MONITORING AND MANIPULATION OF POPULATIONS OF PYTHIUM-OLIGANDRUM, PYTHIUM-MYCOPARASITICUM AND A PAPULASPORA SPECIES IN SOIL

Pythium oligandrum, Pythium mycoparasiticum and Papulaspora sp. were d etected by production of their characteristic resting propagules (oosp ores or bulbils) when soils or soil-sand dilutions were placed on agar colonized by other fungi. Frequency of detection of Papulaspora (on a gar colonized by Botrytis cinerea) was enhanced when metalaxyl was inc orporated into the agar to suppress competition from P. oligandrum; de tection of P. mycoparasiticum (on agar colonized by B. cinerea, Phialo phora sp. or Fusarium culmorum) was enhanced by dilution of soil to re duce the P. oligandrum content. When soil was supplemented with mature wheat flag leaf (but not green grass leaves) the resident population of P. oligandrum was enhanced during 280 days, assessed by most probab le number (MPN) analysis when serial soil dilutions were plated. A sec ond supplement of wheat flag leaf or grass leaf at 150 days significan tly and further enhanced the P. oligandrum population. When oospores o f a metalaxyl-tolerant P. oligandrum were added to soil, this strain c ould be recovered after 240 days and it enhanced the total population of P. oligandrum throughout this time. P. mycoparasiticum was not amen able to MPN analysis because of competition from P. oligandrum in undi luted soils, but its incidence of detection in the higher dilutions su ggested a population density similar to P. oligandrum, and this was su pported by log-probit regression of detection frequency in soils in wh ich P. oligandrum was rare. When cellulose film colonized by B. cinere a, Phialophora sp. or F. culmorum was buried in soil it was colonized mainly by P. oligandrum if the soil was undiluted, but mainly by P. my coparasiticum if the soil was diluted with sand, indicating competitio n between the mycoparasites in soil. In laboratory studies on ''host'' -preference, Papulaspora grew from agar disc inocula across agar colon ies of several fungi, but from bulbil inocula it grew only across B. c inerea and Trichoderma aureoviride, which are the fungi that enable it s detection in soil. In liquid medium, Papulaspora grew only in the pr esence of B. cinerea, but grew well on media that had previously suppo rted growth of F. culmorum or Rhizoctonia solani. In soil Papulaspora was selectively isolated from cellulose film or wheat flag leaves prev iously colonized by Humicola grisea, but not from substrata left uncol onized or colonized by Rhizoctonia oryzae. This ability of secondary c olonizers to invade only some fungal colonies may relate to substrate- possession of crop residues by pathogens in nature. The improved detec tion methods here suggest that P. mycoparasiticum and Papulaspora sp. are common and abundant in soil, and that other mycoparasites might be detected by eliminating competition from fast-growing species such as P. oligandrum. The coexistence of mycoparasites that can compete with one another in soil suggests that they have different ecophysiologica l features (niches) of potential exploitation in biocontrol.