The following points highlight the top five control methods against seed borne pathogens. The control methods are: 1. Regulatory Method 2. Cultural Methods 3. Physical Methods 4. Chemical Methods 5. Biological Methods.
1. Regulatory Method:
There are certain federal and state laws which regulate the conditions for cultivation of crop and their distribution between states and countries. This helps preventing the import and spread of pathogen into country or individual states.
Such regulatory methods are enforced by means of quarantines, inspection of plants in field or warehouses, and occasionally by voluntary or compulsory eradication of certain host plants.
i. Plant Quarantine:
Plant quarantine as a practice covered two basic principles of disease management exclusion and eradication. When germplasm is regulated by quarantine the entry of organism and seeds with infection is prevented by inspection and treatment or the host is banned or otherwise restricted.
The entry of plants, plant parts and their product is conditioned, regulated and restricted at national and international levels through Plant Quarantine Acts. The growth of quarantine regulations has been under way since 1870. Since then each country has its own such organization.
In India, Destructive Insect and Pests Act (DIP Act) was passed by Govt., of India in 1914. The Agricultural Pests and Diseases Acts of different states prevent interstate spread of pests and pathogens. The plant quarantine stations are at major sea and airports.
Pathogens are likely to introduced in or on live plants, seeds, tubers and other planting material, in imported grain, fruit vegetables and other food stuffs, in plant material used in industry (as cotton fibers etc.), in plant material used in packing, and in soil or incidental plant debris present.
The plant and their products must be imported only from disease- free areas. Even if the material is already inspected by the quarantine services of the exporting country, it should be rechecked at the point of import. Special precautions are needed when cultures of pathogenic fungi, bacteria etc. are exchanged for scientific purposes.
In extreme cases it may become necessary to prohibit the import of certain plant material from areas where certain pathogens occur. Effective examination of incoming plant material depends largely on having enough number of adequately trained and experienced inspectors at ports, airports, railway stations and frontier posts. The dramatic increase in air travel has created problems in plant quarantine.
ii. Inspections/Certification:
Several voluntary inspection and certification systems are also in effect in various areas in which appreciable amount of seed, seedling nursery stock, potato seed tubers etc. are produced. Growers interested in sale of these materials submit for voluntary inspection and/or indexing of their crop in the field and in storage by regulatory agency.
iii. Use of Pathogen Free Seeds/Propagating Material:
Crop plants attacked by vectored viruses, mycoplasmas, fungi, bacteria and nematodes can be protected from disease by this method. It must be ensured that the propagating materials (seed, tubers, buds, bulbs, grafts, root stocks, corms, cuttings, rhizomes etc.) are free from pathogens.
Guidelines for Import of Germplasm (seed or others):
a. Import from a country where the pathogen(s) is absent.
b. Import from a country with an efficient plant quarantine service. So that inspection and treatment is done.
c. Obtain plant material from the safest known source within the selected country.
d. Obtain untreated seeds so that detection of seed borne pathogens is facilitated.
e. Obtain clean healthy looking seeds free from impurities.
f. Obtain an official certificate of freedom from pests and diseases from the exporting country.
g. Import the smallest possible amount of planting material, the smaller the amount the less the chance of its carrying infection. It will also simplify post entry inspection.
h. Inspect material carefully on arrival and treat.
i. If other precautions are not adequate, subject the material to intermediate or post entry quarantine.
Problems in Plant Quarantine:
Quarantine serves as a filter against the introduction of hazardous pathogens but still pathogens are introduced.
Possible reasons are that:
i. It is difficult to detect all types of infectious pathogens by conventional methods.
ii. The latent infection may pass undetected under post entry quarantine.
iii. Destruction of all infected or suspected material.
iv. Lack of sensitive methods for testing fungicide treated seeds.
Plant pathogens may be introduced on inert material such as packing material, dried root bits, plant debris, soil clods etc.
2. Cultural Methods:
Adjustment of crop management procedures has been an age old practice with the farmers for prevention of losses in crops due to diseases and other causes. It is an integral part of subsistence agriculture in developing countries. Cultural practices are now being considered as essential back up procedures for management of resistant varieties and also for chemically protected crops.
For development of a disease, contact between the host and the parasite must occur in an environment that is favourable for the pathogen and pathogenesis. Suitable modification in cultural practices can modify the environment that is not favourable for the pathogen but favourable for the host.
On this basis disease control by cultural practices is mainly preventive. Many practices reduce the density and activity of the inoculum. Precautions taken under avoidance are also mainly cultural practices. A large number of fungal, bacterial and virus pathogens are transmitted through true seed or vegetative propagating material. For effective disease control this source of primary inoculum must be taken care of.
Seeds carry the pathogens as:
(i) Internally seed-borne infection,
(ii) Externally seed borne inoculum,
(iii) Contaminants with the seed, and
(iv) Through nursery raised planting stock.
Although the infested or infected propagating material can be made pathogen free by chemical or physical treatments, production of such seed in the field is the first and important step. Following practices are followed to produce and use pathogen-free seed material.
i. Dry Climate for Seed Production:
Control of seed-borne diseases favoured by wet climate can be achieved by raising the crop in dry areas. Some examples are anthracnose of bean (Colletortichum lindemuthianum), anthracnose of cucurbits (Colletortichum lagenarium), Ascochyta blight of pea (Ascochyta spp.).
For producing seed of such crops dry areas are preferred where leaf wetness is avoided. Reasonably good dry climate can be created in wet areas through management of crop canopy facilitating air circulation and entry of sun rays.
ii. Isolation Distance for Seed Plots:
Separation of seed plots from sources of inoculum helps in production of healthy seed. In the production of certified seed, a particular distance between plots is mandatory.
iii. Inspection of Seed Plots:
Periodical inspection of the crop being raised for seed or orchards producing grafts and seedlings of fruit trees for distribution is an important step. Eradication of diseased plants or plant organs immediately follows the inspection. If the crop is badly diseased the plot is rejected for seed. The procedure is followed in production of seed tubers of potato.
iv. Drying and Aging of Seed:
Some pathogens do not tolerate drying of seeds. Viability of some seed is longer than the pathogen present in them. Thus prolonged storage often eliminates the pathogen from the seed. Fusarium solani f.sp. cucurbitae, infecting different cucurbits, is internally seed-borne.
When the infected seed is stored for two years before sowing the fungus is killed. In such method proper conditions for storage must be maintained to avoid harm to the seed.
v. Cleaning of Seed:
Sclerotia and oospores of many fungal pathogens and cockles or cysts of nematodes may be present in the debris mixed with the seed. Common examples are white blisters of crucifers, cyst nematode of sugar beet, and ear cockles of wheat.
Cleaning of seed is done by hot air blast that removes the dust also and by hand. In hand cleaning the seed is submerged in a 20% common salt solution. The debris and nematode cysts or cockles float on the surface and can be skimmed off by hand.
vi. Thermal and Chemical Treatment of Seed:
Heat or chemical treatment of seed before storage is a part of cultural practices for healthy seed production. Chemical seed treatment is a compulsory step in the production of certified seed. Thermal treatment of seed is favored in those cases where the pathogen is deep seated and ordinary protective fungicides control reach the pathogen.
Many systemic fungicides are capable of reaching the internal tissues. Treatment of seeds with vitavax and thiram are the examples of seed treatment for eradication of the pathogen. Bare root dip treatment of seedlings with nematicides is practiced to control root knot and other nematodes.
vii. Site and Treatment of Nursery Beds:
Diseases like club root of cabbage, root knot of tomato are generally carried by seedlings or grafts from the contaminated nurseries. The nursery site should be chosen with care avoiding locations near infested fields and the soil should be periodically treated with heat or chemical.
Soil solarization is feasible method for nursery soil treatment. The other method is to burn a heap of farm trash over the beds. Soil fungicides are also available for chemical treatment.
viii. Adjustment of Harvesting Time of the Crop:
Time of harvesting affects cleanliness of the seed. Delayed harvesting of grain crops in temperate regions gives the pathogens more time for contaminated the seed. Grain crops harvested in wet weather often produce contaminated seed.
Harvesting of potato when the leaves are still green allows the late blight pathogen to contaminate tubers which carry it to the next season. Such situation can be avoided by suitable alteration in the timing of harvest of the crop.
3. Physical Methods:
Different physical methods employed for reduction or elimination of primary inoculum are as follows:
i. Hot Water Treatment of Seeds and other Parts:
Hot water treatment or hot water therapy was first employed by gardeners in Scotland for treating the bulbs of ornamental plants.
Hot water is widely used for control of seed borne pathogens especially bacteria and viruses also fungi. Temperature and period of treatment will vary with the infection or pathogen concerned. In hot water treatment seeds are to be presoaked for four hours at 20-30°C, during which period dormant mycelium develops activity and becomes more vulnerable to exposure in hot water at 50-52°C for a few minutes.
Seeds after treatment are to be dried very carefully before they can be use for sowing. This method is cumbersome. Small quantities can be treated at a time. Besides, temperature has to be exactly maintained during the period of treatment.
ii. Hot Air Treatment:
Hot air treatment is less injurious to seeds and easy to operate but also less effective than hot water. It is use the for treating sugarcane stalk on commercial scale for control Ratoon-stunting Disease (RSD). In this method hot air treatment at 54°C for 8hour effectively eliminate pathogen without impairing germination of buds. Similarly grassy shoot disease of sugarcane has been controlled by hot air at 54°C for 8 hour.
iii. Solar Heat Treatment:
Solar heat treatment is effective in controlling both seed borne and soil borne diseases.
Seed-borne diseases:
Solar heat treatment controls effectively the loose smut of wheat (Ustilago nuda tritici). In this method the seed is soaked in water for four hours (8 a.m. to 12 noon) on a bright summer day. After this presoak, that seed is dried in the sun for four hours from 12 noon to 4 p.m.
Ascochyta rabiei the casual organism of Ascochytosis of chick pea survives in seed. Chaube and Singh studied the effect of sun drying on the survival of the pathogen in chickpea seeds.
The seeds were exposed to bright sun light during the last week of May and the first week of June. The seeds were spread on cemented floor from 8 a.m. to 4 p.m., daily for 15 days. Direct exposure of seeds on cemented floor reduced the recovers of the fungus.
In seeds covered with polythene sheets on cemented floor the reduction in survival of A. rabiei was of higher magnitude. In both the exposure methods, there was no effect on germination of the seed.
4. Chemical Methods:
Seed Treatment by Chemicals:
It is a known fact large number of disease/seed borne pathogen carried/introduced through the seeds. Seed treatment is a process of application of chemical or protectants (with fungicidal, bactericidal, nematicidal insecticidal properties) to seed, that prevent the carriage of diseases and also insects’ causing pathogens in/on the seeds. Seed treatment also enables the seed to overcome seedling infection by soil borne fungi.
The application of chemicals to seeds before planting in other words seed treatment has two purposes:
(i) Control of disease caused by seed borne pathogen/seed infection.
(ii) Protection of germinating seed or seedling from the attack of soil borne pathogen.
Appropriate treatment of seeds can get rid of the seed borne pathogen and can control, to a large extent diseases that would otherwise/harmful result. Besides, incorporation of protective chemical on the surface of seed can reduce the change of infection, consequently harmful effect of many soils borne pathogen which are capable of causing decay of seeds, pre- or post-emergence of damping off or infection in seedlings.
Methods of Seed Treatment and Equipment to be Used:
Methods which are commonly used in India for seed treatment by chemicals is following.
(i) Wet treatment:
Water soluble chemical are dissolved in water in containers of convenient size and seed are dipped for a certain period of time and then dried properly packing, storage, or sowing. If the seed is not properly dried, it is likely to be damaged during storage. Metallic containers should not be used when mercurial chemical are chosen for seed treatment. This method is generally not preferred as its time consuming.
(ii) Dust treatment:
Dry seed treatment is usually carried out in rotary or gravity- fed seed dressers. Bulk of seed is treated by dust formulation which does not require very complicated equipment. Industry made are used for seed treatment which reduce the inhalation hazards these drums treat the quickly and uniformly.
The closed barrel type rotatory drums are available in market and these are also available in Government agencies them on loan or hire.
(iii) Slurry treatment:
The slurry method of seed treatment eliminates, both wet and dry seed treatment difficulties. This method is most suitable for applying wet-table powder formulation. The chemicals are mixed with a small quantity of water and then the mixture is applied with seeds by manual or by automatic, semi-automatic machines.
During the process of treatment slurry gets deposited on the surface of seeds in the form of a thin paste which dries up. This method also suitable, seed treatment for large scale. Seeds treated with dry dusts may be stored for a long period so also slurry treated seeds. But seeds treated by steeping in liquid cannot be stored.
Type of Seed Treatment:
(i) Seed disinfestation:
Seed disinfestation refers to the destruction of surface borne micro-organism that have contaminated the seed surface but not infected seed surface. In other words, chemical applied prior to microorganism infection.
(ii) Seed disinfection:
Seed disinfection refers to the eradication of micro-organism spores that have become established with in seed coat, or in more seated tissue. In other words, chemical applied after microorganism has caused infection.
(iii) Seed protection:
Seeds and young seedling can be protection from microorganism in soil which can caused decay of seed or seedling. That type used chemical called seed protectents.
(iv) Protection:
Seed treatment includes surface treatment with pesticide, fungicide, or insecticide.
Formulations of Fungicide Materials:
(i) Wettable powders:
Wettable powders are easily wetted and disperse well in water. Wet-table powders are most commonly used for spray mixtures. Agitation is generally necessary in the spray tanks to keep a uniform suspension.
(ii) Dusts:
These formulations usually contain 4-10 per cent of the active ingredient and usually applied dry as dust. In dusts formulations active ingredients is very low.
(iii) Liquid/Solution:
True solution is formulations in which the active ingredient or a combination of active ingredient and solvent is dissolved in water. Solutions have the advantage of requiring no agitation after the formulation is added in water. These have high degree of solubility and used knows as quick wet methods.
(iv) Suspensions or slurries:
These are formulations in which a dry form of the active ingredient is mixed with liquid. Slurry treatment in which the chemical is applied in the form of a thick soup (active material dissolved in a small quantity of water or in any harmless solvent), so that during the process of treatment slurry gets deposited on the surface of seeds in the form of a thin paste which dries up.
Such formulations usually have a high percentage of active ingredients similar to wet-table powder. They are mixed with water for final use and require agitation.
(v) Emulsifiable concentrates:
These are liquid in which the active ingredient is dissolved in a solvent. These types liquid not mixed with water, so an emulsifying material is already included. When these emulsifiable concentrates are added to water, a milky mixture is formed which is a suspension of active ingredient and emulsified solvent in the water. However, fungicides are not commonly formulated as emulsifiable concentrates.
(vi) Granules:
These are formed into particles about the size of coarse sugar. The percentage of active ingredients is usually low as in the case of dusts. Rarely, fungicides formulated as granules.
Chemicals which may be used Seed Treatment:
Disinfection of seeds by chemicals was in vogue for a long time. Copper sulphate used for the seed treatment of cereal seed with spores of bunt fungi. Sulphur dust used for the seed treatment of smuts causing fungi.
With the discovery that mercuric chloride can get rid of seed borne infection in the case of fusarium causing fungi, and later the introduction of organomercuric compound for disinfection of seeds a new chapter opened in seed treatment in case of fungal infection.
Organomercuric chemicals have shown effectiveness against a number of diseases in a number of different crops and may be called broad spectrum seed treating fungicides. Seed dressing with organomercurial, it has been to control a large number of diseases of cereals and commercial crops. Later on non-mercurial organic chemicals also appeared in the market and were successfully used for seed treatment.
Such treatments besides controlling infestation and or infection, as the case may be, also afford protection to the seeds or during germination and young seedling in the early stage of growth from soil borne fungi.
The introduction of systemic fungicides for seed treatment has added further possibility:
(a) Control of pathogen located deep inside the seeds.
(b) Control of air borne infection at a later stage of growth of the crops because its systemically translocate to aerial part.
Chemicals Used in Seed Treatment against Pathogenic Fungi may be classified in Table:
Seeds treated with chemicals must be stored dry and treatment should at least by done one week before sowing. In case of treatment with liquids, as seeds cannot be stored the treatment has to be done immediately before sowing. The time required for the purpose as recommended, should be strictly adhered too.
Liquid treatment is usually taken resource to for treating vegetative propagative stocks for example, cuttings, tubers, corms, bulbs, etc., which are not amenable to dry or slurry treatment. Storing under damp conditions after treatment has been reported to damage the viability of seeds. Hence proper storage of treated seeds has to be ensured.
Precaution of Seed Treatment:
1. Many chemicals used for the treatment of seeds are poisonous or toxic to man and animals, so they should be used with caution.
2. Chemically treated seeds should never be used for consumption by man and animals as food and feed.
3. Seed treatment should be carried out in well aerated area.
4. Contract with chemicals through breathing avoided and also avoided skin and eye.
5. Biological Methods:
Bacterial and fungal antagonists may be more effective when applied to seed than applied to soil because of their proximity, in large numbers, to the infection court. Seeds are vulnerable to seed borne and soil borne pathogens, and the period of protection not be long.
The first commercial use of an organism to control a plant disease in soil is Agrobacterium radiobacter to control crown gall in rosaceous hosts. Kerr (1980) isolated strain 84 that was antagonist to A. tumefaciens strain 84 produces an antibiotics called “agrocin 84,” which is one of a group of highly specific antibiotic known as nucleotide bacteriocins.
Since 1973, Agriculture Research Institute has supplied commercial growers with culture of strain 84, but now the organism is marketed commercially in India. Seeds, cuttings, or roots of young plants are dipped into a bacterial suspension; in order for the non-pathogenic strain to be effective, the ratio of pathogenic to non-pathogenic at the root surface cannot exceed one.
Seed treatment with strain 84 is inexpensive, simple, and effective. It is harmless to humans and other mammals; inoculum can be stored in peat and retain a half-life of six months at room temperature, in field tests, B. subtilise was effective when soil was moist.
Seed treatment, usually directed against soil borne pathogens, can also affect seed- borne storage and field fungi (fungi that infect seeds before harvest). Mew and Kommedahi (1972) applied Chaetomium globosum to corn kernels before placing them on an agar medium, and this application reduced the percentage kernels infected with both field and storage fungi.
Rather than adding antagonists directly to seeds, grew inoculum of the pathogen and antagonists on an oat grain medium, and these were mixed with the wheat grain prior to planting. Some of the bacterial and fungal antagonists show potentially promising use, and illustrate various considerations in biological treatment of seed and other infection courts.