ONION SMUT 
This disease is most prevalent in the northeastern part of the United 
States where in certain localities it causes a considerable loss. Because 
of its striking symptoms it has been known to onion-growers for many 
years and has been the subject of study by scientists for at least the past 
forty years. 
SYMPTOMS 
Examine a diseased seedling. OBSERVE :— 
1. The narrow, yellow lesions extending parallel with the leaves. 
These are the first symptoms to appear. 
2. The narrow, black areas which are most numerous near 
the base of the onion and sometimes extend almost to the tip of the leaf. 
The yellow areas will later become darkened like these. 
3. That these black masses are all enclosed within the tissue of 
the leaf, or that occasionally the epidermis is ruptured. 
4. That in cases where the leaf is badly affected, its tip has 
withered and droops. 
Make a DRAWING of a diseased seedling. 
Examine one of the larger diseased onions. OBSERVE:— 
5. That in this case each badly diseased leaf has turned brown 
and has fallen over. 
6. That the black lesions have now broken open, forming long 
black open sori from which a sooty-like mass is sifting. DRaw. 
ETIOLOGY 
The pathogene reponsible for this disease is Urocystis Cepulae Frost, 
a basidiomycetous fungus of the order Ustilaginales. There are two 
fam ilies in this order, the Ustilaginaceae and Tilletiaceae. To the former 
belong those organisms which produce the loose smuts of wheat, oats 
and corn, while to the latter belong the pathogenes like those producing 
the stinking sn-ut of wheat, and the onion smut. 
Life-history. 
The Primary Cycle has its origin in the chlamydospores found in 
the infested soil. 
Pathogenesis. These spores, when near germinating onion seeds, 
also germinate sending out a long branched mycelial thread. This my- 
celiun: probably enters the host directly, or it may possibly produce conidia 
or sporidia borne on the tips of the mycelial branches. In the latter case 
it is the germ. tube which grows from the sporidium, that enters the tissue of 
the host. Infection always takes place under ground and only when the 
seedling is just en erging from the soil. No new infections occur after the 
onion becomes older. This mycelium develops within the leaf-tissues, 
giving rise to the long black sori. Scrape a few spore-balls from a sorus 
and, under the microscope, OBSERVE :— 
7. The size, color and surface markings of the spore-ball. It is 
made up of a central thick-walled resting-spore (one or more celled) 
surrounded completely by thin-walled pseudospores. Only the central 
cells are viable. If the inner structure can not be determined from the 
mount, consult Duggar, Fungous Diseases of Plants, p. 382. 
135 
