Mungbeans vs fungus: two sprays for optimum control

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Key points

  • Applying Folicur 430SC® (active ingredient tebuconazole 430 grams active ingredient per litre) to mungbeans infected with powdery mildew can substantially boost yield and gross margins
  • One spray when the fungus is first detected will help boost yield, but the best balance between application cost and returns is struck when a second spray is applied 14 days after the first, if infection occurs early
  • The value of treating powdery mildew varies depending on the region, timing of infection and season. Incidence and severity will be determined by weather conditions – cooler, humid conditions favour the disease 

When it comes to protecting mungbeans from powdery mildew, one well-timed fungicide spray is valuable, but two can be even better.

Photo of powdery mildew on mungbeans

Severe powdery mildew on mungbeans.

PHOTO: Cox Inall Communications

That’s the finding of a research project across several Queensland research facilities, which looked at the most cost-effective option for limiting the damage Podosphaera fusca (P. fusca) can wreak on a crop.

If powdery mildew infects the crop early and two sprays of tebuconazole (for which there is a permit until 30 June 2017) are applied 14 days apart after the first sign of infection, the study found that net returns on a mungbean crop increased by more than $400 per hectare over an unsprayed crop.

The investigation was led by Sue Thompson from the Centre for Crop Health at the University of Southern Queensland, who, with Queensland Department of Agriculture and Fisheries colleagues, monitored different powdery mildew treatments at three research facilities over two years. “Effective management of mungbean powdery mildew relies on the use of varieties with the highest possible levels of resistance and on the strategic application of fungicides,” Ms Thompson says.

“The variety Jade-AU has the highest level of resistance to P. fusca (moderately susceptible, MS) of the green shiny varieties, with all other Australian varieties apart from Green Diamond being susceptible (S) or highly susceptible (HS).

“As it’s unlikely that significant gains in breeding for resistance to the powdery mildew pathogen will be made in the near future, the targeted use of fungicides is vital to minimising the disease’s impact.”

It is important to note that fungicides containing tebuconazole are currently under Australian Pesticides and Veterinary Medicines Authority permit (permit number 13979, expires June 2017) and is valid only in Queensland and New South Wales for the control of mungbean powdery mildew.

Substantial losses

Nationally, powdery mildew yield losses have been up to 40 per cent in susceptible varieties. Plants are vulnerable from seedling stage onwards, with the first sign of infection being small, circular, white, powdery patches on lower leaves. The fungal disease can develop rapidly on individual leaves and also on mungbean stems and pods.

In Australia, mungbean powdery mildew needs a living host for survival from one mungbean season or crop to the next. Volunteer mungbean plants and possibly other legumes are sources of infection. There is no evidence that the pathogen can survive on plant residues or seeds, or in the soil. Infection occurs when spores produced in the patches of white, powdery fungal threads become airborne, spread in the wind and land on leaves. The cycle of infection can be as short as five to seven days if conditions are favourable.

Management of mungbean powdery mildew relies on planting varieties with the highest levels of tolerance, as well as the strategic application of fungicides. In Queensland, these GRDC-funded trials have investigated the optimum timing of Folicur 430SC® applications using the label rate.

Early detection

In the 2016 trial at the Hermitage Research Facility, the fungus was detected very early, at 24 days after crop emergence. Sixty-three days after emergence, the infestation on untreated plants rated 8 on a severity rating ranging from 0 to 9.

In Kingaroy, the fungus progressed to a mean severity rating of 8.3. In drier Emerald, the mean severity rating only reached 4.5 after the plants matured in unseasonably hot conditions. A fungicide applied when the fungus first appeared checked powdery mildew development at all sites, but with no further sprays, infection generally ramped up to reach similar levels to control plots where no spray was applied.

The most effective treatments all involved two or three sprays, even when the fungicide treatments were applied according to different criteria.

At Hermitage in 2016, virtually the same yield increase came from applying the first spray when the fungus had spread one-third of the way up the plant or at the first sign of infection, with another treatment 14 days after the first. Disease increase was very rapid and reached one-third of the way up the canopy prior to flowering.

In 2015, when powdery mildew came into the trials later and with less intensity, holding off the fungicide application until one-third of the canopy was infected resulted in a lower percentage yield increase than the treatments applied at first sign.

This reinforces the importance of monitoring mungbean crops for the first sign of the disease and applying the first spray at that time. It appears that even if first sign is at the flowering or early pod-fill crop stage, some yield increases will result.

There was a trend of yield increase at all sites by the application of Folicur 430SC®, although at Kingaroy and Emerald the increases were not statistically different.

At Hermitage, the researchers found that the cost-versus-benefits of the two-spray approach were always unequivocally in favour of treating powdery mildew.

Assuming seed costs of $1000 per tonne, and costs of $20 per hectare per spray, the two-spray method delivered returns $412 to $439 better than those from unsprayed plots in a year when powdery mildew infected early.

Returns outweigh costs

Seasonal conditions will vary results, but the researchers concluded that in general, the returns from applying tebuconazole far outweigh the costs, particularly in more humid areas and when crops are maturing into cooler conditions.

“The key to powdery mildew control is spray at first sight and continue monitoring – at least weekly – and then, keeping in mind environmental conditions and crop stage, spray again,” Ms Thompson says.

“Fungicide is cheap and effective and can make a significance difference to yield.”

She says while her research team determined that a single spray when the fungus first appears is useful in boosting yield, a second spray 14 days later produces the optimum balance between cost and return.

Photo of Sue Thompson checking mungbeans for powdery mildew

University of Southern Queensland researcher Sue Thompson checks mungbeans for powdery mildew at the Hermitage Research Station.

PHOTO: Cox Inall Communications

More information:

Sue Thompson, Centre for Crop Health, University of Southern Queensland
0477 718 593
sue.thompson@usq.edu.au

Useful resources:

Fungicide management of mungbean powdery mildew

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GRDC Project Code DAQ00186, DAQ00172

Region North, South