Seed Testing

Seed Health Test

Seed borne disease

Many high yielding varieties have shown susceptibility to different diseases and many of these diseases are seedborne. The seed primordium or the maturing seed may be infected either (i) directly from the infected plant through the flower or fruit stalk and the seed stalk or directly from the seed surface, or (ii)infection from outside may be introduced through stigma or ovary wall or pericarp, and the flower or fruit stalk, and later through the seed coat. A pathogen may penetrate several of these parts of the seed and in turn infect them. The infestation /contamination of the seed may occur during harvesting, threshing and processing. The pathogen may, thus, be carried with the seeds in three ways.

(i) Admixture: Pathogens are independent of seeds but accompany them. Ergot sclerotia are mixed with health seeds during threshing.

(ii) External: The pathogen may be present on seed surface as spores, oospores and chlamydospores as in case of kernel bunt of wheat, covered smut of barley, downy mildew of pearl millet etc.

(iii)Internal: Pathogens establish within the seed with definite relationship with seed parts.

The seed borne pathogens may result in (i) loss in germination )ii) discolouration and shriveling (iii) development of plant diseases (iv) distribution of pathogen to new areas(v) introduction of new strains or physiologic races of the pathogen along with new germplasm from other countries (vi) toxin production in infected seed etc. Visualising the importance of seed borne diseases the Central Seed Certification board has prescribed certification standards for foundation and certified seed for several diseases.

Seed Standards for Foundation and Certified seeds based on maximum percent seed infection

Crop Species	Disease	Causal organism	Certification standards %(by nos.)
			Foundation	Certified
Pearl millet	Ergot	Claviceps fusiformis	0.02	0.04
Paddy	Bunt	Neovossia horrida	0.1	0.50
Sorghum	Ergot	Sphacelia sorghi	0.02	0.04

Ergot Sclerotia seed entirely or partially modified as sclerotia, broken sclerotia or ergotted seed.

Number of seeds to be analysed for different seed certification standards

Percent	Seed Infection	No.of seeds to be analysed/replication
1	None	5000
2	0.02	5000
3	0.04	2500
4	0.05	2000
5	0.1	1000
6	0.25	400
7	0.5	200
8	1.0	100
9	3.0	33
10	5.0	20

The number of seeds to be tested is based on the presence of at least one infected seed in the working sample. Each sample will have two replications. The size of the working sample has been derived from 4 x n , where n = standard specified.

Detection of Insects

Insect damaged seed in purity analysis are classified according to the so-called “One-half seed rule”. If one-half or more of the seed is damaged or consumed by the insect it is considered inert matter. Standard for insect damaged seeds under certification, however, does not allow apparent or visible evidence of damage by insect for both foundation and certified seed ins excess of 1.0% for the seeds of maize and legumes and 0.5% for the other crop seeds unless otherwise prescribed.

Insects infesting seeds and planting materials can be categorized into two groups. The first group includes those insects that infest seeds or vegetatively propogated materials(tuber, corn, rhizome stakes etc,) in the field. They do not multiply during storage or transit and except a few, these are not carried by the infested materials to another field. It includes cut worm, white grub, wire work, gall midges, pink boll worm etc. the number of seeds or planting materials so damaged in the fields remains mostly constant.

The insects belonging to the other group infest seeds or planting materials in the field, godown or any other place where these materials are handled. They continue their life-cycle activities under ambient conditions. the number of insects as well as the damaged seed both increase with the passage of time. It includes most of the insect-pests of stored seed/grain such as weevil, beetle, and moths. The extent of infestation recorded at the time of testing therefore, does not reflect the true infestation present in the seed lot at the time of lifting of the sample unless either the sample is tested immediately or preserved suitably to prohibit further development and multiplication of insects.

Detection of External Infestation: direct Examination

(a) Presence of living insects and its stages

Working sample: Whole submitted sample

Procedure: Visual observation

(i) Seeds are examined in dry state with the help of magnifying glass (10X) or stereoscopic microscope

aided with light.

(ii) Adult weevil, beetle, moth larvae, grubs etc. are separated and counted.

(iii) The result is reported as number of insects (including its stages) per weight of the sample

Presence of insects in the seed sample, so detected, confirms that the seed lot is /was infested. Their absence does not guarantee that the seed lot is free from insect infestation i.e. insect injury to the seeds or internal (hidden) infestation. Thus, it is imperative to carry out further tests to detect external and internal infestation.

(b) Detection of external insect-injury

Working sample : 400 seeds in two replications of 200 seeds each

Procedure: Visual examination

(i) Seeds are thoroughly examined with the help of magnifying glass or stereoscopic microscope aided with light.

(ii) Seeds damaged (less than one-half of its size) by insects including those seeds whose germ(embryo) has been touched or eaten, having escape holes, adhered with eggs etc. are separated and counted.

(iii) Other seeds with no visible symptoms of insect injury are subjected to further test to detect internal infestation

Detection of Internal (hidden) infestation

Some of the stored-seed pests especially weevils and bettles are internal feeder. Detection of their infestation is essential to determine actual infestation. The number of infested seeds, thus obtained is added to the number of seeds found externally damaged by the insects for final calculation.

Alkali or glycerine method

Working sample: Left over seeds of test

Procedure:

(i) Seeds are taken in a beaker (100 ml capacity) and then Sodium hydroxide (NaOH) 10% solution (dissolve 10 gm NaOH pellets in 100 ml water) is poured in it until the seeds are submerged.

(ii) After decanting solution, translucent seeds are washed with water and then examined with the help of magnifying glass.

(iii)After decanting solution, translucent seeds are washed with water and then examined with the help of magnifying glass

(iv) Seeds with visible internal infestation are separated, cut opened to confirm the presence of insect or its stage and counted.

(v) The result is reported after adding the count of damaged seeds obtained in tests as percentage (by number)

Alternatively, the seeds can also be made translucent in lacto phenol solution (dissolve 20 gm phenol crystal in 20 ml luke warm distilled water, and then add 20 ml lactic acid and 10 ml glycerine) following the steps mentioned above.