118 



PLASMODIOPHORALES 



maintained tliat Thysostetjia virginica, Achillea 

 pharmica, Astilbe sp., and Pi/rethrtim have an in- 

 hibitory effect on club root and inactivates the spores 

 within three years. Miiller-Thurgau and Osterwalder 

 ('23) also found that the spores remain viable only 

 three years if beans are rotated with cabbage. Ac- 

 cording to Murphy ('27) turnips should be alter- 

 nated with carrots. Blunck ('29) found that beans 

 were particularly favorable as an alternate crop. 

 Arker ('35) advocated rotation with beets, and 

 Fedorintschik recommended rotation with grass and 

 clover during the last two years of the interval to 

 avoid plowing. 



Eradication of Wild Hosts 



Numerous cruciferous weeds are susceptible to 

 club root, as Halsted ('92-'99), Ravn ('08), Cun- 

 ningham ('I2,'ll.),Ssacharoff ('16),Naumov ('26), 

 Gibbs ('32), Rochlin ('33), Jamalainen ('36), and 

 others have shown, and these hosts may harbor and 

 perpetuate the disease in the absence of cultivated 

 crucifers. Infected weeds have been found in grass 

 pastures, wayside ditches, river beds, gardens, and 

 cultivated fields (Halsted, '98 ; Gibbs, '32), and their 

 presence on infected soil reduces the effectiveness of 

 crop rotation in club root control. Even when only a 

 few weeds are present in infected fields enough 

 spores will be produced and perpetuated to infect 

 subsequent cruciferous crops. Eradication of wild 

 crucifers is therefore highly essential as a control 

 measure and has been advocated and practiced to 

 some extent as such, but in certain places it is not al- 

 ways practical. As Gibbs has pointed out, eradica- 

 tion is impractical in cereal grain crops, grass lands 

 and pastures. In crop rotation on cultivated fields, 

 eradication is obviously important, but unless it is 

 combined with other control measures such as liming 

 and growing resistant varieties of crucifers to keep 

 down spore multiplication, its effect is limited. 



Other special control measures involving winter 

 ridging of the land and hilling up the soil around 

 cabbage stalks have been practiced without consist- 

 ent success. In the autumn of 1898 Halsted plowed 

 infected plots deeply and piled the soil up in long 

 2 ft. higli ridges to expose the spores to the maximum 

 weathering during the following winter months. Less 

 clubbing was present on the ridged land (31 per 

 cent) the following season than on the level plots 

 (38 per cent), but the small difference does not 

 justify ridging as a satisfactory remedy for club 

 root, according to Halsted. He also tested the eft'ect 

 of shading on the disease in turni])s and found that 

 it does not have an inhibitory effect. Hilling up the 

 soil around cabbage stalks leads to increase of ad- 

 ventitious roots on the stalk above the infected por- 

 tion, according to Cunningham ('ll). Such adventi- 

 tious roots are comparatively free of clubbing, and 

 since they occur above the diseased and useless main 

 root the nutriments which tliey absorb are readily 

 available to the developing heads. Cunningham 

 found that liilling increased the yield ten-fold in 



some plots during 1912, but in the following year no 

 beneficial results were attained. 



Resistant Varieties 



Cultivated and wild crucifers vary in degree of 

 susceptibility to P. Brassicae, and several cultivated 

 strains and varieties have been developed which are 

 fairly resistant to club root. A certain measure of 

 control may accordingly be achieved by the cultiva- 

 tion of these varieties. Particularly promising are 

 the results obtained by Olsson ('39, '40) in breeding 

 resistant varieties of swedes and turnips in Sweden. 

 The data on relative degree of resistance, however, 

 are often conflicting, and in certain varieties where 

 some investigators have reported complete immunity, 

 others have found 100 per cent susceptibility. These 

 differences in results are doubtless due in part to 

 variations in experimental conditions and methods 

 employed. As has been shown elsewhere, soil types 

 and moisture, H-ion concentration, number of spores 

 in the soil, etc., are important factors in infection, 

 and unless they are kept constant in experimental 

 work, it is diflicult to determine the inherent degree 

 of susceptibility or resistance of a particular variety 

 or strain. Doubtless many of the reported cases of 

 immunity relate to plants which have escaped infec- 

 tion in tlie field. The literature relating to varietal 

 susceptibility is nonetheless very extensive, and in 

 a brief treatise of this nature no attempt will be made 

 to enumerate and discuss all tlie data relative to this 

 subject. 



The range of susceptibility in turnips is very great 

 and some varieties are reported to vary from 100 per 

 cent susceptibility to almost complete resistance. No 

 varieties, however, have been developed or found 

 which are consistently immune. Southern Curley 

 Top, Rutabaga, and Large Flat Green were re- 

 ported by Cunningham ('14) to be particularly sus- 

 ceptible. In the first named variety clubbing was so 

 extensive that the turnip root was converted into a 

 system of branched hypertrophied rootlets. On the 

 other hand, the following commercial strains have 

 been reported to be relatively resistant : 



Bruce Purple Yellow Top 



Bruce Purple Top Yel- Purple Yellow Top Aber- 



low deen 



Bruce Wallace Rutabaga 



Dale's Hybrid Scarlet Kashmyr 



Early Snowball Seefeld 



Earlv White INIilan Snowball 



Golden Ball Svaliiv's Yellow Tankard 



Green Top Victor 



Hinkenborstel Weibull's Immune 



Irvine's Green Top Yel- Weibull's Sekel 



low White 



May White Fleshed May 



New Bronze Top ^^'hite Milan 



Ostersundom Yellow Aberdeen 



Pomeranian Tankard Yellow Bruce 

 Purple Top Milan 



