Abstract: A trial with different concentrations of aqueous extracts from dry Acorus calamus plants  was conducted to study the effects of the extracts on Microcystis aeruginosa and Chlorella pyrenoidosa growth. Allelopathic effects were observed. H
igh concentration (>40 ml·L-1) extracts inhibited the growth of the two a
lgal species at low inoculation cell density significantly (P<001), with t
he maximal inhibition rate being 98.66% for M. aeruginosa and 9238% for
C. pyrenoidosa, while low concentration (<30 ml·L-1) extracts promote
d the growth of C. pyrenoidosa. Neither high nor low concentration extracts
had significant effects on the growth of the two algal species at high inoculati
on cell density (P>005). The comparison of adding the aqueous extracts onc
e and semi-consecutively showed that the allelochemicals in the extracts were e
asy to be degraded, suggesting that the continuous secretion of allelochemicals
in natural water bodies could play an important role in inhibiting algal growth.

Key words: water bloom control, Acorus calamus extract, Microcystis aerugi nosa, Chlorella pyrenoidosa, inhibitory effect on algae, optimized cropping system, southern China, preventing and avoiding drought disaster, precipitation year type.