- Fruits: berries (blueberries)
- Animal Production: parasite control
- Crop Production: application rate management
- Education and Training: demonstration, extension, on-farm/ranch research, workshop
- Pest Management: cultural control, disease vectors, field monitoring/scouting, integrated pest management, prevention, sanitation
- Production Systems: transitioning to organic
- Sustainable Communities: local and regional food systems
Blueberry is a significant fruit crop in the Southeast, with over 24,000 acres of blueberry in the SSARE region, alone. Recently, confirmation of a fatal virus, blueberry mosaic virus, has threatened blueberry in the region; the virus was confirmed in Kentucky in 2012. Infected blueberry plants cannot be cured, and they often die within 2 to 3 years after the first symptoms occur. Formerly this virus was found only in the northernmost blueberry production areas of the U.S., until 2012, when it was reported in two separate locations in Kentucky. This is the southernmost report of the virus. Little is known about blueberry mosaic virus, but it is believed to spread solely and easily by cuttings; researchers in Michigan and New Jersey have not identified insect vectors. Mechanical means such as tools do not transmit the virus, either.
Besides the unknowns facing researchers and Extension specialists in plant pathology, growers are at critical crossroads. Nursery producers rely on various nursery plants and existing field material as parent stock for propagation, and because virus symptoms can remain latent (dormant) for years, infection can be spread by cuttings. One particular Kentucky grower was affected by such a situation. More than 20 plants in this grower’s field were diagnosed with blueberry mosaic virus in 2012. Early symptoms were observed when plants were approximately 3-years-old, and they began to die within two years after symptom development. Insect infestations were not observed in plantings.
The nurseryman who produced the aforementioned plants did not observe disease symptoms in parent stock, although no records are available of the sources of those plant materials. It is unknown whether virus-infected plants were used as propagation material in this nursery. However, the nurseryman indicated that it was common practice to remove disease-symptomatic plant parts in parent stock (as opposed to discarding entire plants) before they were utilized for propagation. Upon further surveying, it became clear that nursery growers and fruit producers also had a vague and incomplete understanding of virus biology, despite outreach through traditional media outlets.
Our reports of blueberry mosaic virus in Kentucky resulted in immediate grower alarm. Many growers have indicated that they initiated aggressive, broad-spectrum insecticide programs when blueberry mosaic virus was reported in the state. Their rationale was simply that eradication of all insects would eliminate possible vectors. These growers later encouraged fellow growers to adopt similar practices.
The introduction of blueberry mosaic virus into the Southeast resulted in an increased risk for virus infection for blueberry growers. Virus spread through cuttings, whether from commercial nurseries or on-farm propagation practices, can severely impact growers several years after planting. Once infected plants are introduced to fields, it is unknown whether symptomatic plants increase risk for spread within fields. In-field spread can result in significant greater economic losses as plants succumb to the fatal virus.
Project objectives from proposal:
The objective of this project is to promote sustainable management of blueberry mosaic virus in KY by 1) understanding virus spread (or lack of) within established fields, and 2) increasing grower awareness of transmissibility risk in infected propagation materials.
Understand spread under field conditions. The research portion of this project will increase understanding of the spread of blueberry mosaic virus in Kentucky and southeastern regions of the U.S. This project will begin with molecular evaluations of the spread of blueberry mosaic virus by monitoring plants under field conditions. Latent (dormant) infection will be evaluated using quantitative real-time PCR (qPCR). This technique can detect minute amounts (1 to 10 picograms) of the virus, and infection can be determined long before symptoms develop. Detection of field spread during early growth stages can indicate plant-to-plant transmission without the need to wait 3 to 5 years for symptom development.
Regardless of whether virus spread is detected, this study can help growers use research-based recommendations to more wisely manage blueberry mosaic virus. Even though no vector has been identified to date, field spread would be a key indicator of possible spread by insect vectors. On the other hand, failure of this virus to spread through fields could indicate that the highest risk for virus transmission is through infected propagation materials, and that insect vectors play a minor role in disease epidemiology. This can also allow growers to 1) utilize insecticide sprays in a sustainable manner, as opposed to an attempt to eradicate possible vectors and 2) to make educated decisions when choosing propagation materials.
Communicate virus transmission through propagation. Blueberry growers in Kentucky currently utilize unhealthy (and sometimes virus-infected) blueberry plants as propagation materials. Current research indicates that blueberry mosaic virus is spread through grafts and cuttings. However, growers fail to understand the risk for using infected (symptomatic or asymptomatic) propagation materials. As previously mentioned, many blueberry growers have indicated that they prune symptomatic tissue from virus infected plants (not specifically blueberry mosaic virus) instead of eradicating entire plants. A brief survey determined that they sometimes use asymptomatic, infected plants as parent stock for propagation (Ward, unpublished). Demonstration plants propagated from infected bushes will serve as visual indicators of transmission, so that growers can better understand the risk of virus transmission through cuttings. Thus, plant losses from viruses, including blueberry mosaic virus, will be greatly reduced.
Expansive outreach. Growers have various preferences for receiving information, and traditional methods of information dissemination may not be utilized. By employing a wide range of outreach materials, nurserymen and production growers will become more familiar with blueberry mosaic virus. Basic virus-management concepts will be introduced, so that growers will better understand virus infection and spread. Traditional fact sheets, as well as at least one video, will serve to educate growers with visual details for virus identification and production practices. Presentations will include Kentucky Horticulture Society meetings, and then digital versions will be made available, as well. Thus, as growers better understand infection and spread of blueberry mosaic virus, they can more easily adopt sustainable practices for virus management.
Approach and Methods
Specific aims of this project are to 1) evaluate the potential spread of blueberry mosaic virus in Kentucky under field conditions and 2) demonstrate transmissibility by cuttings.
Evaluation of the spread of blueberry mosaic virus. A local blueberry farm, Rainbow Harvest Farm, was the site of the first confirmed cases of blueberry mosaic virus in Kentucky and the Southeast. The grower, John Thaman, preserved symptomatic plants after diagnosis, and more than 20 symptomatic plants (cultivar ‘Bluecrop’) are available for the purpose of this project. One project component will monitor spread of blueberry mosaic virus under field conditions in 2013 and 2014. This spring, ten symptomatic blueberry plants from across the 5-acre field will be selected as hot spots. Next, three noninfected blueberry plants adjacent to each infected target hot spot will be designated as test plants for evaluations throughout the study.
Test plants will be analyzed again during autumn 2013, spring 2014, and fall 2014 in order to determine whether infection in neighboring (test) plants occurred during the experimental period. Identification of virus spread may indicate that insect vectors are present. Throughout the experiment, insect traps will be positioned and collected biweekly at each hot spot location. Traps will be labeled and stored. In the event that virus spread is detected in the field, insect captures and concentrations will be analyzed under the supervision of entomologist Dr. Ricardo Bessin. During each of the sample periods, five shoots will be selected from each test plant; those with symptoms suggestive of blueberry mosaic virus will be included. Shoots from each plant will be pooled for analysis by qPCR, but individual plants will be tested separately. Latent (dormant) infection will be monitored using quantitative real-time PCR (qPCR). This molecular technique allows minute amounts (1 to 10 picograms) of the pathogen to be detected, even if symptoms have not yet developed. Plant material will be ground using liquid nitrogen, and then DNA will be extracted using standard protocols. Primers and a probe specific to the single-stranded RNA virus will be used to amplify sequences through reverse transcription PCR (Polashock, unpublished). This protocol was developed by J. Polashock and collaborators at USDA-ARS GIFVL in New Jersey, and a memorandum of understanding will be established in order to proceed with experiments at the University of Kentucky.
Multiple asymptomatic blueberry plants that were located adjacent to symptomatic plants were tested in 2012 in a preliminary survey. All tested asymptomatic plants were confirmed to be noninfected. There was no indication of in-field spread at this time. However, it is possible that as virus titer (concentration) increases in infected plants, field spread can increase, as well. Thus, field spread, if it occurs, it may do so at this stage of infection/symptom development.
Detection of latent infection can indicate delayed disease development and serve as an advanced indication of infection. Regardless of whether in-field virus spread is detected, this study can help growers use research-based recommendations to reduce infection by blueberry mosaic virus. Field spread could be a key indicator of possible insect vectors, although no vector has been identified to date and none are suspected to transmit it. On the other hand, failure of this virus to spread through fields may indicate that the highest risk for virus transmission is through infected propagation materials, and that insect vectors are not present in the field. This will also allow growers to utilize insecticide sprays in a sustainable manner, as opposed to applications that have little to no effect on the epidemiology of disease progress. Likewise, growers will be better able to make educated decisions when choosing propagation materials. Overall, a better understanding of virus spread can allow growers to make effective, sustainable virus-management decisions.
Monitor Symptom Development and Demonstrate Transmission by Cuttings. Infected plants will be identified at Rainbow Harvest Farm and utilized for propagation of demonstration plants. At least 20 softwood cuttings from each 1) symptomatic, 2) asymptomatic infected, and 3) noninfected healthy plants will be grown and maintained in a greenhouse at the University of Kentucky. A separate noninfected control will also be obtained from a separate source. Plants will be maintained in insect-proof cages to assure that insect vector transmission of blueberry mosaic virus does not occur. No other blueberry plants will be located in the facility, and no commercial blueberry fields are located for several miles from the university campus. Plants will be maintained for at least two years, and time of symptom development will be recorded. Through the course of this experiment, insect sticky traps will be positioned in each cage, collected biweekly, and stored in the event that results suggest vector transmission. Latent virus infection will be monitored using qPCR as described above. It is possible that symptom development of infected plants (positive results by qPCR) may be delayed for longer than the timeline of this experiment; plants will then be maintained for several years in order to determine time required for symptom development. Photographic documentation of symptom development will supplement quantitative records of virus infection.
Demonstration plants propagated from infected blueberry will serve as visual indicators of transmission by cuttings. Upon symptom development, these plants will be utilized for demonstration purposes at meetings and field days. See outreach section below for details.