Australian Mungbean Association

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Disease management in mungbean

Varietal disease resistance

Powdery mildew

Powdery mildew on mungbean is caused by the fungus Podosphaera fusca (also known as P. xanthii).

Symptoms: Infected plants have a greyish-white powdery growth on the surface of leaves, stems and pods. Late infections during the pod filling stage can cause leaf drop but do not appear to seriously affect yield.

Conditions favouring development: Favoured by cooler conditions, and is often widespread in late-planted crops. The fungus survives on other living hosts, and is spread by wind. Infection typically becomes apparent during February, and usually increases in severity during March and April. Significant yield loss can occur if powdery mildew develops before or at flowering, particularly if the crop is under moisture stress.

Management: Jade-AU and Green Diamond offer the best  levels of resistance to powdery mildew, and are the preferred varieties for late plantings sown after the end of December. While mildew is slower to develop on these varieties, the disease can still be quite damaging if it occurs prior to, or at flowering.

Foliar sprays with a registered fungicide, or one under permit, is warranted where infection occurs at or prior to flowering. DAF trials have shown that fungicide sprays, when applied early, can result in up to a 30% yield increase. Recent research indicates that spraying at the first sign of disease and then again 14 days later, if needed, is often the most effective management strategy. High water rates and thorough coverage are critical. Some of these are protectant fungicides, and must be applied during the early stages of disease development to be effective. Spraying late in the afternoon or at night will help minimise any leaf burn. Consult an agronomist about spray options and timing.

 

Tan spot

Tan spot is caused by the bacterium Curtobacterium flaccumfaciens and is usually present at a low level in most crops. It can cause widespread and serious losses.

Symptoms: Infected plants have large, irregular, dry, papery lesions on leaves that coalesce to form large brown dead areas, commonly with yellow margins around the dead areas. These areas usually tear and fall out, giving the leaf a ragged appearance. Infected flowers usually die. Early infection results in stunting, yellowing, and poor set seed.

Conditions favouring development: Tan spot is seed borne, and while it may develop in the seedling stage, the disease is more commonly seen from the second trifoliate leaf stage onwards. The bacterium is spread from infected seedlings to other plants in the crop by wind-blown rain (particularly hail) and mechanical damage (machinery and abrasion from dust storms).

Symptoms develop rapidly if the crop is subjected to adverse growing conditions, such as heat or moisture stress.

Management: While it is not possible to totally eradicate this disease, using low risk seed and practising good crop rotation will minimise its impact. Alternate hosts, such as cowvine and bellvine, must be controlled. Cowpea and soybean also host the disease.

While seed lines are often inspected for the disease, it is not possible to guarantee total freedom from the disease. Infected plants may not develop symptoms under favourable growing conditions, and consequently the disease can go undetected. Crystal, Jade and Satin II have the highest levels of resistance to tan spot.

 

Halo blight

Halo blight is a seed-borne bacterial disease caused by Pseudomonas savastanoi pv. phaseolicola. Halo blight has caused significant losses in all areas and can be particularly severe in spring-sown crops.

Symptoms: On young leaves there is an extensive yellow halo surrounding a smaller (1–2 mm) area of brown, shiny tissue. The halo may be less pronounced on older leaves and lesions may appear similar to bacterial blight (Xanthomonas sp.). Circular watersoaked spots develop on pods.

Conditions favouring development: Symptoms develop during cool, humid conditions following rain, most notably in spring crops. Significant yield losses can result if the disease develops before flowering or during pod development.

Management: Planting low risk seed is the most effective control measure. Rotation with non-hosts (summer and winter cereal crops and cotton) and incorporation of residue will be effective only if seed-borne transmission is minimised.

 

Charcoal rot

Soil-borne infection by the charcoal rot fungus Macrophomina phaseolina ultimately causes a dry rot of the stem and often plant death. Charcoal rot infection of seed intended for the sprouting market results in a soft, wet rot of the sprouts during the germination process. Many of our overseas sprouting markets specify that seed lots must be charcoal rot-free.

Symptoms: An early sign of stem infection is the presence of brown lesions at the base of the plant and/or where the branches join the main stem. Infected plants usually die prematurely, and the stems turn ashy-grey, often with minute black specks (microsclerotia) evident within the affected area. Yield can be significantly reduced.

Conditions favouring development: Charcoal rot generally occurs after flowering during a period of heat and/or moisture stress, and results from infection of the roots by soil-borne microsclerotia.

By contrast, evidence suggests that seed infection occurs during rain periods, when microsclerotia are splashed from the soil surface onto developing pods. The disease can be particularly severe after sorghum, but stress at, and after, flowering is the key driver.

Management: Avoiding paddocks where charcoal rot has been a problem in the past, planting on good soil moisture, and practicing agronomic practices to minimise stress, can reduce the risk from charcoal rot.

 

Fusarium wilt and root rot

Fusarium wilt and root rot is caused by the fungi Fusarium oxysporum and F. solani. The disease has caused significant losses in affected paddocks.

Symptoms: Infected plants often wilt and die. There is often a basal rot present and the roots are rotted. Surviving plants remain stunted and leaves defoliate from infected plants, often leaving behind only the plant stem. The internal vascular tissues inside the basal stem turn brown.

Conditions favouring development: Fusarium wilt often occurs after periods of stress, in particular excess moisture. The pathogens are soil-borne and will survive in the soil for many years.

Management: Avoid paddocks with a history of Fusarium wilt, and those prone to waterlogging. Agronomic practices that minimise stress will help to reduce the risk of developing the disease.

 

Gummy pod

This bacterial disorder (Gluconobacter spp.) occurs following the fermentation of sugars secreted by the floral nectaries on the mungbean plant.

Symptoms: Stems, pods and pod-stalks become covered with a sticky white froth that exudes from the nectaries. This can be followed by collapse of the stalks supporting the pods, and then pod drop.

Conditions favouring development: While the bacterium is commonly found in mungbean flowers, the production of froth occurs when crops are severely heat and moisture stressed. It tends to be a more widespread and serious problem in the drier western areas.

Management: There are no practical in-crop control measures. Regular cleaning of harvest equipment to remove froth build-up is often required. Crop desiccation will often help reduce harvest difficulties.

 

Puffy pod disorder

Symptoms: Pods develop a blotched, puffy appearance. In the most severe cases, up to 50% of the pods within affected crops may display symptoms of the disorder. Seeds in infected pods do not mature properly, often turn brown, and may develop secondary rots.

Conditions favouring development: This disorder appears to be due to a physiological stress (most likely moisture stress) because no pathogens have been isolated from affected plants.

Management: If the level of puffy pod in a crop is significant, harvest affected pods before they mature. Desiccate promptly, and use a low drum speed and high air flow to eject un-threshed puffy pods out of the header.

 

Legume little leaf phytoplasma

Symptoms: Affected plants develop a spindly, erect growth habit with small, ‘cupped’ leaves. Flowers are distorted, with green petals. If pods develop they are usually distorted, and seeds in these pods either fail to develop or turn brown. This discoloration can be a major cause for downgrading of the sample and discounted returns to growers.

Conditions favouring development: The disease, caused by a phytoplasma (a specialised bacteria), is spread by leafhoppers, which have a wide host range on other crops and weeds. Dry seasonal conditions promote the movement of leafhoppers from infected weeds to mungbean crops.

Management: Control is generally not warranted, but it is recommended to monitor leafhopper numbers.

 

Tobacco streak virus

Tobacco streak virus (TSV) was identified in 2006 in mungbean crops in the Central Highlands region of Queensland. The impact ranged from minor to severe. TSV has not yet been found in other mungbean growing areas.

Symptoms: TSV-infected mungbean plants are usually stunted and wilted with dead (necrotic) stems and tips. Yellowing of leaves followed by spreading necrosis is common, sometimes with necrotic line patterns. Plants that are infected early have the worst symptoms and often die prematurely. Major weed hosts of TSV show no symptoms.

Conditions favouring development: Thrips are the only known vector of TSV through the transmission of virus-infected pollen. Several common broadleaf weed species are hosts of TSV, with parthenium weed being a widespread and key host of the virus in central Queensland. The physical damage caused by thrips feeding on the plant enables infected pollen to enter the mungbean plant where the virus multiplies.

Management: While it is difficult to predict the impact of TSV in mungbean, there are strategies to minimise the risk of infection:

  • maintain farm hygiene, in particular the control of parthenium weed
  • avoid planting mungbeans close to pastures, particularly those containing dense infestations of parthenium and other broadleaf weeds
  • if crops are planted close to parthenium infested pastures, either spray out pasture close to the mungbean crop or leave a buffer
  • avoid planting mungbeans into marginal soil moisture, especially near parthenium infested pasture
  • TSV is potentially seed-borne, so ensure planting seed is from crops with no signs of TSV.

For more information: Managing TSV risk in Mungbean crops

 

Fungicides commonly used in mungbean

Use only registered fungicides to manage fungal diseases in mungbean.

Powdery mildew.

Tan spot.

Halo blight on leaves.

Halo blight on pods.

Legume little leaf phytoplasma on mungbean.

Tan spot on seedlings.

TSV affecting leaves and tips.

TSV affecting leaves and pods.

Fusarium wilt affected plant.

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