167 
ETIOLOGY 
This disease is caused by the myxomycete, Plasmodiophora Brassicae 
‘Woronin. It is apparently an obligate parasite, saprogenesis being merely 
a period of rest. It has never been cultured saprophytically. 
Life-history. This pathogene has, normally, an annual life-history 
but its spores may evidently lie dormant in the soil for several years in 
the absence of a cruciferous crop. : 
; The Primary Cycle is initiated bv spores that have remained dormant 
in the soil from a former season. ‘ 
Pathogenesis. When the affected roots rot, the spores of the patho- 
gene, mixed with the decayed host-tissue, remain in the soil. Selecting a 
root in an advanced stage of the disease, remove a bit and crush it ina 
drop of water on a slide; cover and examine with the high-power. oB- 
SERVE -— 
9. The minute globose hyaline spores, abundant among the 
crushed tissue-fragments. DRAW. 
These spores germinate in the spring after the manner of those of the 
saprophytic slime-molds. The spore-wall cracks and the naked protoplast 
creeps out. Study the process as illustrated in Vermont Bul. 173, fig. 6. 
copy to show several stages in spore-germination and the development 
of the swarmspore or myxameba. 
These myxamebae, thus set free in the soil water, come in contact 
with nearby roots of susceptible plants and infection results. The single 
uninucleated swarmspore enters through a root-hair. Here by rapid 
nuclear division and growth, a multinucleate ameba or plasmodium is 
formed which penetrates into the cortical tissues. By direct migration 
through the cell-walls and by the division of the meristematic host-cells 
into which the ameba have penetrated, invasion of the various tissues is 
effected. 
The plasmodium increases in size, destroys the protoplast of the cells 
in which it is lodged, and gradually fills the lumena of the cells. Make thin 
sections (or use prepared slides) through roots in the early stages of the 
disease. OBSERVE :— 
10. The finely and densely granular multinucleate protoplasm 
of the pathogene in certain of the enlarged cortical or medullary ray-cells. 
Compare with the protoplasm of uninvaded cells nearby. (See Vermont 
Bul. 175, fig. 34.) DRaw an uninvaded and a normal host-cell, with 
contents, to show contrast. : 
These multinucleate plasmodia, when mature, divide into spores, each 
one-celled. Cut sections (or use prepared slides) of diseased roots con- 
taining mature spores. OBSERVE :>— 
11. That the infested cells are packéd with spores; uniform size 
of spores; and absence of capillitium, characteristic of most saprophytic 
slime-molds. The host-cell serves as a sporangium. DRAW an invaded 
cell with spores. 
12. In which tissues of the root the pathogene is most abundant. 
Saprogenesis. The affected tissues gradually decay and the 
mature spores go into a resting- or dormant-period. No saprophytic 
activities, between spore-germination and infection by myxamebae in the 
spring, are known to take place. j : 
Secondary Cycles. It is known that the spores will germinate at 
once or very shortly after they are formed. Therefore, if set free by the 
