Pythium Species and Isolate Diversity Influence Inhibition by the Biological Control Agent Streptomyces lydicus

Weiland, J. E. 2014. Pythium species and isolate diversity influence inhibition by the biological control agent Streptomyces lydicus. Plant Dis. 98:653-659. Disease control of soilborne pathogens by biological control agents (BCAs) is often inconsistent under field conditions. This inconsistency may be partly influenced by pathogen diversity if there is a differential response among pathogen species and isolates to selected BCAs. The responses of 148 Pythium isolates obtained from soil at three forest nurseries and representative of 16 Pythium spp. were evaluated in the presence of Streptomyces lydicus strain WYEC108 in an in vitro assay. Percent growth inhibition, inhibition zone distance, mortality, and growth rate were recorded for each isolate, and data were analyzed for effects of species and isolate. Responses of three Pythium spp. (Pythium irregulare, P. sylvaticum, and P. ultimum) were further analyzed for a location (nursery) effect. Although S. lydicus inhibited all Pythium isolates, differences in percent growth inhibition, inhibition zone distance, and mortality were observed among Pythium spp. and isolates. Small but significant location effects were also noted. Growth rate also varied among Pythium spp. and isolates and was found to strongly bias percent growth inhibition and, to a lesser degree, inhibition zone distance; depending on which measure was used, slowergrowing isolates appeared less sensitive (growth inhibition) or more sensitive (inhibition zone) to S. lydicus than faster-growing isolates. Results illustrate the importance of using multiple, representative pathogen isolates in preliminary BCA inhibition assays as well as accounting for the effect of pathogen growth rate on pathogen inhibition by BCAs. Future studies should take pathogen diversity into account when evaluating biological control efficacy. Pythium spp. cause damping-off and root rot in forest seedling nurseries worldwide and are considered one of the most important soilborne pathogens affecting tree seedling production (41,44–46). In the Pacific Northwest (PNW) region of the United States (Idaho, Oregon, and Washington), forest nurseries rely on periodic fumigation with either methyl bromide or dazomet for control of Pythium spp. and other soilborne pathogens (43,45,46). Chemical fumigants have long been preferred for soilborne disease control in forest nurseries because of their broad-spectrum, nonselective activity against numerous soilborne plant-pathogenic genera, and for their additional activity against soilborne insect pests and weed species. Nursery managers may also apply specific fungicides such as mefenoxam during the growing season for supplemental disease control. As state and federal regulatory agencies place greater restrictions on the use of fumigants for tree seedling production, alternative measures of disease control are increasingly desired by the forest nursery industry. Ideally, these disease control measures would have minimal environmental impact and pose little to no risk for human health. Forest nursery managers have been interested in using biological control agents (BCAs) against soilborne pathogens because biological control is considered more environmentally sustainable and less toxic than conventional chemical fumigants and pesticides (2,9). Unfortunately, there have been few successes in the use of BCAs in forest tree seedling production (7,8,17,21,25,28,36), and many growers cite inconsistent results or a lack of disease control as the primary reason for not using biological control in their nursery (interviews with 14 forest nursery managers by the author in 2011, personal communication). In general, BCAs do not have the same degree of broad-spectrum activity against the wide variety of soilborne pathogens that has been exhibited by chemical fumigants. Although biocontrol is sometimes thought to be more effective under conditions with relatively low biodiversity (12), including low pathogen diversity (37), many individual BCAs are labeled as having relatively broad-spectrum activity against multiple soilborne phytopathogenic genera or species. As a result, researchers and growers have frequently applied single BCA strains in attempts to control soilborne diseases that are caused by multiple phytopathogenic agents such as damping-off or root rot (10,19,21,27,31,35). In some cases, subsequent disease control was successful (10,19,31,35) whereas, in others, it was not (21,27,35). Most of the research regarding biocontrol efficacy has focused primarily on factors associated with the BCA or its interaction with the rhizosphere and edaphic environment. These factors include the BCA application dose and resultant population size, rhizosphere competence, survival, effects of BCA genetic diversity on disease control, mechanisms of pathogen suppression (e.g., antibiotics, competition, and so on), substrate interactions (e.g., soil versus compost), and BCA nutritional requirements (6,18,37,47). In comparison, very little research has been conducted to quantitatively determine how much variability within pathogen communities (among species) or populations (among isolates of the same species) contributes to the outcome of biological control (32,37). Research that specifically and systematically evaluates the responses of representative isolates from local, naturally occurring pathogen communities or populations to selected BCAs is also lacking (32). Although there are many BCA studies that assess several pathogen species or isolates, the number of species or isolates evaluated is usually extremely limited and, as a consequence, there is limited statistical inference about the contribution of pathogen species or isolate diversity to BCA inhibition (5,6,11,28,35–37,48,50). Furthermore, the pathogen taxa used in many biological control assays are often not representative of the greater genotypic and phenotypic diversity existing in the field. In many cases, the pathogen isolates were collected from different geographic locations, plant hosts, or culture collections (9,11,22,26,29,32,35,37,38,42,48,50). Knowledge about the degree to which BCA inhibition is influenced by pathogen species and isolate diversity is particularly relevant to Corresponding author: J. Weiland, E-mail: [email protected] Accepted for publication 2 December 2013. http://dx.doi.org/10.1094/PDIS-05-13-0482-RE © 2014 The American Phytopathological Society