What was it all about?

This investment ran from 2016 to 2019 to characterise a new carlavirus found infecting Fabaceae crops in South East Queensland. The carlavirus called cowpea mild mottle virus (CPMMV) was first detected in Australia on bean and soybean crops in south-east Queensland in 2016, causing potential economic loss to the state’s $70 million green bean industry.

The research team sought to investigate this new virus to understand how it might spread and to develop and help growers adopt management strategies for the virus, resulting in improved pack-out, increased marketable yield and a reduction in the impact of the disease.

The majority of host species for CPMMV are legumes (family Fabaceae) with French bean, soybean and cowpea being most affected. Mung bean, asparagus bean, adzuki bean, lima bean, and Fabaceae weed species are also susceptible.

Symptoms include mottled leaves, discolouration, and poorly developed, deformed pods. The virus is transmitted from plant to plant by the silverleaf whitefly, with early inoculation time found to have a very significant effect on pod yield and quality.

Surveys in the Fassifern Valley, Queensland indicated that bean crops grown during windows over the spring-summer season are less likely to be affected by CPMMV.

CPMMV has not been found outside of Queensland and would only be a potential problem where the specific vector, silverleaf whitefly, is established. It can transmit the virus in relatively short feeding periods of about 10 minutes, with the Fassifern outbreaks demonstrating how quickly an epidemic can develop when whitefly populations are high, a virus source is available and susceptible bean plants are at a high risk growth stage.

Recommendations for growers

The research team recommend the following management tools for growers:

• Purchase bean seed of a known variety from a reputable supplier
• Where possible, avoid planting bean crops adjacent to crops of soybean, cowpea and mung bean
• Monitor crops for whitefly weekly. Apply registered insecticides and/or biocontrol agents as appropriate for whitefly control. Only low to moderate whitefly populations are needed to result in considerable virus spread if virus sources are available; these populations are less than those that would normally result in crop damage.
• Crop damage through yield and quality reductions is likely to be greater when virus infection occurs in young crops. Disease close to harvest is unlikely to cause major damage.
• Destroy crops as soon as practicable after harvest, especially if virus infection is suspected. If whitefly are present, spray with an oil prior to destroying the crop as this will limit movement of whitefly to other crops.
• Plant virus tolerant varieties in high risk periods or locations, selecting them based on performance and market requirement
• An alternative or complementary strategy is to plant a proportion of other crops, such as pumpkin which is a preferred host for whitefly and should limit movement into bean crops. Pumpkin is not a host of CPMMV and would have a positive impact of reducing virus spread into bean crops, particularly during high risk windows.

ACT NOW

Learn more with these materials developed by the project team:

• Read more in the Jan/Feb 2019 VegeNotes from AusVeg
• View a presentation about the team’s work on CPMMV, shared during the project
• Learn more with the Fact Sheet: Bean Carlavirus, Cowpea mild mottle virus shared by the Department of Agriculture and Fisheries, Queensland.