Seed health testing in pulse crops

Many important diseases of pulses can be seed-borne. Pulse growers can minimise losses from these diseases by using high quality seed.

Seed testing is required to establish whether seed is infected. Seed health tests are currently available to detect the most important seed-borne pathogens of pulses. Only seed that is pathogen-free should be used for sowing.

Testing seed before sowing will identify potential disease problems and allow steps to be taken to reduce the disease risk. Laboratory testing is usually required, as infected seed may have no visible disease symptoms.

Importance of seed-borne diseases

Results of seed health tests in Australia and overseas have shown that many serious diseases of pulses can be seed-borne and significant crop losses can result from the use of infected seed.

Also the uncontrolled movement of infected seed between regions can result in the rapid expansion of the area affected by these diseases.

Pathogens can adversely affect germination, cause seedling infection and damage mature plants. The transmission of fungal and bacterial pathogens from seed to crop can vary considerably depending on growing conditions.

Diseases caused by viruses usually have higher transmission rates than those caused by fungi or bacteria, and are less affected by seasonal conditions. Consequently, there are different tolerance levels for seed infection by different pathogens.

Seed-borne diseases often strike early in the growth of a plant causing poor crop establishment and reduced plant vigour which results in lower yields, for example cucumber mosaic virus in lupins.

However, some diseases like Ascochyta in chickpea for example, can cause total crop loss.

Symptoms of seed-borne diseases

Viruses

Alfalfa mosaic virus (AMV)

Plants may develop a bright yellow leaf mottle, tip necrosis, reddening, stunting, pod flattening and blackening. Yields are reduced through plant death, the production of small seeds, and seeds with brown coat discolouration. A high incidence of AMV is found in lentil, lupin and chickpeas.

Find out more about alfalfa mosaic virus.

Bean yellow mosaic virus (BYMV)

Plant leaves may develop leaf mottle or a distinctive yellow mosaic pattern, stunting and reduced leaf size. Early infections can seriously reduce plant growth and grain yield. BYMV is an economically important virus for lupins.

Find out more about bean yellow mosaic virus.

Cucumber mosaic virus (CMV)

Plants may develop leaf chlorosis (yellowing), stunting, distortion or bunchy appearance. Pods may be flattened and turn purple-brown. CMV causes problems in lentil, lupin and chickpea.

Find out more about cucumber mosaic virus.

Pea seed-borne mosaic virus (PSbMV)

Plants may develop downward rolling of leaf margins and slight clearing of the veins in young leaves. Production of small seeds with distinctive brown staining and 'tennis ball' marking is common. Seed discolouration can significantly reduce the commercial value of grain. PSbMV is an economically important virus for field peas.

Find out more about pea seed-borne mosaic virus.

Fungi

Ascochyta blight

Ascochyta fabae, Ascochyta lentis, Ascochyta pisi, Ascochyta rabiei, Mycosphaerella pinodes, Phoma pinodella

Tan coloured lesions form on leaves, stems and pods. Infected leaves may drop prematurely. Yields are reduced through plant death and crop damage. Infected seed can be shrivelled or badly stained.

Find out more about:

Grey mould and chocolate spot

Botrytis cinerea, Botrytis fabae

Stem infection can cause the damping-off of young seedlings. Later infection causes grey mould or chocolate spot on foliage and flowers. Yields are reduced through plant death, crop damage and flower abortion. Infected seed can be small and badly discoloured.

Find out more about:

Lupin anthracnose

Colletotrichum lupini

A severe foliar disease that causes bending and twisting of stems, with a lesion in the crook of the bend (walking stick). Stem infection often results in the death of plants and major yield losses. Affects both narrow leaf lupins and albus lupins.

Anthracnose is an exotic disease in Victoria and there are restrictions on seed movement.

Phomopsis stem blight

Phomopsis leptostromiformis

Causes yellow-brown lesions on leaves stems and pods. Severe infection can kill plants. Infected seed can be covered with a web-like grey mould. Toxin produced by infected stubble can kill animals.

Brown leaf spot

Pleiochaeta setosa

Can cause both root rot and leaf spotting. Affected roots develop large dark brown lesions. Irregularly shaped dark brown lesions develop on infected leaves. Can seriously reduce plant growth and grain yield.

Find out more about brown leaf spot of lupins.

Bacteria

Bacterial blight

Pseudomonas syringae pv pisi, Pseudomonas syringae pv syringae

Important seed-borne disease of field peas. Both these pathogens may be carried by the seed either internally or externally. Water-soaked leaf, stem and pod lesions may occur at any growth stage. Yields are reduced through plant death, crop damage and the production of small seeds. Brown discolouration of the seed coat can sometimes occur.

Find out more about bacterial blight of field peas.

Epidemiology of seed-borne diseases

Infection of seed

Seed infection levels are determined primarily by weather conditions between flowering and maturity. Warm, humid conditions during this period often results in heavy pod and seed infection.

Dry weather between flowering and maturity minimises pod infection and is essential for the production of pathogen-free seed.

There is often considerable variation between genotypes in their resistance to seed infection. Virus infection in seed depends on the amount of spread of virus between plants during the growing season, often by aphid vectors, and the genetic susceptibility of the host plant.

Seed to plant transmission

Crop losses are likely to be higher for pathogens that invade the roots soon after germination than for pathogens that affect the shoots of young plants.

The extent of seed to plant transmission of fungal and bacterial pathogens is known to vary considerably depending on infection conditions. Poor germination and diseased seedlings can result from the use of infected seed lots.

Transmission from seed to seedling is usually highest for viruses and varies from around 0.1-5% for AMV/CMV, up to 100% for PSbMV.

Transmission rates for fungal and bacterial pathogens are affected more by the environment and some diseases can be carried over in soil as well as on crop residues.

Survival on seed

Seed-borne pathogens can often survive for several years in and on seed.

Some pathogens are carried on the seed coat while others can be harboured deep inside the seed. Fungi and bacteria are mostly located on the seed coat and embryo infection is uncommon. Viruses are not carried on the seed coat and are only found in the seed embryo or tissues of the seed coat.

Infestation levels of most pathogens decrease rapidly during storage and long-term storage can eliminate some pathogens from seed. Unfortunately, there is likely to be a marked reduction in the viability of seed stored for a long period and this may negate any benefits from lowering disease levels in seed.

Management of seed-borne diseases

Measuring the amount of seed-borne inoculum

The amount of inoculum may be expressed in terms of the proportion of infected seeds, the degree or severity of infection (inoculum per individual seed) or the viability of the inoculum (that is the infectivity of the pathogen in seed).

Most seed tests measure the proportion of infected seed. Because low levels of seed-borne inoculum can lead to considerable disease, the most sensitive test should be used to determine the level of seed infection.

Fungi may be detected using a standard blotter test or an agar plate test, the latter being the more sensitive. Seed-borne bacteria can also be detected using an agar plate test.

While results of standard tests indicate the proportion of infected seed, they provide no information on the amount of inoculum per seed. Where a high percentage of seed is infected there is often more inoculum per seed associated with larger infections and deeper penetration.

Seed-borne viruses are usually detected using ELISA or PCR tests. It is important that the diagnostic tests are conducted on germinated seed (seedlings) as virus may sometimes infect the seed testa without infecting the embryo or seedling, for example PSbMV.

There is a wide range of tolerance levels for different pathogens.

For viral diseases a threshold of  less than 0.1 per cent seed infection is recommended for sowing in high risk areas and  less than 0.5 per cent seed infection for sowing in low risk areas.

For most fungal pathogens a threshold of less than 1 per cent seed infection is acceptable. However, there is a nil tolerance for the most serious fungal diseases such as Ascochyta rabiei in chickpea.

Preventing the spread of seed-borne diseases

The best method of to reduce the risk of disease damage is to source pathogen-free seed. Testing seed before sowing will establish whether seed is free of disease.

The next best option is to select seed from crops that show no sign of disease. Seed with high levels of seed-borne disease should not be used for sowing.

For some fungal diseases, it may be possible to reduce the risk of disease by applying a fungicide to seed prior to sowing.

Seed treatments are not available for the control of virus or bacterial diseases.

Table 1. Seed health tests currently available and tolerance levels for seed infection

(Note: thresholds given are a guide only)

Viruses

Laboratory

Sample size submitted

Number of seeds tested

High risk area — Seed infection threshold for acceptance of seedlot

Low risk area — Seed infection threshold for acceptance of seedlot

AMV

AgriFood

AGWEST

TASAG

3kg

1000

Less than 0.1%

Less than 0.5%

BYMV

AGWEST

TASAG

3kg

1000

Less than 0.1%

Less than 0.5%

CMV

AgriFood

AGWEST

TASAG

3kg

1000

Less than 0.1%

Less than 0.5%

PSbMV

AGWEST

TASAG

3kg

1000

Less than 0.1%

Less than 0.5%

Bacteria

Laboratory

Sample size submitted

Number of seeds tested

High risk area — Seed infection threshold for acceptance of seedlot

Low risk area — Seed infection threshold for acceptance of seedlot

Pseudomonas syringae pv pisi

AGWEST

AsureQuality

1kg

1000

Nil tolerance

Less than 0.1%

Pseudomonas syringae pv syringae

AGWEST

AsureQuality

1kg

1000

Nil tolerance

Less than 0.1%

Fungi

Laboratory

Sample size submitted

Number of seeds tested

High risk area — Seed infection threshold for acceptance of seedlot

Low risk area — Seed infection threshold for acceptance of seedlot

Ascochyta and Botrytis

AgriFood

1kg

400

Less than 1%

Less than 5%

Ascochyta fabae

AGWEST

SARDI

1kg

400

Less than 1%

Less than 5%

Ascochyta lentis

AGWEST

SARDI

1kg

400

Less than 1%

Less than 5%

Ascochyta pisi

AGWEST

1kg

400

Less than 1%

Less than 5%

Ascochyta rabiei

AGWEST

SARDI

1kg

1000

Nil tolerance

Nil tolerance

Botrytis cinerea

AGWEST

SARDI

1kg

400

Less than 1%

Less than 5%

Botrytis fabae

AGWEST

1kg

400

Less than 1%

Less than 5%

Colletotrichum lupini

AGWEST

1kg

1000

Nil tolerance

Nil tolerance

Mycosphaerella pinodes

AGWEST

1kg

400

Less than 1%

Less than 5%

Phoma pinodella

SARDI

1kg

400

Less than 1%

Less than 5%

Growers should discuss the likely disease risk on their farm with local advisers.

Wherever possible, only pathogen-free seed should be used. This is most important when a crop is being grown in a new area. It is important to note that a negative result from a seed test does not guarantee that a seed lot will be free from disease.

Seed testing laboratories

  1. Crop Health Services, Agriculture Victoria, (03) 9032 7515
  2. DDLS Specimen Reception, DPIRD, (08) 9638 3351, DDLS@dpird.wa.gov.au
  3. AsureQuality 3-5 Lillee Crescent (PO Box 1335) Tullamarine Vic 3043, (03) 8318 9024
  4. SARDI Seed Testing Service, Adelaide, (08) 8303 9585 or (08) 8429 2214.
  5. TASAG ELISA and Pathology Testing Service Department of Primary Industries, Parks, Water and Environment, New Town, Tasmania 7008, (03) 6165 3777. Please check with the TASAG laboratory if the test is required to satisfy phytosanitary or export purposes.

Further references

Contact

Field Crops Pathology
Grains Innovation Park
110 Natimuk Rd
Horsham 3400
(03) 4344 3111

Or call the Customer Service Centre, 136 186

Acknowledgements

Mohammad Aftab, Angela Freeman, Trevor Bretag, Frank Henry. Support by the Grains Research and Development Corporation is gratefully acknowledged.

Page last updated: 23 Jul 2021