Stem and bulb 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. The 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. There is no standard for infestation in the UK Field Bean Seed Certification Scheme.
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:
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:
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:
|Field beans and broad beans||Vicia faba|
|Vetch||Vicia and Lathyrus spp.|
|Corn buttercup||Ranunculus arvensis|
|Field bindweed||Convolvulus arvensis|
|White dead nettle||Lamium album|
|Red dead nettle||Lamium purpureum|
|Dead nettle||Lamium amplexicaule|
|Sterile oat||Avena sterilis|
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.
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.
(Edition: Number 9 : CB0509)
Field bean plant showing stem
swelling and discolouration
Infested field bean plant