May 17, 1924 
A Fusarium Bulb Rot of Onion 
691 
RELATION OF TEMPERATURE TO OCCURRENCE AND DISTRIBU¬ 
TION 
The final interpretation of field observations in the light of these experimental 
data must be delayed until a thorough survey of Fusaria associated with bulb 
rot and a study of their morphological and physiological characters has been 
completed. However, it is significant to note that at least in the Middle West, 
where this disease is now common, it does not ordinarily appear until July or later. 
An examination of soil-temperature records (see fig. 3) taken at 1-inch depth in 
an onion field at Racine, Wis., on two extreme seasons, 1915 and 1916, shows that 
in this rather northerly section the daily mean does not ordinarily reach 15° C., 
the lower limit for this disease, for any considerable period until May 15 or later, 
while the optimum temperature is not reached until July 1 or later. It would 
therefore seem plausible to believe that soil temperature limits the progress of 
the disease, so far as this species of Fusarium is concerned, during the early half 
of the growing period. 
Fjg. 3.— Chart from data collected at Racine, Wis., during 1915 and 1916, showing the daily mean soil 
temperature at a depth of 1 to 2 inches 
The distribution of the Fusarium bulb rot is not sufficiently well known to 
interpret any limits on the basis of soil factors. It is significant, however, to note 
that it has appeared perhaps in its most severe form in the Walla Walla section 
of Washington and in the Uncompahgre section of Colorado, both of which regions 
are subject to very high temperatures during the last part of the onion-growing 
season. Moreover in a recent survey of onion diseases in Europe, the senior 
writer found Fusarium bulb rot doing severe damage only in the irrigated section 
at Valencia, Spain. Here again the disease appears in the hot midsummer and 
the general conditions of soil and air environment are very similar to those of 
the Walla Walla section. A repetition of the soil temperature experiments 
described above with two pathogenic strains of Fusarium collected in the 
Valencian section showed that these forms coincide very closely with Fusarium 
cepae in their temperature reaction. 
RELATION OF STORAGE TEMPERATURE TO THE DISEASE 
A thorough study of storage conditions as affecting the progress of a bulb rot 
should include a study of humidity as well as temperature. Since facilities for 
controlling humidity were not available, some preliminary experiments were con¬ 
ducted in which only the temperature was varied. 
Experiment 1: Sixty small white onion sets were prepared for inoculation in 
the usual manner and divided into two equal groups. One group remained with¬ 
out further treatment to serve as controls while the other 30 bulbs were inoculated 
by insertion of macroconidia of the organism into the wound. The two groups 
were next placed in separate moist chambers at 22° C. for 48 hours and were 
