Seedborne diseases of cereals: tests and thresholds

It is important to understand the quality of cereal seed intended for drilling, especially the presence of seedborne diseases. This page includes the tests and thresholds available for certified, farm-saved and organic seed sources of wheat and barley.

Cereal quality standards are published in the Seed Marketing Regulations.

The regulations state: “Harmful organisms that reduce the usefulness of the seed shall be at the lowest possible level.”

Certified seed sources

All seed bought and sold in the UK must be certified. By law, seed must be officially sampled and tested before it can be certified.

Regulatory standards exist for loose smut and ergot and advisory thresholds have been set for many other key diseases (Table 1).

Countries may prescribe stricter standards than the EU minimum. The UK sets a Higher Voluntary Standard (HVS) for seed. Sold at a premium, HVS seed has higher standards for varietal and species purity, ergot and loose smut.

Although not a legal requirement, most certified seed is treated (unless it is organic). If certified seed is supplied untreated, a grower or merchant should test it for seedborne diseases not covered under the regulations to decide on the need for a seed treatment.

Farm-saved seed sources

High-quality farm-saved seed can be grown and processed on or off farm. Best practice is to save seed from a crop grown from certified seed.

Monitor seed crops regularly and keep grain separate from other grain bulks.

Before cleaning or drying, send a representative sample to a laboratory to test for germination and the presence of seedborne diseases. This will determine if the seed is suitable for use and whether a seed treatment is needed.

Where disease levels exceed set standards or thresholds (Table 1), apply an appropriate seed treatment or do not use the seed.

Although these thresholds and standards are not a legal requirement, the use of farm-saved seed with high infection levels will cause inoculum to build up and spread.

Farm-saved seed information

Table 1

Regulatory standards and advisory thresholds for key seedborne diseases of wheat and barley

Crop	Disease	Method	Duration	Results	Regulatory standard (S) or advisory threshold (A)
Wheat	Bunt Tilletia tritici	Wash	48 hours	Spores per seed	(A) Treat if one spore/seed or more
Wheat and barley	Ergot Claviceps purpurea	Visual	24 hours	Number of pieces in 500 g or 1,000 g	Maximum pieces: (A) (S) Three pieces/500 g (minimum standard) (S) One piece/1,000 g (Higher Voluntary Standard)
Wheat and barley	Loose smut Ustilago tritici (wheat) Ustilago nuda (barley)	Embryo extraction	48 hours	% infection in 1,000 embryos (advisory) or 2,000 embryos (certification)	Maximum infection: (A) (S) 0.5% (minimum standard) (S) 0.2% (Higher Voluntary Standard)
Wheat and barley	Head blight Microdochium spp.	Agar plate Molecular	7–10 days 48–72 hours	% infection Either over or under 10% infection	(A) Treat if over 10% (wheat) (A) Treat if over 30% (barley)
	Head blight Fusarium graminearum	Agar plate	7–10 days	% infection	(A) Treat if over 10%
	Foot rot Cochliobolus sativus	Agar plate	7–10 days	% infection	Risk is low (wheat) (A) Treat if over 30% (barley)
	Septoria nodorum Parastagonospora nodorum	Agar plate	7–10 days	% infection	(A) Treat if over 10% (wheat) Risk is low (barley)
Barley	Covered smut Ustilago hordei	Wash	24–48 hours	Spores/seed	(A) Treat if present
Barley	Leaf stripe Pyrenophora graminea	Agar plate Molecular	7–10 days 48 hours	% infection Presence/absence	(A) Treat if over 2% (A) Treat if present
Barley	Net blotch Pyrenophora teres f. sp. teres	Agar plate Molecular	7–10 days 48 hours	% infection % infection	(A) Treat if over 15%

Wheat

Test wheat for bunt and microdochium seedling blight. Also conduct tests for ergot, loose smut, septoria nodorum and fusarium seedling blights, if a problem is suspected (e.g. if it is present in the growing crop).

Barley

Test barley for the presence of loose smut, leaf stripe and net blotch (where not previously present). Tests for ergot, covered smut and Fusarium graminearum should also be conducted, if a problem is suspected (e.g. if it is present in the growing crop). Microdochium is less of a concern in barley.

*Treat for seedling blights when sum of infection levels exceeds 10%.

Seed treatment

If disease levels are below those shown in Table 1 and germination is greater than 78%, a fungicide seed treatment is not usually required. However, consider all disease threats, especially if drilling late into cold soils.

For seed treament options, visit our fungicide perfomance pages

Organic seed sources

Test all seed for organic production for germination and seedborne diseases. Organic-certified seed must meet the same quality standards as conventionally produced seed. Seed merchants are usually able to provide the seed test certificates for each lot.

How to sample grain

Central to success is the segregation of different grain qualities, and the extraction of representative samples with the appropriate equipment. 300–500 g is normally a sufficient sample size for germination and disease testing.

How to sample grain (the basics)

Note: Diseased seed lots can contaminate healthy lots. Brush sampling equipment with water and detergent, before and between lots, and allow it to dry.

Number of primary (incremental) grain samples required*

Lot size (tonnes)	Primary samples required
5**	10
10	20
30	40

*based on International Seed Testing Association Rules.

**For smaller seed lots, it may be cheaper to treat than to test.

Bunt balls (left) and healthy grain (right)

Seedborne diseases

The consequence of planting infected seed depends on the pathogen in question. Seed can be an important source of some pathogens. However, for some seedborne diseases, other sources of the pathogen (e.g. soil or crop debris) are more important. Symptom expression also occurs at various growth stages. Some diseases may cause the seedling to die before it emerges. In an emerged crop, some diseases may show symptoms in the seedling. For some other diseases, however, visible symptoms may not show until a much later growth stage (e.g. after flowering).