaal rl dl., l!».");;i. They toiiiid thai, in 

 colls adjacent to the pcfidcrni, chloroj^-cnic 

 aciil was pccsciit in lai-.tici- (luaiititics in po- 

 tato tul)(M-s of scah-r(>sislant xarictics tlian 

 ill those of suscoptil)Ic xarictics. A test that 

 would not re(|uife destruction of the whole 

 tulter was de\ised so the tested tuber could 

 l)e usetl for seed purposes. The test was 

 based on the fact that cliloi-ogenic acid is 

 localiz(>d in the cells dii-ectly und(M' the c\n- 

 derinis and coi'ky co\-ei'ing and in cells 

 around the lenticels of th{> highly r(>sistant 

 \-arieties. Scab resistance was found to be 

 associat(Hl with the amounts of chlorogeiiic 

 acid present. 



The test consisted in placing several drops 

 of 2 per cent aciiieous ferric chloride solution 

 on the surface of a tuber and macerating 

 with a stainless steel knife the tissue covered 

 by the test sohition. By pricking this area un- 

 der a drop of ferric chloride solution, the 

 pre.sence of chlorogenic acid in or near the 

 lenticels could be determined. The distribu- 

 tion of chlorogenic acid in the tuber was 

 measured by spreading the ferric chloride 

 .solution o\-er the freshly cut surface of a 

 half of a tuber. In resistant varieties a green 

 color reaction was found near the surface. 

 In some highly resistant varieties the chloro- 

 genic acid was present throughout the tubei-, 

 its greatest concentration being in the cells 

 directly under the corky co\'ering. The color 

 reaction was found to l)e greater in the im- 

 mature than in the mature tubers. Different 

 varieties showed differences in color reaction. 



The effect of six phenolic compounds on 

 the gi-owth of S. scabies was tested at pH 

 0.0, 7.5, and 8.5. Four orthodihydricphenols 

 (chlorogenic acid, caffeic acid, catechol, and 

 tetrahydroxybenzoin) were effective upon 

 autoxidation in inhibiting the growth of *S'. 

 scabies in culture medium. The inhibition 



increased with increases in the pll. These; 

 results were said to support the theory that 

 the mechanism of scab resist aiure in {)otato 

 tubers iiixolves enzymatic oxidation of chlor- 

 ogenic acid, which produces a (|uinone toxic 

 (o the scab organism. 



Th(> mechanism was believed to depend 

 on the amount of chlorogenic acid in the 

 periderm and on the presence of tyrosinase 

 in the same tissue. The localization of this 

 acid around the lentic(>ls that serve as the 

 normal entrance points for the scab organism 

 was important. A tendency for the acid to 

 accumulat(> in cells adjacent to injured areas 

 was observed. Tyrosinase was found in high 

 concentration in the tissue contahiing chloro- 

 genic acid and was found to become oxidized 

 when the tissue was injured. The oxidation 

 products formed, such as (juinones, were be- 

 lieved to be toxic to the invading organism. 



Johnson and Schaal (1957) brought out 

 further that the concentrations of chloro- 

 genic acid and of total o-dihydricphenols 

 were much higher in the periderm of tubers 

 of certain scab-resistant varieties than in 

 tubers of scab-susceptible varieties. The 

 greatest difference between the \'arieties was 

 found at the stage where the tubers were 

 growing most rapidly. The tubers were 

 analyzed at three stages of growth and after 

 5 months of storage at 35 to 38°F. The re- 

 sults indicated that the ferric chloride spot 

 test for scab resistance could be made on im- 

 mature or on freshly harvested mature tu- 

 bers. 



Effect of Treatment 



Addition of stable manure to soil has usu- 

 ally been found to increase the incidence of 

 scab, because of the resulting alkaline reac- 

 tion and th(> accumulation of humus. Similar 

 results were also reported from excessive use 

 of potassium. Application of certain fresh 

 organic materials, such as green manures, 

 results, however, in the reduction of scab. 

 Alillard and Taylor saw in this effect a kind 



