Characterisation of a Carlavirus of French Bean
A sudden and severe outbreak of a virus disease in fresh green beans in the Fassifern area of south east Queensland in the autumn of 2016 caused considerable economic impact in a matter of a few months. The disease caused leaf mottling and, importantly, distortion of pods, resulting in considerable losses through yield reductions and downgrading of product.
Preliminary work identified the pathogen as a Carlavirus and a member of the Cowpea mild mottle virus (CPMMV) section of this virus group. Carlaviruses have not previously been identified from legumes in Australia. The detection of this virus at a high incidence in one important bean production area was significant and a potential economic threat to bean production in Queensland and other States. Green bean production in Australia has an annual value of $74 million with Queensland producing approximately 80% of the crop. The major Queensland production areas are spring to autumn crops in the Lockyer and Fassifern valleys in south Queensland and winter production in the BowenBurdekin area of north Queensland. Project VG 15073 was initiated to determine the cause, nature and management of the disease caused by this Carlavirus.
Project VG15073 met all four of the contracted outcomes:
- Identify and characterise a new Carlavirus identified from beans crops in Queensland
- Identify the potential distribution and incidence of the virus in other French bean production areas of Australia
- Develop management strategies for the virus in and modify these as further information becomes available on virus host range, epidemiology, transmission and bean varietal reactions
- Provide information on the virus to growers, the vegetable industry, biosecurity agencies and plant virologists
CPMMV occurs in all Queensland bean production areas, including the Lockyer and Fassifern valleys, Bowen and Bundaberg. Disease incidence recorded during surveys in these areas from 2016 to 2019 were usually less than 5%.
The exceptions were levels exceeding 50% in some crops in the Fassifern area growing during the autumn production period from March onward in 2016 and 2019. A consistent result from intensive surveying in this district over four years was no virus detections in bean crops grown from spring through to December. By contrast, virus did occur each year in crops growing into and during the autumn-early winter period, with low incidences in 2015 and 2017 and very high incidences in 2016 and 2019 Whitefly numbers varied throughout the district by location and potentially management practices. The numbers also varied by season and year with the highest populations on French bean generally observed during autumn, particularly in 2019. CPMMV has not been found outside of Queensland and would only be a potential problem where the specific vector silver leaf whitefly (SLW) is established.
SLW can transmit the virus in relatively short feeding periods of about 10 minutes and the Fassifern outbreaks have demonstrated how quickly an epidemic can develop when whitefly populations are high, a source of virus is available and bean plants of a susceptible variety are at a high risk stage of growth. Most host plants of CPMMV are in the legume family. In Queensland, the known field hosts are bean, soybean, mung bean, cowpea and the perennial legume species Siratro, Glycine and Phasey bean.
Glasshouse inoculation experiments have found that CPMMV has a relatively broad host range within the legume family, particularly in the genera Phaseolus, Glycine, Macroptilium and Vigna.
Some strains of CPMMV are known to be seed transmitted in other countries. However, extensive testing of commercial seed lots of bean, soybean and mung bean failed to demonstrate seed transmission. Grow out tests using seed produced from CPMMV inoculated bean and soybean plants also failed to detect seed transmission. Lack of virus detection in seedlings during intensive surveys of commercial bean crops in the Fassifern also indicate bean seed was not a pathway for virus introduction into these crops.
The major green bean varieties currently used are highly susceptible to the virus. Thirty four bean varieties were assessed for virus tolerance and agronomic characters in three field trials. Of these 18 were ranked as virus tolerant, with mild or no leaf or pod symptoms. Five of these varieties were selected for more extensive testing in several areas. These virus tolerant varieties are an important component of a management plan to reduce the economic impact of CPMMV in green beans.