Despite the loss of chlorothalonil (CTL), growers still have effective options for controlling chocolate spot, one of two dominant diseases in beans. Capable of completely defoliating crops and causing yield losses of 50% or more, it is a potentially margin-sapping disease.
Two dry springs have kept chocolate spot pressure low in recent years.
Yet experts are reminding growers not to become complacent and to check crops regularly as treatments are preventative rather than curative.
Dr Lea Herold, plant pathologist at the Processors and Growers Research Organisation (PGRO), says: “Chocolate spot [Botrytis cinerea, Botrytis fabae] is the earlier of the two dominant diseases in beans, the other is rust.
“In winter beans, chocolate spot is generally seen from early May onwards, in spring beans it’s a little later.
It favours cool and wet conditions.
The earlier infections in winter varieties are due to the earlier sowings, the denser canopies and microclimates they produce creating ideal conditions for the disease.
“In some years, like this one, chocolate spot can occur much earlier; we have had reports already in some of the winter bean crops from this year.
While in February and March control is very rarely justified – most crops will outgrow the infection and it is unlikely to impact yield.
It’s important to note that there are already inoculum sources around the country,” Dr Herold adds.
She encourages growers to keep a close eye on crops by crop walking and monitoring the forecast.
“Recently we’ve had such dry springs that chocolate spot didn’t develop and applications probably weren’t economical.
But there’s no guarantee we’ll get the same conditions this year and with predominantly preventative tools in the toolkit, if you leave it unchecked and conditions are suitable for disease development, you could be facing significant yield losses.
“Chocolate spot survives on crop debris from previous years.
The first symptoms are small, brown, circular spots, which can coalesce to form greyer coloured lesions that extend over the leaf surface.
Stems and pods can also develop a covering of spots or flecks.
Infections tend to start at the bottom of the crop, so if newer leaves have pustules, it is time to get applications on,” she says.
Will Foster, independent agronomist based in Lincolnshire, recognises the impact that losing CTL will have for growers.
“Up to now, CTL had been a cheap and effective option.
For some winter bean growers, this may be the first season without the chemistry.
With product in stock, and a May 20 cut-off date for applications, some winter bean crops potentially had a CTL-based treatment in 2020.” BASF business development manager Iain Ford agrees: “Losing CTL has had a big impact.
It’s never been the only active but it’s been a useful way of keeping on top of disease.
It offered growers a different mode of action, it was effective and relatively cheap.” But growers still have options, notes Mr Foster.
“The mainstays are Signum, tebuconazole, azoxystrobin and metconazole.
Most tebuconazole and azoxystrobin products are generics so they are cheaper than Signum, which is a premium product.
Signum uses SDHI and strobilurin chemistry, differing from the triazoles and offers help in resistance management.
“If you’re also looking to apply a pyrethroid, you can also use Signum at flowering, which you couldn’t with a triazole as you’d be increasing the risk of damaging bees,” says Mr Foster.
PGRO trials, the greatest yield responses from applications have come from chocolate spot applications at mid-flower, when the first pods are developing in the crop.
“Sometimes in winter beans if infection is very high – and only if infection is early and very high – then an application at T0, for example at early flowering, could be justified,” says Dr Herold.
“As a guide, ‘very high’ is where you have more than 10% leaf area infected.
The problem is the products are mainly preventative and early curative, meaning you can’t wait until there’s 30% infection.
It’s just too risky.
It’s why we say if chocolate spot is visible and the forecast is for cool, yet humid weather, don’t risk it, apply it.” Mr Foster agrees: “Get in early at the first signs of disease at robust rates to ensure it doesn’t get established.
Beans can get quite a dense canopy, so to get best coverage through the canopy and reach those lower leaves, keep speeds down and water volumes up.
In PGRO trials conducted in 2014, a year in which chocolate spot pressure was high, Signum gave the best yield response – up 17% in comparison with the untreated crop.
In comparison, Alto Elite (chlorothalonil + cyproconazole) gave only a 3.5% yield uplift versus the untreated.
“The yield uplift is partly due to the absence of disease, but it’s also a result of the greening effect of the pyraclostrobin,” explains Dr Herold.
Last year BASF took the trials a step further.
Conducted by PGRO and OAT, the two trials sought to assess the yield uplift and return on investment.
Two applications of Signum were compared with tebuconazole plus azoxystrobin treatments in commercial crops of Tundra and Vertigo.
Signum was applied at mid- to late-flowering and again at the end of flowering to the beginning of ripening.
Mr Ford says: “The mean, untreated yield of the trials was 4.2 tonnes per hectare.
The mean yield of the azoxystrobin plus tebuconazole programmes was 4.8t/ha and the mean yield from the Signum programmes was 5.1t/ha.
“At market prices, the cost of Signum more than paid for itself, giving a margin over fungicide costs of more than £100, £30/ha greater than a standard programme of azoxystrobin plus tebuconazole.
This was a noteworthy result in a low chocolate spot, high rust year.
“Signum is widely recognised for being the best product for chocolate spot control, but this trial also showed how it offers good protection from rust too.
“Whichever disease it is preventing, by keeping crops greener for longer and maximising the time that they are photosynthesising, Signum protects yields,” he explains.
After two dry springs with little chocolate spot disease pressure, Mr Foster warns growers not to get complacent.
“A sensible strategy in beans is always a two-spray programme with a robust programme at earlyto mid-flowering, followed by another three to four weeks later.” “Conditions so far this year have met the brief for ‘cooler and wetter’,” adds Mr Ford.
“We had a really cold snap in February which will have checked any chocolate spot already in crops, but is unlikely to have killed the disease completely.
March has been cooler and wetter with largely overcast conditions, making it ideal for the disease’s development,” Mr Foster says.
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