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Rust spots stop bean fill

Agronomy Issues
Rust on beans

Conditions over the past few seasons have seen rust develop to become the most dominant disease in field beans.

Control is essential when conditions are suitable for its development, with the need to keep leaves clean through to the end of pod fill, according to PGRO specialist, Dr Lea Herold.

Lea Herold PGRO

“Hot days and humid nights can see rust develop very quickly,” she warned.

“From first symptoms to full coverage of the leaf can occur in just a few days.”

In severe cases, Dr Herold reported yield loss from rust can reach 70% - the most serious of common bean diseases she outlined for growers and agronomists at this year’s on-line PGRO Syngenta Pulse Roadshow meetings.

Missed the PGRO Syngenta Roadshow 2021? Catch up with the latest on disease control here

Where conditions are cooler and wetter, however, chocolate spot is the most common disease in field beans, she pointed out.  Winter beans are more susceptible than spring sown, exacerbated by early autumn drilling and dense crops creating humid conditions where disease thrives.

Chocolate spot on beans

Growers comments at the event highlighted chocolate spot had already been seen in over wintered bean crops. PGRO trials had indicated optimum fungicide spray timing of a first application at mid-flowering, followed by a second treatment three to four weeks later, to typically coincide with an application to target rust.

New fungicide research

To better tackle rust and chocolate spot, Syngenta Technical Manager, Michael Tait, outlined a new fungicide option for legumes had been extensively trialled and was now in the registration process - with plans to introduce for the coming season if approved.  

Michael Tait

Incorporating Solatenol and prothioconazole, it had provided excellent control of both rust and chocolate spot, he reported.

The approval application was seeking one application per season in field bean and combing peas, so the agronomic decision would be to understand where it offered most in the fungicide programme, he suggested.

“Research indicates the active is most effective as a preventative application to target disease before it gets into the leaf or becomes established in the plant,” he advised.

“Particularly with rust that can develop so quickly.”

Bean rust spore and germination

Electron microscopy of bean rust spores (above, left) had demonstrated just how effectively it could prevent disease damage in the leaf even under high-risk pressure (below, right).

Bean rust infection and control

Trials have shown the product is best used as the first application in a two-spray programme, typically followed by Amistar, he advocated.

Disease watch

Dr Lea Herold also urged growers and agronomists to be on the lookout for ascochyta and cercospora infection on leaves over the growing season. The challenge is that they are virtually indistinguishable on the leaf – showing as brown lesions with white centre spots. Both diseases are more likely to occur in winter beans.

“We highly recommend that if you see theses symptoms in the field to send a sample to the PGRO Crop Clinic,” she urged.

“It is important, since ascochyta is a seed-borne disease, whilst cercospora is not.”

 She highlighted the fact that seed testing rarely identified the pathogen had showed how effective the screening process had been, but was crucial growers remained vigilant for the future.

“From an overall disease perspective, we advocate a rotational break of at least five years between field bean crops,” advised Dr Herold. “However, trials have shown that some legume species in cover crops doesn’t count to the five years,” she added.

PGRO Roadshow banner

Read more on Pest management and IPM strategies from the PGRO Syngenta Pulse Roadshow here

Recordings of the Pulse Roadshow 2021 webinars will be available on the SyngentaUKTV YouTube channel ( and the PGRO website (

Click on any of the links below to watch now:

2021 Market for Pulses

Pest Management and IPM strategies

Disease control in Field Beans

Weed Control Options in Pulses