98 



PLASMODIOPHORALES 



coli may become badly clubbed in soils adequately 

 provided witli lime. In 193 1, however, they reported 

 that turnips and swedes are unaffected by club root 

 in soil the pH value of which was raised to 6.76 and 

 7.02 by liming and concluded tliat pH 6.6 is the prob- 

 able limit for the disease. Larsen and Walker (31) 

 also observed that the addition of calcium hydroxide 

 and calcium or magnesium carbonate in doses suffi- 

 cient to raise the pH to 7.1 and above did not gener- 

 ally inhibit development of club root in silty clay 

 loam soils. In the greenhouse, however, infection was 

 reduced by the addition of these substances suffi- 

 ciently to bring the pH up to 7.0. At the pH 7.2 or 

 above infection was completely inhibited. Whitehead 

 ('36) likewise noted that the disease is generally less 

 prevalent in alkaline soils but he also found a high 

 percentage of infection in cabbage, cauliflower and 

 Brussel sprouts in soils with pH ranges of 7.4.5 to 

 7.81. 



It is obvious from the data presented above that 

 H-ion concentration in the soil is not the sole deter- 

 mining factor in infection, development, and severity 

 of club root. As Naumov and others have pointed out 

 the intensity of infection is intimately associated 

 with many external and internal factors, such as de- 

 gree of soil infestation, moisture content, anatomical 

 structure of the hosts, specific and varietal suscepti- 

 bility, etc. 



Temperature does not appear to be as important 

 as other factors in spore germination, infection, and 

 development of club root, because these processes 

 may occur under a fairly wide range of temperature. 

 It was commonly believed that outbreaks of disease 

 were most severe in cold countries and during the 

 cool seasons in warm regions, but this belief was not 

 based on exact experimental data. As to spore germi- 

 nation Chupp (17) found that it does not occur at 

 room temperature (16° to 21° C) and that the opti- 

 mum lies between 27° C and 30° C. He, nevertheless, 

 obtained host infection at room temperatures, which 

 indicates that temperature was not the only impor- 

 tant factor in his experiments. Wellman, on the other 

 hand, found that spores will germinate within a 

 range of 6° C to 27° C, with an optimum range of 

 18° to 25° C. Honig ('31) likewise found that spores 

 may germinate readily below 21°. As to the direct 

 effect of soil temperature alone on infection and club 

 root development very little experimental evidence is 

 available, but temperature doubtless operates indi- 

 rectly in conjunction with other soil factors. In care- 

 fully controlled tests Montieth ('2i) showed that 

 club root develops at 9° to 30° C. One case of club- 

 bing was found at 35°, but it occurred on the main 

 stem near the surface of the soil where contact with 

 air probably lowered the temperature. Clubbing was 

 most severe at 25°. ISIontieth concluded that the tem- 

 perature range over which the disease occurs is more 

 or less the same as that required by the host and that 

 temperature itself is not a limiting factor in club 

 root development. In similar controlled tests Well- 

 man found that no clubbing occurs below 12° and 

 above 27°. The optimum temperature for greatest in- 



fection and disease production ranged from 18° to 

 2i°, with the peak for severity slightly above 2i°. 

 In soil temperatures of 12°, 15° and 27° a fair per- 

 centage of plants became infected, but clubbing was 

 distinctly inhibited. The optimum temperatures for 

 spore germination, infection and development of the 

 disease determined by Wellman are 5° higher than 

 those which Tisdale (Jour. Agric. Res. 25 : 55) found 

 to be optimum (20°) for cabbage root development. 



Soil moisture is more significant than temperature 

 in relation to infection and severity of club root. The 

 early student of this disease as well as later investi- 

 gators, including Halsted, Ravn, Cunningham, 

 Chupp. Whitehead, Reed and others, noted that the 

 disease is most prevalent in low lying, poorlv drained 

 soils and severest after periods of wet weather, and 

 concluded that soil moisture is perhaps the most im- 

 portant determining factor. These conclusions, how- 

 ever, were based more on general observations than 

 on direct experimental evidence. Montieth demon- 

 strated experimentally the dependence of club root 

 on high soil moisture and sliowed that cabbage could 

 be grown free of the disease in heavily infested soil 

 by keeping the moisture down to t5 per cent of the 

 total water holding capacity. At 60 per cent club 

 root was uniformly present. He believed that the 

 failure of club root to develop in infested soils with 

 low moisture content is probably due to insufficient 

 water for spore germination. Montieth's results have 

 been by and large confirmed by Wellman and Nau- 

 mov. Wellman, however, demonstrated that con- 

 tinued high soil moisture is not necessary for infec- 

 tion and development of the disease. Plants which 

 had been exposed only 18 hours to infested soil with 

 80 per cent moisture content became badly diseased 

 when transplanted to relatively dry infested soil. He 

 believed that even in a dry season a heavy rain or a 

 few moderate rains at short intervals might raise the 

 moisture content sufficiently to insure infection. 

 Wellman's results may be the explanation of Motte's 

 report that club root is prevalent during the drv sea- 

 sons in Denmark. Naumov ('33) likewise found that 

 cabbage seedlings became infected within a range of 

 soil moisture from 45 to 100 per cent of the total 

 water-holding capacity, with the optimum at 80 per 

 cent. At 30 per cent no development of the disease 

 occurred. 



The physical character of the soil has also been re- 

 garded as a significant factor in club root infection, 

 development, and severity. Sandy, humus-rich, 

 clayey soils favor the disease, according to McAlpine 

 ('03), Bos ('04), Janson ('20), and Naumov ('28). 

 In Belgium, Vanderyst ('04) reported that club root 

 occurs abundantly on sandy soils, is generallv ])res- 

 ent in soils rich in shale, sparse on limey and clavev 

 soils, and unknown on soils rich in lime. Huniphrev 

 ('92) and Read (11) found the disease to be most 

 severe on heavy soils and those rich in humus. Ravn 

 ('08) reported that turnips were more susceptible 

 than cabbage on sandy soil, while on clayey soil the 

 degree of susceptibility was reversed. Hayunga 

 ('19), Jan.son ('20) and Bremer ('23) found club 



