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
The collection and identification of nectarine fruit have been completed and the majority of isolates have been identified using genotypic and phenotypic methods. These isolates are being evaluated for biocontrol activity using an in vitro membrane system. Significant progress has been made in developing an artificial membrane system to study the interaction between the resistant infection structure (appresoria) of the brown rot pathogen, M. fructicola, and biocontrol agents. This system is being utilized to identify bicontrol agents that may be effective against latent infections. An apple microarray, suitable for use in the study of apple, pear, peach, and other Rosaceae fruit crops, has been utilized to identify genes induced or suppressed in immature peach fruit inoculated with either a pathogen (M. fructicola) or a non-pathogen (P. digitatum). Characterization of these genes is in progress and will help to identify how the brown rot pathogen overcomes host resistance in peach fruit and what biochemical pathways are affected by the infection process. The development of antibodies directed against enzymes (polygalacturonases) produced by the pathogen and that play a role in virulence is being pursued as a strategy to manage postharvest diseases. Significant progress has been made in isolating genes responsible for the production of the antibodies in mice spleen tissues.
1. Mechanism by which postharvest pathogens infect fruit is identified. In order to develop new disease control strategies, a better understanding is needed of how pathogens overcome innate host resistance mechanisms. In a study of citrus, we found that the green mold pathogen, P. digitatum, is able to suppress the production of hydrogen peroxide by the host and that this suppression helps the pathogen to infect and colonize the fruit tissue. Hydrogen peroxide is an oxidant that is detrimental to the pathogen and acts a signaling molecule to trigger a wide variety of host resistance genes. The role of reactive oxygen species, such as hydrogen peroxide, is being evaluated in other host pathogen systems and may represent a common mechanism by which pathogens overcome host resistance. Such knowledge can be used to develop alternative strategies for managing postharvest diseases in fruits.
Droby, S., Wisniewski, M.E., Macarisin, D., Wilson, C. 2009. Twenty years of postharvest biocontrol research: Is it time for a new paradigm?. Postharvest Biology and Technology. 52:137-145.