91 



PLASJIODIOPHORALES 



regularly spindle-shaped, but when several infec- 

 tions occur togethtr the swellings may fuse and pro- 

 duce irregular growths or compound spindles (fig. 

 3). According to Kiister ('11) and M. T. Cook ('23) 

 these galls are kataplasniic, since the affected tissues 

 usually remain parenchymatous and do not undergo 

 differentiation. Other root symptoms have also been 

 reported. According to Appel and ^^'erth ('10), no 

 hypertrophy occurs in radishes, and the disease is 

 here characterized only by darkened and decayed 

 areas. Honig ('31) found similar symptoms on Lu- 

 naria biennis. Ravn ('22) and Pape ('25) likewise 

 reported tlie occurrence of deep wounds or lesions on 

 turnip roots which were filled with spores. Accord- 

 ing to Pape, such symptoms appear when the galls or 

 nodular excrescences on the roots decay. 



In a study of 101 species from 28 genera, Cun- 

 ningham ('li) found definite types of hypertrophy 

 and symptoms more or less characteristic for certain 

 crucifers and classified them accordingly : 



1. Complete clubbing of main and lateral roots. 



Brassica oleraceae. 



2. Clubs on main root, laterals free. Sisi/mbriiim 

 altissitnum. 



3. Clubs on lateral roots, main root free. Sisi/m- 

 brium officinale and Erysimum cheiranthoides. 



4. Clubs on main and lateral roots with club-free 

 rootlets above the diseased portion. Lepidium 

 sativum. 



5. Clubs as tumors of the roots. Raphanus sati- 



I'US. 



6. Dark, decomposing spots on the roots. Rapha- 

 nus satii'us. 



In the last category true hypertrophy does not 

 occur. The disease is here characterized by cracks, 

 fissures, and darkened areas in the host tissue whicli 

 turn black, decay, and serve as sites of secondary 

 infections by other fungi. As has been noted above, 

 Appel and Werth claimed that these are the charac- 

 teristic symptoms of the disease on radishes, but 

 Cunningliam found them only on the Everlasting 

 radish, in addition to spindle-shaped swellings of the 

 rootlets. 



Club root disease may also stimulate branching of 

 the roots and shoot and lead to the production of buds 

 where they do not normally occur, as has been de- 

 scribed by Caspary, Woronin, Favorski, and Kunkel. 

 The secondary roots may attain a length of several 

 inches or become stunted as short knobs. On tlie other 

 hand, the production of secondary rootlets may be 

 greatly inhibited, according to Laubert ('0.5) and 

 Schlumberger ('11). The diseased buds on infected 

 roots and shoots are often unable to respond normally 

 to fi;ravitv, and they may grow downward and hori- 

 zontally as well as upward. In the latter instances the 

 infected buds may push uji above the surface of the 

 ground and give rise to tiiick, distorted, fleshy, and 

 abnormally succulent leaves and petioles, so that 

 the disease may occasionally manifest itself above 

 ground in the shoot, petioles, and leaves. In addition 

 to tliese above-ground symptoms, club root causes 



yellowing of the leaves, wilting on hot days, and in 

 tlie case of cabbage, atrophy, or complete lack of 

 head development. Seedlings which are infected 

 earlv usually die within a few weeks. The wilting 

 of large diseased plants is partly due to hypoplasia 

 of the xylem region and to splitting up of the woody 

 cylinder by infection and expansion of the medullary 

 rays. 



All galls or swellings on roots of crucifers, how- 

 ever, are not due to P. Brassicae. Nematodes, insects, 

 and other factors may cause malformations which 

 are superficially very similar to club root, and unless 

 microscopic examination of the tissues is made, these 

 galls may be easily mistaken for those of the finger- 

 and-toe disease. 



Anatomically, the causal organism of club root 

 affects the cortical parenchyma most conspicuously, 

 but it also produces marked changes in the cambium, 

 xylem, and medullary rays. When roots of consider- 

 able size are infected the amoebae and small Plas- 

 modia migrate through the cortical parenchyma into 

 the cambium. Here they follow the path of least re- 

 sistance, according to Kunkel and Larsen, and 

 spread up, down, and around the central cylinder 

 through the delicate thin-walled cambium cells and 

 form tlnis a cylinder of infected tissue. From the 

 cambium they may travel laterally into the cortex, 

 medullary rays, and xylem. Their migration up and 

 down in the cambium ceases after a while, and the 

 distance of the infection in these directions deter- 

 mines the ultimate length of the spindle-shaped club. 

 Each club, in Kunkel's opinion, is a morphological 

 unit which has resulted primarily from the abnormal 

 growth of the cambium. In comparatively old in- 

 fected roots the medullary ray cells divide a number 

 of times and enlarge and thus form large bands of 

 pathological tissue which split and force the xylem 

 tissues apart, until the latter becomes distorted and 

 shifted out of their natural position. Separated from 

 each other in this manner, the vascular bundles grow 

 out fan-wise instead of remaining wedge-shaped and 

 are no longer able to function normally. Plasmodia 

 and amoebae have frequently been found in the tra- 

 cheids, but they do not seem to have any appreciable 

 effect on the normal functions of such differentiated 

 cells. In young roots medullary ray infection is less 

 common, and most of the abnormal growth occurs in 

 the region of the cambium and the cortex. The xylem, 

 nonetheless, may fail to differentiate properly and 

 is often supplanted by a mass of partially differen- 

 tiated cells. 



As is shown by figure 4, one of the most strik- 

 ing appearances in sections of diseased roots and 

 shoots is the presence of more or less isolated groups 

 of hypertrophied infected cells which Nawaschin 

 named "Krankheitsherde." He believed that these 

 groups arise by repeated anti- and periclinal division 

 of one or more originally infected cells, whereby the 

 ])lasmodia are passively distributed in a radial di- 

 rection around the region of infection. Cliupp also 

 reported that a single amoeba might give rise to as 

 many as six such groups by multi]ilication and migra- 



