Soil nematode test breaks new ground

Soil nematode test breaks new ground

The PGRO has recently developed a new molecular test to help growers tackle stem and bulb nematode in beans.

In a first for UK growers, the novel test can differentiate between nematode species in the soil to inform better rotational choices.

Dr Becky Howard, PGRO research and development manager, said: “Stem and bulb nematodes are one of the most important problems in both spring and winter field beans.

“The PGRO has provided laboratory seed tests for this pest for many years but up until now there has been no way for growers to quickly determine the species in soil samples before planting.

“The new molecular test represents a great step forward to help growers improve crop performance and reduce the presence of the pest in rotations.”

Infestation

“Stem and bulb nematodes are both seed and soil-borne and can result in significant yield losses for bean growers,” Dr Howard explained.

“The principal routes of introduction are from the use of infested seed and bulbs. But the pest can then remain viable in the soil for up to 10 years placing future crops of beans at risk from damage.”

Affected plants may appear in isolation or in larger patches of the field with damage often first seen as the plants reach the flowering stage.

Symptoms include:

• Stunted plants with thickened and twisted stems
• Thickened and brittle leaves with bronze discoloration in the petiole
• Brown or rust red stems which may swell, twist and break
• Uneven pod fill and poor seed development
• Black and shrivelled mature pods and blackened seeds

By testing the soil and determining what species is present, growers can determine field risk prior to planting and implement specific management practices.

Determining species

The new molecular soil test can reveal information on the nematode species.

Dr Howard explained: “There are two species of stem and bulb nematode.

“Ditylenchus gigas is the most common and damaging nematode species in field and broad beans, but Ditylenchus dipsaci can also be found in both types of bean and is known to affect a broader spectrum of plants including brassicas, oats, potatoes and maize.”

She says the two species are similar but testing to identify type will help growers to manage crop rotations more effectively to avoid other potential plant hosts to reduce incidence on farms.

“If Ditylenchus gigas is identified beans should be avoided in the affected fields for 10 years as this can have a high impact on yield and grain quality.

“If Ditylenchus dipsaci is identified then it’s more of a rotational problem affecting other crops in the cycle and measures need to be taken to prevent transmission of nematodes to clean soils and reduce levels across the farm,” she said.

A standard nematode test costs £33 and will indicate whether nematodes are present. To determine species, a molecular diagnostic test costs £77.

Testing

Between 10% - 20% of seed samples tested by the PGRO each year are found to be affected by stem and bulb nematode and Dr Howard explains the 2020 harvest season looks set to follow this trend.

“Damage to yield and grain quality can be severe when nematodes numbers in soils are high and wet conditions during spring and early summer can allow the pest to move more freely.

“Because of the significant threat to UK bean growers, in collaboration with other seed testing stations we changed our testing protocol for the 2019 harvest season.

“We now test – a larger quantity of seed, which provides a more representative sample, increases detection rate and reduces the likelihood of producing false negative results.”

Mitigation

Nematodes are able to endure sub-zero winter temperatures, summer soil temperatures of 55c and can survive in a desiccated form in plant debris, seed and soil for decades.

“An adequate crop rotation and good weed control will, in many cases, help to prevent the pest building up in the soil - and beans shouldn’t be grown more than one year in five.” Becky said.

“Removal and destruction of bean straw will also significantly reduce the return of nematodes from the plants to the soil.”

Where the crop has been diagnosed as being infested, the produce should not be used for seed and a break of at least ten years should elapse before beans are grown.

Although infested seed is unsuitable for drilling, beans are still usable in animal feed compounds and blemish-free produce may still be suitable for export.

Best practice

“Best practice would be to save seed from a crop sown with certified seed and to pay attention to the crop in the field,” Dr Howard said.

“When a field is suspected of being infested, harvested seed should be kept separate from other bulks and should be sampled prior to cleaning or drying.”

Given that the pest can also infest clean land, only tested bean seed where the pest is not detected should be used and farm-saved seed should be carefully sampled and tested.

There is no standard for infestation in the UK Field Bean Seed Certification Scheme, so purchasers of certified seed should ensure that the test has been carried out on the seed they are buying.

Sampling and Testing

Before sending a seed sample for testing it is important that a representative sample is taken from the seed bulk.

“Preferably a sample should not represent a bulk greater than 30 tonnes for seed test results to be representative,” she said.

BOX: Quick Guide to Seed Sampling:

• Take samples when seed arrives in the store in accordance with methods in AHDB grain sampling guidance. If seed is already in store sample using a spear.
• Collect 20+ random samples from across the seed lot / heap.
• Bulk samples to form a single composite sample of at least 1200 grams and send to PGRO lab.

BOX: Quick Guide to Soil Sampling:

• Nematodes move with the water table in the soil profile so sample in autumn or spring after rainfall.
• Collect 20+ random samples from across the field.
• Bulk samples to form a single composite sample of at least 2kg and send to PGRO lab.

Dr Howard adds that if seed is from more than one field, growers should consider taking a separate sample of seed from each field prior to bulking. If an issue in a particular field is suspected seed should be stored and sampled separately.
If nematodes are detected by the lab, field and broad beans should not be grown in the rotation for 10 years.

Testing advice and additional guidance is also available from PGRO, the UK’s centre of excellence for peas and beans.