Monitoring and manipulation of populations of Pythium oligandrum, Pythium mycoparasiticum and a Papulaspora species in soil

Pythium oligandrum, Pythium mycoparasiticum and Papulaspora sp. were detected by production of their characteristic resting propagules (oospores or bulbils) when soils or soil-sand dilutions were placed on agar colonized by other fungi. Frequency of detection of Papulaspora (on agar colonized by Botrytis cinerea) was enhanced when metalaxyl was incorporated into the agar to suppress competition from P. oligandrum; detection of P. mycoparasiticum (on agar colonized by B. cinerea, Phialophora sp. or Fusarium culmorum) was enhanced by dilution of soil to reduce 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 probable number (MPN) analysis when serial soil dilutions were plated. A second supplement of wheat flag leaf or grass leaf at 150 days significantly and further enhanced the P. oligandrum population. When oospores of a metalaxyl-tolerant P. oligandrum were added to soil, this strain could be recovered after 240 days and it enhanced the total population of P. oligandrum throughout this time. P. mycoparasiticum was not amenable to MPN analysis because of competition from P. oligandrum in undiluted soils, but its incidence of detection in the higher dilutions suggested a population density similar to P. oligandrum, and this was supported by log-probit regression of detection frequency in soils in which P. oligandrum was rare. When cellulose film colonized by B. cinerea, 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. mycoparasiticum if the soil was diluted with sand, indicating competition between the mycoparasites in soil. In laboratory studies on "host"-preference, Papulaspora grew from agar disc inocula across agar colonies of several fungi, but from bulbil inocula it grew only across B. cinerea and Trichoderma aureoviride, which are the fungi that enable its detection in soil. In liquid medium, Papulaspora grew only in the presence of B. cinerea, but grew well on media that had previously supported growth of F. culmorum or Rhizoctonia solani. In soil Papulaspora was selectively isolated from cellulose film or wheat flag leaves previously colonized by Humicola grisea, but not from substrata left uncolonized or colonized by Rhizoctonia oryzae. This ability of secondary colonizers to invade only some fungal colonies may relate to substrate-possession of crop residues by pathogens in nature. The improved detection 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 ecophysiological features (niches) of potential exploitation in biocontrol.