Enhancing the sustainability of fresh-market snap bean production in the Northeast by identifying and promoting cultivars with tolerance to Cucumber mosaic virus (CMV).

Enhancing the sustainability of fresh-market snap bean production in the Northeast by identifying and promoting cultivars with tolerance to Cucumber mosaic virus (CMV).

Summary

Crop damage inflicted by a disease complex of aphid-transmitted viruses continues to threaten the profitability of snap bean production in the northeast region of the United States. This project seeks to advance the potential to reduce crop damage by identifying and deploying snap bean cultivars that are capable of maintaining acceptable levels of yield and quality while infected with the most prevalent of the viruses, Cucumber mosaic virus (CMV). The project has assembled a large and diverse panel of snap bean cultivars representing varying origins, dates of release, and market classes. This germplasm is being characterized under controlled conditions for each cultivar’s symptom expression in response to infection with CMV, as well as other viruses. Based on the severity of CMV-induced symptom expression, a subset of eight cultivars with the most severe symptoms, and a subset of eight cultivars with the mildest symptoms were selected for field-based research. A split-plot design with cultivar as the main plot and CMV infection status (+/-) as the sub-plot was planted in five replications and harvested during 2011. The data from this and future experiments will allow for a better characterization of the impact of CMV infection on yield, whether there is a relationship between symptom expression and tolerance to infection, and whether tolerant cultivars can be identified and deployed to reduce crop damage.

Objectives/Performance Targets

The objectives remain identical to the original plan of work. The objectives are listed below followed by a brief description of what has been accomplished to date.

1. Characterize symptom expression in response to Cucumber mosaic virus (CMV) infection in a diverse group of snap bean cultivars

This objective has motivated a thorough examination of the natural genetic variation available in snap beans for symptom expression in response to infection with important viruses of the Northeast. The panel of snap bean germplasm has been expanded to include 190 snap bean cultivars with relevance due to varying origins, dates of release, market classes, and potential virus resistance, and is now also complimentary with the snap bean panel used for research by the bean community and the BeanCAP. The second of two replications of the experiment to characterize symptom expression in response to CMV infection was carried out, and the response of each cultivar was assessed for symptom incidence, symptom type, and symptom severity. The disease screen was highly successful based on the high efficiency of inoculation (100%) and pronounced symptoms on all known susceptible controls and cultivars, with no symptoms evident in uninoculated controls. In addition to the stated objective, the panel of snap bean cultivars was also evaluated for response to infection with Bean yellow mosaic virus (BYMV), and Clover yellow vein virus (ClYVV), two additional viruses of increased prevalence in snap beans in the Northeast. As expected, there was a wide range of symptom expression in response to virus infection among the snap bean cultivars. Symptom severity varied from very mild to very severe, and allowed for the identification of snap bean cultivars with divergent symptom expression. None of the cultivars screened however, lacked symptoms completely. Digital images of the first unfolded trifoliate leaf of uninfected, CMV infected, BYMV infected, and ClYVV infected snap bean plants of the majority of the cultivars were obtained and compiled into a booklet for future reference/publication. The second replication has confirmed the repeatability of the methods and the symptom expression responses. The set of complete information and digital images will be compiled for future dissemination.

2. Conduct field-based evaluation of agronomic characteristics and yield responses to CMV infection in a subset of snap bean cultivars with diverse symptom expression in order to:
-(2a.) Determine the impact of CMV infection on the yield of snap bean cultivars with a range of symptom expression.
-(2b.) Identify snap bean cultivars capable of producing acceptable yields and quality when infected with CMV.
-(2c.) Characterize the relationship between patterns of symptom expression and yield loss.

The overall purpose of this objective is to obtain accurate and objective data on the impact of CMV infection on the yield components and total yield of snap bean cultivars. A subset of eight snap bean cultivars with the most severe symptoms and eight snap bean cultivars with the mildest symptoms were chosen based on the results of Objective 1. Seed of these sixteen snap bean cultivars was increased during the early spring of 2011 and was subsequently treated and prepared for field planting in the early summer of 2011.
A split-plot design with cultivar as the main plot and CMV infection status (+/-) as the sub-plot was planted in five replications on July 7. The subplots were either inoculated with CMV or mock inoculated with buffer and carborundum only using a mist-blower on July 20. Each of the five replications was then covered with insect barrier to prevent endemic aphid-transmission of CMV into uninfected subplots. The entire experiment was surrounded by guard rows of a blend of 6 snap bean cultivars. At flowering, the insect barrier was removed to assess the efficiency of the inoculation and the efficacy of the insect barrier. To accomplish this, 5 leaves were harvested at random from each sub-plot of each replication and virus status was determined by DAS-ELISA. The efficiency of inoculation was adequate, though perhaps less than ideal at a mean of 65% infection of those inoculated. The insect barrier was almost completely effective at preventing transmission of the virus between inoculated and uninoculated subplots (>1% infection), though it did have a uniform effect on increasing plant size compared to the uncovered guard rows. Given these results, the yield data can be analyzed with confidence that insect transmission of the virus is not confounding the results. The field plots were allowed to mature to physiological maturity and ten plants were harvested at random from within each stand of each subplot of each cultivar of each replicate. The pods are currently being harvested from each plant, bulked into each subplot and weighed for initial data. Data collection from the harvest is currently ongoing due to the labor required to hand process 1600 plants, and therefore the data is not yet fully available for the analysis to address Objectives 2a. through 2c., though it will be in the very near future. Given that additional data beyond that of 2011 will need to be obtained to confirm any results, serious consideration is being given to including a controlled environment evaluation of tolerance, as well as the planned completion of another field experiment in 2012.

3. Publish and Disseminate Results

The results to date do not yet provide enough data for publication. The images and data of the cultivar symptom expression response experiment are being updated and will be incorporated into a publication in the future. An initial booklet was printed and shared with a number of snap bean growers and processing industry representatives from New York in the summer of 2010 for their information.

Accomplishments/Milestones

The project has made significant progress by completing Objective 1 and by advancing towards the completion of Objective 2. Through the expansion and completion of Objective 1, the project has now characterized the symptom expression response to virus infection (CMV, BYMV, ClYVV) in a large and diverse panel of snap bean cultivars that is now even more relevant to the bean research community. The results of this characterization suggest that though none of the snap bean cultivars tested have resistance to CMV, they do vary significantly in the severity of their symptom expression. The characterization with the other viruses (BYMV and ClYVV) has lead to similar conclusions and additional hypotheses that are slightly beyond the scope of this project, though may ultimately be very informative in the larger overall research effort.
The project has also made considerable progress towards accomplishing the goals of Objective 2. All of the constraints of 2010 were successfully overcome in 2011 to allow for the seemingly successful completion of the first field-based experiment. The field experiment was planted, inoculated, maintained, and harvested successfully in line with the appropriate experimental technique, though the data collection and analysis is ongoing. One aspect however that still needs attention is the optimization of the methods to both inoculate plants with the virus as well as to prevent transmission between subplots, and thus new techniques may be tested and considered prior to repeating the experiment. Given that additional data beyond that of 2011 will need to be obtained to confirm any results, serious consideration is being given to including a controlled environment assessment of tolerance to infection, as well as the completion of another field experiment in 2012.

Impacts and Contributions/Outcomes

The project has now assembled and characterized a particularly relevant panel of snap bean germplasm for symptom expression in response to CMV infection. The characterization has demonstrated that although no snap bean cultivars are resistant to CMV, they do vary significantly in their response to virus infection and in how they express symptoms, which is valuable information for both growers and particularly those involved in serving growers through snap bean cultivar development. This valuable information will also be employed in additional future research to understand the genetics of CMV symptom response.
It is anticipated that such a diverse collection of cultivars will vary significantly for how CMV infection impacts yield. The project goals of determining this impact and in identifying snap bean cultivars that are capable of producing acceptable yield and quality when infected with CMV will eventually lead to accurate and objective information on which cultivars may be best to grow in areas that experience snap bean virus epidemics. It is hoped that such information will help to enhance the sustainability of fresh-market snap bean production in the Northeast by reducing the economic losses caused by aphid-transmitted virus epidemics.

Collaborators: