What options do growers have to manage this disease?
by Trudy Kelly Forsythe
As global faba bean consumption grows, and work on finding domestic and export markets for Canadian faba beans continues, producers are naturally interested in getting the best return for their investment. One of the areas under examination is the economic value of managing chocolate spot.
Chocolate spot is a Botrytis disease species that thrives under warm, humid conditions. Jessica Pratchler, Research Manager with the Northeast Agriculture Research Foundation in Saskatchewan, says if the weather is conducive to the disease it can spread rapidly, especially after heavy rains.
“If it develops early, it reduces the amount of photosynthetic area by causing reddish brown flecks or spots on the leaves and can cause seed staining if severe,” she adds. “In the last two years, the disease has not been at elevated levels until later July and early August. At this time, the plant is starting to naturally drop its leaves, so the disease has not been severe. Also, a little disease may be beneficial to help the plant reach maturity.”
One of the best ways to manage chocolate spot in faba beans is to use quality seed and seed treatment. There are currently no disease-resistant varieties of faba bean seeds available for Canadian producers at this time, however, research and breeding is underway, says Kim Stonehouse, a Regional Crops Specialist with the Saskatchewan Ministry of Agriculture.
Stonehouse also recommends testing seed and not using seed with infection levels above 10 per cent. He explains this value has been extrapolated from Botrytis in lentils since the critical level is currently unknown in faba beans. To aid in disease management, producers should not exceed the recommended seeding rates since denser canopies are able to promote disease development under the right conditions.
A general recommendation for planting pulses is to pay attention to field selection and maintain a four-year rotation between pulse crops. However, it should be noted that chocolate spot has been observed in fields that have never had faba beans.
“Scouting and correct identification of chocolate spot is currently the best estimate of the damage potential and successful management depends on immediate fungicide application when the disease is noted and environmental conditions are conducive to disease development,” says Stonehouse. “However, we do not have a really good idea of how much disease must be present.”
While fungicide application is the best option for controlling chocolate spot in faba beans, economic thresholds and optimal application timing are currently unknown. The current recommendation is for fungicide application at early and/or at late flowering. However, the recommended timing may not be the most economical depending on the environment and disease pressure, says Pratchler.
Stonehouse adds that depending on when the infection occurs, chocolate spot can be yield limiting or affect quality of the seeds. “Therefore, fungicide application timing will depend on which of these factors are deemed to be the most threatened and economically damaging.”
Pratchler is currently doing her Master of Science project on Optimal Fungicide Products and Application Timing for Disease Control in Faba Bean Varieties. This research was conducted in 2015 and 2016 in seven different locations across Saskatchewan from more typical areas (northeast) to non-typical (southwest). To date, her research has revealed a mixed bag of responses to fungicide application.
“At some sites, we did see a variety response with CDC Snowdrop having slightly more disease than CDC SSNS-1,” she says. “At other sites, fungicide application at 50 per cent flowering reduced disease severity.”
However, with CDC Snowdrop, the researchers found that Propulse® applied at 50 per cent flowering also resulted in a significant yield increase of approximately100 kilograms per hectare, at four of 14 site years. “So, to summarize, the benefits provided by fungicide application for the control
of chocolate spot and faba bean diseases, is highly dependent on the environment,” says Pratchler.
Pratchler says there is another year of data to collect of the Optimal Fungicide Products and Application Timing for Disease Control in Faba Bean Varieties trials. “Ideally, I would like to research the effects of intercropping with cereals, rotational effects, and further optimal fungicide application timing work to determine the effects these variables have on disease management,” she adds.