Colorado Plant Diseases 
45 
Affected plants, as the name indicates, are characterized by the 
yellow color of their outer leaves. The plants are stunted and the 
lower leaves show a tendency to drop at the slightest touch. Young 
seedlings are often affected in which case the plant rapidly wilts, 
turns black and dies. 
Cabbage yellows is due to a fungus that lives from year to year 
in the soil, gaining entrance to the roots and checking the flow of 
water from the roots to the rest of the plant. Crop rotation and 
the use of resistant varieties are the most practical means of con- 
trolling the disease. The Wisconsin Agricultural Experiment Sta- 
tion has had remarkable success in selecting strains of cabbage 
resistant to yellows. 
Club-Root (Plasmodiophora brassicae) (46) — Club-root is not 
common in Colorado. In the eastern states, however, it is a very de- 
structive disease. The roots of infected plants become much enlarged 
and swollen, each infected root taking on a peculiar spindle shape. A 
dwarfing of the remainder of the plant results. In warm, dry 
weather infected plants wilt sooner than healthy ones. In severe 
cases the plant may die. Club-root is due to infection by a slime 
mold which gains entrance into the roots from the soil. The little 
slimy masses of protoplasm, that make up the body of the organism, 
work their way from cell to cell in the root, stimulating it to ab- 
normal growth. Late in the season these masses of protoplasm inside 
the root break up into small round spores. When the root decays 
these spores are set free in the soil. Under proper conditions these 
spores germinate to form new masses of protoplasm which may affect 
new cabbage plants. 
Since the organism causing club-root lives in the soil, crop rota- 
tion is one of the best control methods. All roots of infected plants 
should be destroyed. A heavy application of lime (100 bushels per 
acre) every few years is said to be an effective means of control. 
CELERY 
Late Blight (Septoria petroselini var. apii) (47) — This is by far 
the most serious disease of celery in Colorado. Few fields are free 
from the trouble and many are seriously damaged. 
Both leaves and stems are affected. Small light brown spots are 
first formed on the lower outer leaves. The spots later turn black 
and if the leaf is badly affected it will dry up and die. Many small 
black spore cases (pycnidia) are found over the surface of the spots 
and imbedded in the leaf. Many small but long needle-shaped 
spores are set free from a round opening in the tops of the pycnidia. 
