Seed Borne Pathogen Diseases Affecting Crops: Introduction, Transmission and Detection Techniques

In this article we will discuss about:- 1. Introduction to Seed Borne Pathogen 2. Ways of Infection of Seed Borne Pathogen 3. Transmission 4. Detection Techniques 5. Prevention Methods.

Introduction to Seed Borne Pathogen:

Seed is the basic unit in crop production technology. It has attracted the agriculturist even in early days. Seed plays a vital role in associating micro-organisms which prove hazardous for the seed or new plant created from it. Important seed borne pathogen/ micro­organism are various fungi, bacteria, viruses, nematodes etc.

The associated micro-organism may be pathogenic, weak parasite or saprophytes. They may be associated internally or externally with the seed or as concomitant contamination as sclerotia, galls, fungal bodies, bacterial ooze, infected plant parts, soil particles etc. mixed with the seed. Seed borne pathogen generally plays a negative in human welfares as well as agriculture production.

Seed borne pathogens causes diseases at various stages of crop growth from germination of seed up to crop maturity and heavy losses have been observed, caused by seed borne pathogen in various crops. Seed borne pathogens causes seed and seedling rots, i.e. pre- and post- emergence losses, diseases at various stages of crop growth like root rot, stem rot, fruit rot, wilt, blight, leaf spot etc.

influence the crop stand and ultimate yield. Therefore, the good seed must not be affected by any seed borne pathogen. Pathogen free seed is a factor which needs the maximum attention of farmer for an increase crop production. Thus, detection of plant pathogen from seed and their estimation and management is very important for agriculture production/yield.

Ways of Infection of Seed Borne Pathogen:

I. Externally Seed Borne Pathogen:

The seed inoculum in such cases is superficial and confined to the surface of seed, usually as adhering propagules, e.g., spores sclerotia, mycelium, bacteria, nematodes, virus particles etc. Contamination of seed surface, especially by fungi is often detectable by direct observation under microscope or by examining seed washing.

II. Internally Seed Borne Pathogen:

The inoculum lies with the tissues i.e., this pathogen are carried inside the seed, usually as adhering by vegetative cell, spores, pycnidia, nematodes or virus particles. Dry seed may look perfectly healthy when examined under a binocular microscope and no signs of infection. Seed borne pathogen established with seed coat, testa, pericarp, endosperm and embryo.

III. Concomitant Contamination:

The inoculum is present as contamination mixed with seed in the from of infected debris fungal sclerotic, bacterial ooze, nematode cysts, infected soil particles etc. Such contamination is difficult to detect.

Transmission of Seed Borne Pathogens or Disease:

Seed plays a vital role in the transmission of pathogens directly or indirectly. It is essential to understand precisely how the organisms are associated with the seed and get transmitted. The type of pathogen transmitted includes seeds of plant (phanerogamic plant parasite), nematodes, fungi, bacteria and viruses. 

Plant pathogens are seed transmissible by

(i) adhering to seed surface

(ii) becoming internally established with in the seed and

(iii) Accompanying the seed lot as infected plant debris, soil clad or adhering to containers or otherwise.

Detection Techniques of Seed Borne Pathogens:

Several methods have been developed to detect seed borne micro flora. The method of detection may be general or specific for individual pathogen.

The selection of seed testing method for a particular study is based on certain Objectives. Doyer, (1938).

(i) Testing for quarantine purposes.

(ii) Testing for national seed certification schemes.

(iii) Testing for evaluating the planting value of the seed.

(iv) Testing for storage fungi.

Generally, according to the International seed Testing Association (ISTA) (1999) until and unless otherwise stated, a minimum of 400 seed should be tested for each sample.

Prevention Methods against Seed Borne Pathogens:

An outline of the measures of prevention of diseases due to seed borne pathogens, as modified from Baker (1972), is enumerated below:

(a) Management Practices:

1. Seed source-pathogen free seed.

2. Selection of seed-production area and season when and where the seed is not

3. Likely to carry pathogens.

4. Seed-field inspection.

5. Seed certification.

6. Quarantine.

The measures 1^st to 4^th attempt elimination and 5^th is for avoidance.

(b) Cultural Practices in Seed-Production Fields:

1. Sowing methods, e.g., deep sowing and planting.

2. Pathogen control in seed field- control of weed hosts, production including pre- harvest earhead spray with fungicides as in wheat or rice if there is rain.

3. Avoidance of overhead watering -Ditch irrigation, rather than sprinkler irrigation is favourable for seed crops. This is particularly true in semiarid areas where foliage would otherwise remain uninfected.

4. Harvesting method- Delaying harvesting, and various cares taken during harvesting.

5. Eradication of infected host plants- Applicable when a disease is newly introduced in an area.

6. Ageing of seed to utilize the phenomenon that some seeds remain viable for a period longer than the period of survival of the pathogen, as in cucurbits.

7. Treatment of field soil-Occasionally effective.

The measures are against the directed reduction of established inoculum. The measures 3^rd arid 4^th also reduce inoculum build up.

(c) Curative or Eradicative Measures for Seed already Contaminated:

1. Seed indexing.

2. Separating procedures.

3. Chemical seed treatment.

Seed treating chemicals with low mammalian toxicity such as antibiotics (aureofungin, blasticidin, etc. against rice blast and brown spot), organic sulphur, systemic fungicides and their combinations (thiram+carboxin; thiram+bavistin, etc.) have been developed to replace organic mercury.

4. Thermotherapy of seed:

The above measures are intended for reduction of established inoculum.

(d) Breeding for Resistance against Seed Transmission:

That the amount of primary inoculum should be limited in seed should be the chief objective of control of the diseases due to seed borne pathogens. Thus, breeding disease resistant varieties is likely to be a very successful measure against these diseases. The integration of different measures would be necessary for the evaluation of a recommendation for any specific case.