104 



PLASMODIOPHORALES 



Control of Club Root 



Because of the great economic importance of club 

 root extensive attempts to control the disease have 

 been made for almost a century and a half, but so 

 far no completely effective measures have been de- 

 veloped. The resting spores of the causal organism 

 are produced in prodigious quantities, have a fairly 

 high degree of resistance, and may remain viable in 

 the soil without the presence of host plants for seven 

 to eight years ; all of which makes effective control 

 very difficult. Control is furthermore hampered by 

 the wide range of wild and cultivated hosts which 

 harbor P. Brassicae and the fact that crucifers are 

 susceptible as long as they are alive. 



Control measures against club root have involved 

 sanitary practices, sterilization of the seed, disin- 

 fection of soil in seed beds, applications of fungi- 

 cides to the soil in fields, adjustment of the soil re- 

 action, addition of lime, judicious use of basic fer- 

 tilizers, soil drainage, crop rotation, eradication of 

 wild cruciferous hosts, and the development of resist- 

 ance varieties or races of cultivated crucifers. 



One of the main factors which makes club root dif- 

 ficult to eradicate in the soil is the longevity of the 

 resting spores. Longevity is not influenced by graz- 

 ing, crop rotation, plowing, or the application of car- 

 bonate of lime and sulphur to the soil, according to 

 Gibbs ("39). Fedorintschik ('35) found that soil 

 from fields wliich had not been sown to crucifers for 

 seven years contained enough viable spores to infect 

 26.6 per cent of aseptically grown cabbage seedlings 

 after transplantation. In cabbage fields rested one 

 year the viability of the resting spores was reduced 

 from 81.2 per cent to 13.7 per cent, but in one field 

 rested five years the reduction was only 40 per cent. 

 Plowing the fields two or three times a year has no 

 effect on resting spore viability, according to Fedo- 

 rintschik. In badly infested fields up to 100 million 

 spores per cc. of soil have been found (Naumov, 

 '28). which may extend to and infect plants at depths 

 of 10 to 30 cms. in soils of various types (Gibbs, 

 '32; Motte, '34; Potts, '35; Fedorintschik). The in- 

 tensity of attack is directly correlated with the lumi- 

 ber of spores in tlie soil, according to Gibbs ('31b) 

 and Fedorintschik, but Naumov ('28) found but 

 little evidence of correlation in Russia. Gibbs found 

 that one plant out of 42 became infected when there 

 were approximately 25,500 spores per seed box, and 

 43 out of 44 when 530,000,000 spores per box were 

 present. According to Fedorintschik's calculations, 

 less than 10,000 spores per cc. of soil cause isolated 

 attacks on lateral roots but does not reduce the crop 

 weight of cabbage. More than 10,00 spores may cause 

 50 per cent infection of lateral roots but no reduction 

 in crop weight, while 300,000 spores per cc. of soil 

 usually leads to over 50 per cent infection of the 

 whole root system and reduces the crop weight to 50 

 per cent. 



In the soil the spores are also fairly resistant to 

 fungicides in concentrations low enough to avoid in- 

 jury to the host. Bremer ('35) found that a 0.5 per 

 cent solution of uspiilun poured over spores in the 



soil killed only 24 per cent to 38 per cent, and that 

 5 days were required to kill the spores *hen im- 

 mersed directly in a 0.25 per cent solution of the 

 same fungicide. Likewise, relatively strong solu- 

 tions of formalin were ineft'ectual. Fedovota ('29) 

 found tliat treatment with 0.1 per cent mercuric 

 chloride has little or no toxic effect on the spores. On 

 the other hand, Honig ('31) reported that 0.001 

 per cent mercuric chloride when applied directly to 

 the spores caused general plasmolysis, while higher 

 concentrations were more or less ineffective. He also 

 found that solutions of MgS04, NaCl, KNOo, and 

 NH4CI in molar concentrations of 1:100, 1:10,000, 

 1:100.000 plasmolysed tlie spores within 4 weeks. 

 Immersion of spores for 30 minutes in 70° C. water 

 and heating the soil 5 to 30 minutes at 60° to 80° C, 

 renders them inactive (Naumov, '28 ; Vladimirskaya, 

 '30; Anony., Ger., '39). Polyakoff ('39) reported 

 that immersion of spores for 5 minutes in condensate 

 (containing 5 per cent formalin) kills the spores, 

 and that this solution added to the fields at the rate 

 of 1.8 by volume of soil reduces infection 70 to 100 

 per cent. Desiccation has a marked effect on spore 

 viability, according to Naumov ('25). Spores kept 

 in a relative dry cellar over winter caused infection 

 of seedlings the following spring, but a year later 

 they were no longer viable. If desiccated completely 

 the spores lose tlieir infective power within a year. 



SANITARY PRACTICES 



Since the spores of P. Brassicae will survive pas- 

 sage through the digestive tract of animals and may 

 be carried to the fields in contaminated manure, it is 

 obvious that diseased roots should be thoroughly 

 boiled before being fed to livestock. Stable and liquid 

 manure should be avoided as much as possible, since 

 it is conducive to club root development if applied 

 directly to a crop of crucifers. If it is to be used at 

 all it should be applied during the season preceding 

 a susceptible crop. If contaminated it should be steri- 

 lized or disinfected before application to the soil. 

 ^'incent. Herviaux, and Coic ('38) advocated the ad- 

 dition of 90 kg. nitrogen in the form of cyanamide 

 to stable manure before using. It is interesting to 

 note in this connection that Naumov ('28) reported 

 that, contrary to all expectations, the addition of 

 stable manure to the soil exerted a slight detrimental 

 action on the parasite. 



Other sanitary practices involve collecting and 

 burning diseased plants. These should not be allowed 

 to rot in tlie soil or in piles, since this liberates the 

 spores in the soil again. Plowing under of diseased 

 plants to various depths has been advocated. Frank 

 ('96), Potter ('97), L. R. Jones ('01), Laubert 

 ('05a), Kock ('11) and Lindner ('11) recommended 

 a depth of 1 meter; Naumann ('13), Neger ('17), 

 Trieschmann ('17) and Ludwigs ('25) 80 cms.; and 

 Miillers (Honig, '31) 20 to 30 cms. The latter depth 

 is obviously inadequate, since it has been sliown that 

 infection may occur at 30 cms. Esmarch ('24) con- 

 tended that burial is worthless and that burning is 

 the only safe method of disposal. 



