The biological control activity of Pythium oligandrum against black scurf of potato caused by Rhizoctonia
solani AG-3 was evaluated in field experiments after treatment of potato seed tubers with P. oligandrum.
Seed tubers infected with black scurf sclerotia were dipped for a few seconds in a suspension of 103, 104
or 105 mL1 P. oligandrum oospores and were then air-dried. Each level of P. oligandrum-treatment
significantly reduced the disease rates of stolon at a level similar to that achieved by chemical control.
When P. oligandrum populations adherent to the surface of seed tubers were determined, oospore counts
on tubers treated with 104 or 105 oospores mL1 were about 540/cm2 or about 22,000/cm2 just after
dipping and decreased to about 170/cm2 or 2900/cm2 after a 3-week incubation, respectively. Confocal
laser scanning microscopic observation with an immuno-enzymatic staining procedure showed that
P. oligandrum hyphae had colonized the sclerotia and established close contact by coiling around the
R. solani hyphae present on the surface of seed tubers, in a manner similar to that observed in the
dual-culture test. Quantification of R. solani DNA by PCR indicated that the R. solani population was
reduced on the seed tubers treated with P. oligandrum compared to untreated tubers. Furthermore, the
ability of P. oligandrum to induce resistance against black scurf was determined using a potato tuber disk
assay. Treatment of tuber disks with the cell wall protein fraction of P. oligandrum enhanced the
expression of defense-related genes such as 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase,
lipoxygenase and basic PR-6 genes, and reduced disease severity upon challenge with R. solani compared
with untreated controls. These results suggest that biocontrol mechanisms employed by P. oligandrum
against black scurf involve both mycoparasitism and induced resistance.