POTASH FROM KELP. 33 
had checked their growth. Comparable to these stunted plants 
were a number of plants of elk kelp (Pelagophycus) observed on 
the edge of the La Jolla bed in November, 1917. ‘These specimens 
were fully matured and decaying plants, yet they had never attained 
the surface, but stood erect, with their cysts 10 to 20 feet under 
water. 
During the cool, stormy weather of November sind December, 
1918, the kelp beds improved noticeably. During the following 
months they gradually returned to their original density, responding 
to changes in water temperature and weather. Unfavorable con- 
ditions later on resulted, however, in the nearly total disappearance 
of kelp beds along the San Diego coast, with the exception of the 
bed off Point Loma where there is an unusually free circulation of 
water, and where the temperature of the water is low. 
There is little doubt that black rot is the most destructive enemy 
of kelp. This disease attacks all the beds along the south California 
coast, including those around the islands, and destroys more kelp 
than storms do. It does no appreciable damage in the winter, but 
attacks the spring and early summer growth which would otherwise 
be the best to harvest. Between April and July, 1918, it ruined a 
fine bed of kelp at Naples; thus, where 3,000 tons could have been 
eut in April, only 300 were available in July when the bed was 
opened. The United States Government reserve bed at Summerland, 
as stated above, became a veritable bed of black rot. By August, 
wherever the harvesters had not cut, the disease had possession of 
practically every frond, even in the wind-swept beds of San Nicolas. 
The disease was reported to have destroyed much kelp in the summer 
of 1917, and it probably also destroyed the San Nicolas beds in the 
summer of 1915, and the heavy Mexican beds in the late winter and 
spring of 1919. 
Black rot is a bacterial disease, but the species of bacteria has not 
yet been classified. However, sufficient information has been ob- 
tained, from experiments in the laboratory and observations in the 
kelp beds, to enable us to formulate a plan of control. The organism 
does not grow readily at a temperature as low as 15° C., and, conse- 
quently, practically disappears during winter. It thrives in a tem- 
perature of 18° to 20° C. It has been found to require a great deal 
of air. When infected plants were placed in a vessel of sea water 
and covered to exclude all air but what was dissolved in the water, 
the bacteria of black rot were speedily displaced by ordinary fermen- 
tation bacteria. This behavior of the organism in the laboratory is 
paralleled ky its not working under water to any noticeable extent 
in the kelp beds. It apparently requires more air than is dissolved 
in the water. 
The plants which black rot has been observed to attack are long 
bladder kelp (Macrocystis pyrifera), elk kelp (Pelagophycus porra), 
and ribbon kelp (/'gregia laevigata). Of these, the last is seldom 
attacked, because it floats for the most part under water, except in 
the tidal zone, where the surf beats off and carries away any weak- 
ened or decayed material. Pelagophycus, while very frequently 
attacked, is not destroyed in such quantities as Macrocystis, probably 
because the more delicate parts are usually under water. The dis- 
ease mainly attacks leaf blades, which present many wounds and 
