Chocolate spot on faba bean a minor concern
Fungicides worked, but weren’t economical.
April 12, 2023 By Bruce Barker
Chocolate spot on faba bean can leave a bad taste in your mouth, but research has found that even though the disease is frequently seen in Alberta and Saskatchewan, yield losses haven’t been significant. And, while research has found that fungicides do help to reduce disease levels, fungicide application hasn’t proven to be economical.
“We did five years of fungicide research and really didn’t feel any reason to continue with any further research on the disease,” says Robyne Davidson, research scientist with Lakeland College at Vermilion, Alta.
The research was conducted at Falher, Edmonton, Lacombe and under irrigation at Lethbridge, Alta., from 2016 through 2020. Six fungicides were compared including Lance (boscalid; Group 7), Acapela (picoxystobin; Group 11), Vertisan/Fontelis (penthiopyrad; Group 7), Priaxor (fluxapyroxad + pyraclostrobin; Group 7 + 11), Headline (pyraclostrobin; Group 11), and Delaro (prothioconazole+ trifloxystrobin; Group 3 + 11). Application was at the early signs of chocolate spot or at mid-flower at recommended label rates.
The fungicides targeted chocolate spot and ascochyta diseases. Chocolate spot is caused by two pathogens infecting faba bean, including Botrytis fabae or Botrytis cinerea. Typically, both pathogens are found across Alberta and Saskatchewan, but usually causing only low levels of the disease. Botrytis cinerea is of minor concern in terms of impact on faba bean on the Prairies, but B. fabae can be more problematic. Botrytis fabae is a serious disease in Europe and Australia where the growing season is longer, and is the pathogen of most concern on the Prairies. Symptoms of chocolate spot appear first as small, brown coloured lesions on faba bean leaves, but expand and merge into larger spots. Under severe infestation, the whole leaf may turn brown. Larger spots may eventually turn gray, and look like botrytis or gray mould in other crops. Davidson’s research covered both wet and dry years. In dry years of 2017 and 2018, the disease levels were low, and so the response to fungicide application was small. In the wetter years of 2016, 2019 and 2020, disease levels were higher, and fungicide application lowered disease levels slightly. All the fungicides worked, with no product being superior to others. Fungicide application did increase yield, but the increase wasn’t large enough to provide a payback.
“The disease will show up in a wet year, but if you’re selling faba bean to the elevator for around $8 a bushel, you won’t likely see an economic benefit of fungicide application,” says Davidson. “But, if you’re growing faba bean for seed under irrigation and selling the seed for $20, you might see an economic benefit.”
For the time being, Davidson’s research program is looking at lupin agronomy, with faba bean agronomy on the back burner. The drop in Alberta faba bean acreage from 120,000 acres to 30,000 acres has also meant less emphasis on faba bean. Going forward, she will continue to monitor chocolate spot infestations, and if the market or disease pressure changes, more focus could be put on faba bean.
“We have the growing issue of Aphanomyces root rot in pea and lentil. If farmers can’t grow those crops, they are looking to lupin and faba bean as possible alternatives, so those are the crops we continue to work with,” says Davidson.
In the meantime, for faba bean growers, Davidson says fungicide application is not necessary – for now. “This can and may change, and there are always exceptions.”