1a. Objectives (from AD-416)
The objective of this project is to develop novel, biologically-based disease-control strategies for temperate fruit crops in order to reduce the use of chemical pesticides. This will be done by identifying microbial antagonists that are effective against latent and wound-induced infections of stone fruit, determining the genetic factors that make the brown rot pathogen so virulent by comparing host response to pathogenic and non-pathogenic organisms, and determining the role of fungal polygalacturonases as a virulence factor for postharvest infections of pear and peach.
1b. Approach (from AD-416)
The project will utilize a broad range of approaches to develop new biologically-based methods of postharvest disease control. Naturally-occurring yeasts and bacteria will be isolated from stone fruit and screened for activity against latent and wound-induced infections of stone fruit caused by the brown-rot organism, Monolinia fructicola. As part of the evaluation, select microbes will be tested for their ability to degrade melanized fungal structures such as appressoria using a model membrane system. Subtractive-suppressive hybridization of cDNA libraries will also be utilized to better understand the genetic basis of resistance mechanisms in stone fruit. This will be done by comparing host response at different developmental stages to both pathogens and non-pathogens. Lastly, the role of fungal polygalacturonases (PGs) as a virulence factor will be studied by utilizing recombinant antibody technology. The effect of the recombinant antibodies on conidial germination and the infection process will be evaluated.
3. Progress Report
Significant progress has been made in identifying bacterial and yeast that are capable of colonizing various structures of the fungus causing brown rot of stone fruits. These potentially beneficial microorganisms are being tested for their antagonistic activity against the brown rot fungus on nectarine and plum fruits. Polygalacturonases are enzymes that are produced by the brown rot pathogen of stone fruit and play a role in virulence. The development of antibodies directed against these enzymes produced by the pathogen is being pursued as a strategy to manage this major postharvest disease of stone fruit. Significant progress has been made in selecting clones expressing antibodies with good affinity to the polygalacturonases prouced by the brown rot pathogen, Monilinia fructicola. The simple compound, beta-aminobutyric acid (BABA), was demonstrated to induce resistance to brown rot infection in mature peach fruit. Both disease incidence and lesion diameter were reduced in treated samples by 50% compared to untreated, inoculated wounds. The ability to use BABA in combination with yeast antagonists to improve biocontrol efficacy is being evaluated. The importance of reactive oxygen species (ROS) in plant defense responses against certain pathogens is well documented. There is some evidence that microbial biocontrol agents also induce a transient production of ROS in a host plant which triggers local and systemic defense responses to pathogens. Using laser scanning confocal microscopy, it was observed that the application of M. fructicola and Candida oleophila into citrus and apple fruit wounds correlated with an increase in H(2)O(2) accumulation in host tissue. Living yeast cells were detected in fruit wounds at this time point indicating the ability of M. fructicola to tolerate host ROS, which has been reported to be an intrinsic characteristic of efficient yeast antagonists. Results indicate that the yeast-induced oxidative response in fruit exocarp may be associated with the ability of specific yeast species to serve as biocontrol agents for the management of postharvest diseases. An apple germplasm collection is maintained at the USDA-ARS-PGRU in Geneva, New York. This collection represents a diverse apple gene pool and was evaluated for resistance to blue mold and bitter rot. Resistance to blue mold was confirmed in select accessions over multiple years. Maturation patterns and quality indices for soluble solids and acidity, which may also affect pathogenicity, were highly variable and reflect the genetic diversity of the germplasm collection. Resistance in four accessions to bitter rot (caused by fungus Colletotrichum acutatum) and two accessions resistant to both diseases are reported for the first time.