1a. Objectives (from AD-416)
The fundamental goal of this research program is the development of sustainable methods fro the control of soilborne diseases of fruit trees. Thus, the objectives are to define the mechanism(s) of action for brassicaceae seed meals in the control of the pathogen/parasite complex which incites orchard replant disease, and develop and evaluate in the field novel formulations or integrated systems to attain the necessary spectrum of pathogen/parasite suppression for control of orchard replant disease.
1b. Approach (from AD-416)
This program will develop a systems approach to disease management which exploits the biological resources resident to orchard ecosystems and utilizes minimal inputs beyond that commonly employed during orchard renovation or management. This will be achieved through acquiring an understanding of the biological and chemical mechanisms involved in the brassicaceae seed meal induced suppression of the pathogen complex contributing to apple replant disease. New tools will be developed to efficiently monitor fungal community structure in these soils to garner a more complete understanding of the functional elements contributing to disease suppression and the temporal nature of the response. Enhanced efficacy of bio-active plant residues for the control of soilborne pathogens will be obtained by clarifying the role of soil microbial communities in eliciting the inhibitory activity of these plant tissues. Information from these studies will be utilized to develop and field validate biologically sustainable management strategies for control of replant diseases in conventional and organic production systems. Replacing 5350-22000-012-00D (11/07).
3. Progress Report
Unique fungal/oomycete cultures and DNA clones of the ITS region representing approximately 200 different taxa were identified. These served to generate and comprise the initial library of clones used in construction of the fungal DNA array. Studies were conducted to validate the approach of utilizing the entire internal transcribed spacer (ITS) region (ITS1/5.8S/ITS2) of fungal ribosomal DNA as both the target and the probe in an array-based detection system. A mini-array consisting of 10 different Pythium and Phytophthora species as the array targets were probed with individual single species Pythium probes. These tests performed demonstrated that taxonomic lineages could be distinguished when ITS DNA sequence similarity differed by greater than 5-10%. The relative sensitivity of fungi and oomycetes that comprise the biological complex inciting apple replant disease to allylisothiocyanate, the dominant isothiocyanate released in response to Brassica juncea seed meal amendment, was assessed. Differences in sensitivity were observed among disparate members of this complex, and even among species within a genus; differences were particularly noteworthy among species within the genus Pythium. Seed meal amendments increased populations of bacterivorous protozoa, which could have negative effects on the biocontrol organism Pseudomonas fluorescens strain SS101 which suppresses Pythium spp. Studies demonstrated that production of the cyclic lipopeptide massetolide A protected the bacterium against protozoan grazing. Effects of brassicaceous seed meals and apple rootstock genotype on the activity and composition of Pythium spp. and Pratylenchus penetrans populations were assessed. Geneva series rootstocks were less susceptible to infection by Pythium spp., whereas M26, MM106 and MM111 were highly susceptible. Geneva rootstocks consistently supported lower populations of P. penetrans than did rootstocks of the Malling or Malling-Merton series. Significant rootstock x seed meal interaction was detected, and nematode suppression in response to B. napus or S. alba SM was only observed when used in concert with a tolerant rootstock while B. juncea SM suppressed nematode root populations irrespective of rootstock. In field trials that employed a Brassica napus/Brassica juncea composite seed meal amendment in a commercial organic orchard, the growth response attained was rootstock-dependent. Root growth of G30 rootstock planted in seed meal amended soil was superior to all other treatments.These findings demonstrate that use of brassicaceous seed meals for replant disease suppression must employ an appropriate rootstock in order to achieve optimal disease control. This progress significantly advances goals outlined in Component 1, Pre-Plant Soil Fumigation Alternatives, of National Program 308, development of new technologies for alternatives and integration into commercial crop production systems currently dependent upon methyl bromide soil fumigation.
1. Cyclic lipopeptides produced by Pseudomonas fluorescens confer resistance to protozoan grazing. Cyclic lipopeptides (CLPs) are proteins produced by certain bacteria and function in the biological control of various plant pathogenic fungi. This study was undertaken to assess the possible non-target effects of CLPs produced by biocontrol bacteria. CLPs produced by strains of the biocontrol bacteria Pseudomonas fluorecens were demonstrated to be toxic to the protozoan Naegleria americana. Populations of Pseudomonas that do not produce CLPs exhibited a more rapid decline in soil containing the protozoan relative to survival of the CLP producing Pseudomonas strains. These results show, for the first time, that CLPs produced by Pseudomonas contribute to survival in soil and are potent metabolites in the bacterial defense against protozoan predation. This addresses National Program 308, Component 1, Problem Statement 1b.
2. Efficacy of brassicaceous seed meal amendments for replant disease suppression occurs in a rootstock dependent manner Studies were conducted to assess the interaction between apple rootstock and the capacity of brassicaceous seed meals to provide control of two components of the pathogen complex that incites replant disease; Pratylenchus penetrans and Pythium spp. ARS scientists in Wenatchee, WA discovered that Geneva series rootstocks were less susceptible to root infection by native populations of Pythium spp. and supported lower populations of P. penetrans than did rootstocks of the Malling or Malling-Merton series. Significant rootstock x seed meal interaction was detected, and nematode suppression in response to B. napus or S. alba SM was only observed when used in concert with a tolerant rootstock while B. juncea SM suppressed lesion nematode root populations irrespective of rootstock. These findings demonstrate that utilization of brassicaceous seed meal amendments for replant disease suppression must employ an appropriate rootstock in order to achieve optimal disease control. National Program Component and Problem Statement Addressed: Problem Statement 1A: Development of New Technologies for Alternatives and Integration into Commercial Crop Production Systems Currently Dependent upon Methyl Bromide Soil Fumigation.
5. Significant Activities that Support Special Target Populations
Contacts with orchardists regarding non-fumigant alternatives for control of soilborne plant pathogens, and in particular apple replant disease, occur on a regular basis. Contacts included the establishment of a demonstration in a small family operated organic orchard and interaction with growers at industry sponsored meetings. Information provided to growers is based upon findings from this CRIS concerning the development of novel non-fumigant methods for the management of replant disease that can be utilized in organic and conventional orchard production systems.