Columbia subject headings:
Harvard subject headings:
Citrus fruit industry--California, Southern.
Citrus fruits--Research--California, Southern.
Citrus--Diseases and pests--California, Southern.
Horticulturists--California, Southern.
Powell, George Harold--Correspondence.
Princeton subject headings:
California, Southern--Biography.
Citrus fruit industry--California, Southern.
Citrus--Diseases and pests--California, Southern.
Horticulturists--California, Southern--Biography.
Powell, George Harold--Correspondence.
Powell, Gertrude Eliza Clark, 1870-1957--Correspondence.