Crop Update 09:  Sampling and testing for Stem and bulb nematode: Guidance 10th June 2020

(Edition Number 8: CB20-8)

Stem and bulb nematode in field beans – guidance note for sampling and testing seed 2020

Stem and bulb nematodes can be seed-borne and can also survive in the soil in a free-living form. The principle routes of infestation on farms are from the use of infested seed, infested bulbs or from contaminated soil. Testing of farm saved seed is therefore highly advisable.

There is no standard for infestation in the UK Field Bean Seed Certification Scheme.

The nematodes are slender, transparent and virtually microscopic. They can be found in large numbers within the stem or leaf tissue or in seed, by microscopic examination. Ditylenchus gigas is the most common and damaging species in field and broad beans, but Ditylenchus dipsaci can also be found in both.

PGRO has now developed a molecular test that can also detect the presence of the nematodes in soil samples as well as seed and can also differentiate between the two species. For details see the Summer 2020 edition of the Pulse Magazine, page 9: http://www.graphicgeneweb.co.uk/2020-summer/mobile/index.html

This pest has become one of the most important problems in both spring and winter field beans. Following introduction to fields and establishment in soils, nematodes can remain viable for many years, placing future crops of beans at risk from damage, which is often first seen as the plants reach the flowering stage. Earlier symptoms may be found at any stage after crop emergence.

The plants may be stunted, and the stems thickened and twisted. Leaves become thickened and brittle with a bronze discolouration occurring in the leaf petioles. Later, the stems turn brown or rust red in colour and may swell, twist and break. Pods fail to fill evenly, and seeds are poorly developed, becoming black and shrivelled as they mature.

Affected plants may appear singly, or in larger patches of the field. The appearance of single isolated plants across the field may indicate an infested seed source, while more general patchy crop damage would indicate a pre-existing field infestation.

Multiplication of nematodes is greatly enhanced during a wet spring and crop loss from this pest can be substantial. Infested seed is unsuitable for drilling, but beans are still usable in animal feed compounds and blemish-free produce may still be suitable for export.

Good practice is five years free of growing beans prior to a seed crop, and an adequate crop rotation and good weed control will, in many cases, help to prevent the pest building up in the soil.

Removal and destruction of bean straw will 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.

Best practice would be to save seed from a crop sown with certified seed and to pay attention to the crop in the field. When a field is suspected of being infested, harvested seed should be kept separate from other bulks. This seed should be sampled prior to cleaning or drying for disease, pest and germination testing. Preferably a sample should not represent a bulk greater than 30 tonnes for seed test results to represent the bulk.

Given that the pest can infest clean land, only tested bean seed where the pest is not detected should be used. Farm-saved seed should be carefully sampled and tested before planting and purchasers of certified seed should ensure that the test has been carried out on the seed they are buying.

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

The following guidance based on best available advice (PGRO/NIAB/SASA) will help to reduce the chance of on-farm problems:

Sampling:

• It is preferable to take samples when seed arrives in the store, in accordance with methods in the AHDB grain sampling guidance. However, if seed is already in store sample using a spear.
  
  
• Collect a minimum of 20 random samples from across the seed lot / heap. The size of the random samples should be roughly equal and should be bulked to form a single composite sample of at least 1200 grams. Reducing the number of random samples or weight of the composite sample will lead to a reduction in detection of nematodes, potentially leading to false negative results. Send 1200 grams of seed to a specialist laboratory offering stem nematode analysis services.
  
  
• If seed is from more than one field, consider taking a separate sample of seed from each field prior to bulking in order to collect a representative sample.
  
  
• If an issue in a field is suspected, store and sample seed from it separately.

Seed testing:

• Laboratories should test at least 600 grams of seed per sample received
  
  
• Test three replicates of 200 grams per sample
  
  
• Reducing the weight of sample tested may lead to reduction of detection of nematodes, potentially leading to false negative results
  
  
• If nematodes are detected, field and broad beans should not be grown in the rotation for ten years

Although it is possible to detect the presence of nematodes using this guidance, due to the variability of the pest within the seed lot and between seeds it cannot accurately determine the level of infestation or the percentage of infested seed. Therefore a ‘Clear’ test is an indication and not a guarantee that the crop or seed supplied will be free of nematodes.

In addition, very low levels of infestation may be present below the limit of detection of the test, even though seedsmen take all precautions to reduce the risk of seed being infested with nematodes.

Host range and field management:

Ditylenchus gigas
D. gigas is thought to be the principal species affecting field beans in the UK. It causes significant damage in field beans but has a limited host range that includes the following plant species:

Source: Stawniak, 2011.

Avoidance of beans in the same rotation as the species listed will help to reduce the chances of rapid build-up of D. gigas populations.

Ditylenchus dipsaci
D. dipsaci is thought to be the less common species affecting field and broad beans in the UK, but with many host plant species. A full list of species can be found at https://www.cabi.org/isc/datasheet/19287. The species is known to affect more than 450 plant species, including Alliums, Brassicas, bulb flowers, field and broad beans, oats, sugar beet, hemp, strawberries, lucerne, tobacco, Phaseolus beans, phlox, peas, rye, potatoes, clover, maize and some weeds. As such, management of rotations is more difficult, although some crops in the rotation may be treated with nematicides, reducing populations overall.

The information in this publication must not be reproduced without the express written permission from the PGRO. Information disseminated by the Processors & Growers Research Organisation is given after the exercise of all possible care in compilation, preparation and issue, but is provided without liability in its application or use.