Inhibitory Effect of Pentachlorobenzyl alcohol (PCBA) on Sporulation of Piricularia oryzae Cavara

Pentachlorobenzyl alcohol (PCBA) is one of the most effective fungicides for the control of rice blast. When blast-infected rice seedlings on a nursery bed outdoors were sprayed with PCBA at 1000μg/ml, spore scattering from the infected seedlings was markedly inhibited from 2nd to 10th day after the spraying. Sporulation on lesions of the sprayed leaves was greatly suppressed during this period. When the leaves, on which PCBA had been sprayed, were detached and incubated at 28°C in a moist chamber, spores were produced as well as on leaves from unsprayed seedlings. The same result was obtained with the test using rice seedlings grown and inoculated under greenhouse conditions. On the contrary, phenylmercuric acetate (PMA, Hg-30 or Hg-20μg/ml) and blasticidin-S (Bc-S, 20 or 10μg/ml) inhibited the sporulation both on leaves of growing plants and on detached leaves. When these three compounds were sprayed on mycelial mats grown on a agar medium in petri dishes, PCBA did not inhibit the sporulation, whereas PMA and Bc-S, each at 40μg/ml, inhibited the sporulation by about 100 per cent. These results clearly indicate that sporulation-inhibiting effect of PCBA is not caused by the direct contact of the compound with the fungus but is caused by an interaction between the compound and the growing rice plant.