CERCOSPORA LEAF-SPOT OF BEETS 
Although this disease is found more often on sugar-beets, it also affects 
the common garden-beet. ‘The disease is typical of a great many Cerco- 
spora leaf-spots on different hosts. 
SYMPTOMS 
Lesions are usually present only on the leaves, though they have been 
reported on the petioles, the bracts, peduncles of the flowers and on the 
seed-pods. 
In the material provided, oBSERVE :— . 
1. The variously sized spots with brown or grayish centers 
surrounded by a zone of red or purple (brown in dried specimens); their 
shape and distribution over the leaf; differences in the character of the 
spot on the upper and the lower surface. Make prawincs to show these 
oints. 
‘ As the diseased leaves drop off, new ones are formed, thus producing 
an elongated crown on the root. (See photographs or Duggar, Fungous 
Diseases of Plants, p. 310.) 
ETIOLOGY 
The pathogene responsible for this disease is Cercospora Beticola 
Saccardo, one of the Hyphomycetes. Only the asexual form of the fungus 
is known. 
Life-history. 
The Primary Cycle is initiated by conidia developed on overwintered 
tops and crowns left in the field. They are blown or spattered by the rain 
to the first young leaves. 
Pathogenesis. Scrape several of the spots with a scalpel and make 
a mount in water of the material thus procured. Under the microscope, 
OBSERVE :— 
2. The thin, long, needle-like spores, which are slightly thickened 
at one end and tapering at the other. How many septain each? pRaw. 
These conidia germinate and the germtubes enter the leaf through 
the stomata. Study the germinating spores. OBSERVE:— 
3. The number of germtubes developed from each spore; num- 
ber from each cell. praw. Study leaf-invasion through the stomata, 
as shown by Pool and McKay, Jour. Agr. Research 5, pl. LXXXI. copy 
one or more of the figures, A, B, C, or F. Label in detail. 
The mycelium developed from these germtubes spreads through and 
kills the tissue. A leaf-lesion or spot thus results. Very shortly this 
mycelium develops the fruiting structures on the surface of the lesion. 
Cut out a spot from the leaf including a margin of healthy tissue. 
Macerate in a casserole for half a minute in potassium hydroxide over a 
microburner. Mount in water and oBsERVE:— 
_ 4, At the edges of the lesion, the almost hyaline branching 
mycelium, which may be uniform in width or have occasional cells thick- 
ened, with granular vacuolated contents. DRAW. 
5. The tufts of brown conidiophores closely crowded at the base 
and widely separated at the top. 
6. That very few of the conidiophores are straight but appear 
knotted or angled. 
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