9ti 



PLASMODIOPHORALES 



Entrance and Spread of P. Brassicae 

 in the Host 



Actual penetration of P. Brassicae into the host 

 was not observed by the early workers, but most of 

 them assumed that it occurs only when the plants are 

 young and susceptible. Honig and Rochlin, however, 

 subsequently demonstrated its entrance through the 

 walls of root hairs and epidermal cells, although 

 Woronin, Chupp, Cook, Schwartz, and others had 

 previously held that the amoebae gain entrance 

 through the root hairs (fig. 28, 29) and migrate into 

 the deeper lying tissues. W. G. Smith ('SI), on the 

 other hand, maintained that the parasite enters as a 

 Plasmodium. Favorski reported that infection may 

 take place through ordinary epidermal cells and 

 stated that Woronin's figures of amoebae in root 

 hairs relate to Olpidiiim Brassicae. Kunkel found 

 that old plants are as susceptible as young ones and 

 that infection of old roots is very common. He fur- 

 ther refuted the claim that root hairs are of any im- 

 portance as avenues of infection and concurred witli 

 Favorski's belief that Woronin had figured thalli of 

 0. Brassicae and O. borzii in the root hairs instead 

 of P. Brassicae. Cook and Schwartz, Honig. Roch- 

 lin, and otiiers, however, have subsequently dem- 

 onstrated quite definitely that P. Brassicae occurs 

 in root hairs and thus confirmed the observations of 

 M'oronin and Chupp. Kunkel, nonetheless, showed 

 that old plants are susceptible and may become in- 

 fected as long as they live. Infection through me- 

 chanical wounds and ruptures caused by adventitious 

 roots and by the removal of lower leaf petioles at the 

 time of transplanting is fairly common, according to 

 Larson ('S-l). The enlargements, however, which are 

 formed at the region of injury on the stem are defi- 

 nite spheroid galls in contrast to the spindle-shaped 

 clubs on the roots. 



As to the spread of the parasite in the host tissues 

 and the channels involved, it is now generally agreed 

 that it occurs in two ways : by migration of amoebae 

 and young plasmodia from cell to cell, and by passive 

 distribution of the parasite through repeated divi- 

 sions of infected cells. Woronin contended that amoe- 

 bae and Plasmodia migrate only througli pits and 

 sieve plates, while Atkinson believed that amoebae 

 are able to spin out into sucli fine tlireads that they 

 can enter the roots along with nutrients in solution. 

 Eycleshymer found plasmodia in xylem vessels and 

 thought therefore that tliey may travel in the fibro- 

 vascular bundles. Nawaschin believed that migra- 

 tion of amoebae from cell to cell is impossible after 

 secondary thickening begins in the roots, and hence 

 distribution by division of infected cells is the princi- 

 pal method of dissemination in old roots. Subse- 

 quently, Lutman figured and described tlie passage 

 of small plasmodia from cell to cell, and since that 

 time Cluipp, Kunkel, Honig, Rochlin, and others 

 (fig. 31-33) have demonstrated its occurrence. Cook 

 and Schwartz, more than a decade later, however, 

 still expressed doubt as to its occurrence. Fedorint- 

 schik ('3.5) believed that in the early stages of the 



disease, migration of amoebae is the principal method 

 of distribution in tlie host tissues, but after the plas- 

 modia have formed and begun to mature, further 

 spread is by division of infected cells. While it is now 

 generally believed that division of the host cell 

 greatly increases the number of infected cells, it 

 nevertheless appears to play a minor role in distrib- 

 uting the parasite throughout the roots and shoots.' 



Dissemination of P. Brassicae in Nature 



The club root organism is readily disseminated in 

 nature in various ways and by numerous agents. It 

 was formerlj' believed (Atkinson, '89; Carruthers, 

 '93; Miiller and Osterwalder, '19) that the motility 

 of the zoospores in moist soil spread the disease, but 

 Chupp ('17) lias presented evidence to show that 

 zoospores and amoebae rarely travel more than five 

 inches. It has also been claimed (Carruthers. Ravn, 

 '08, and otiiers) that wind is an important agent of 

 dissemination, but this factor apparently operates 

 only in the case of light, dry, loose soils and where 

 strong winds prevail. It has been demonstrated in 

 heavier and more compact soils that unless the patho- 

 gen is transferred by some other agent, wind does not 

 usually spread it from one field to another. Rains and 

 water are doubtless more important, particularly on 

 rolling land where the water following a heavy rain 

 runs off quickly and carries the spores to lower-lying 

 fields. According to Naumov ('2.5), however, disper- 

 sal in a radial direction by such means is not very 

 extensive. Miiller-Thurgau and Osterwalder ('23) 

 reported tliat in the course of a year club root does 

 not spread laterally more than 1 Vo to 2 meters in the 

 ground. Earthworms have also been found to be ac- 

 tive in the dissemination of club root in small gardens 

 (Gleisberg, '22; Bremer, '2-t; Fedorintschik, '3.5). 

 The spores may be carried in the mucilage on the 

 skin or in the intestinal tract, and virulent forms of 

 P. Brassicae have been found in the excreta of worms. 

 Ground moles, root nematodes and insects feeding on 

 diseased roots doubtless spread the disease to some 

 extent (Favorsky, '10; Esmarch, '2-1; Beyer. '25; 

 Chupp, '25; Erickson, '26), but how important they 

 are as active disseminators is not known. 



1 In a paper presented before the meeting of the Ameri- 

 can Phytopatholofjical Society at Dallas, Texas, December 

 19+1, Walker reported system infection of cabbage and dis- 

 tortion of buds, stem, and leaves as follows: "Under green- 

 house conditions when cabbage seedlings are grown in soil 

 infested with P. /Jivi.v.fiVfic the pathogen, after infecting the 

 root, may migrate through the cambium into the stem. 

 There is relatively little camhial proliferation in the inter- 

 nodal regions abo\'e the third or fourth leaf. Dormant buds 

 at the leaf sears, however, are stimulated to grow and be- 

 come invaded by the pathogen. They become malformed 

 due to hyperplasia. The organism may reach the growing 

 point in young |)lants and cause extreme distortion of stem 

 and leaves. Wlien ]ilants are incK'ulated at above ground 

 leaf nodes, the pathogen may migrate down the stem, leav- 

 ing no evidence of proliferation in its path until the hypo- 

 cotyl is reached, where a typical club is formed. There is 

 evidence that the reaction of the host is influenced by the 

 nutrient sujiplied to it." (Phytopath. 'iJ: 18) 



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