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The following points highlight the top three successive phases of seed transmission. The successive phases are: 1. Seed to Plant Transfer 2. Plant to Seed Transfer 3. Seed to Seed Transfer.
Seed Transmission: Successive Phase # 1. Seed to Plant Transfer:
Neergaard (1977) has classified seed to plant transmission of seed borne pathogens in to eight groups.
(i) Intra-Embryal Infection Followed by Systemic Infection:
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In an infected embryo the pathogen becomes active with the germination of seed and continues to follow the growing points of the plant and finally reaches the floral organ.
(ii) Intra-Embryal Infection Followed Buy Local Infection:
The embryo is infected but the pathogen is spread from primary infection to leaves, petioles and stem to produce local lesions.
(iii) Extra-Embryal Infection Followed by Systemic Infection:
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The pathogen gets established outside the embryo i.e., in the pericarp, seed coat or endosperm and remains viable in the seed for a considerable period. It grows in to the young plant during the germination of the seed and gets systemically distributed as the plant growth continues.
(iv) Extra-Embryal Infection Followed by Local Infection:
The seed is infected outside the embryo (Pericarp, seed coat, endosperm). The pathogen is carried on the cotyledons or in the seed coat during germination. They may be transmitted to the young plant by air, rain, insects etc., and penetrates into the host to cause local infection.
(v) Seed Contamination Followed by Systemic Infection:
The pathogen is externally seed borne which contaminate the healthy seeds. Infection of the very young seedlings occurs during germination of the seeds. The pathogen continues to grow upward with the apical meristem.
(vi) Seed Contamination Followed by Saprophytic Dormant Stage and then by Local Infection:
The pathogen is transmitted through seed as a contaminant and lives for a period of time saprophytically or parasites in a resting stage in the soil or in plant debris and infect the host at a later time.
(vii) Seed Contamination Followed by Saprophytic Stage and then by Systemic Infection:
The pathogen is transmitted through seed as a contaminant and lives saprophytically for a limited period. Under favourable conditions if the host is present causes infection.
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(viii) Seed Contamination Followed by a Non-Parasitic Phase Leading to Organ Specific Seed Infection:
The ovary is transformed into a sclerotium or a gall and gets mixed with seeds during threshing. After sowing, the pathogen lives for a period of time saprophytically or saprozoically or as a resting stage in the soil. The pathogen infects the host later at the time of flowering.
Seed Transmission: Successive Phase # 2. Plant to Seed Transfer:
According to DeTempe (1979) Pathogen is plant to seed transfer by following five types:
(i) Pathogen Spreads into the Seed from the Fruit:
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They are usually internal and transmit through vegetative cells, spores, pycnidia, nematodes, viruses and bacteria.
(ii) Pathogen Penetrates into the Seed through the Vascular System:
A few pathogens are known to infect seeds through the vascular elements. The pathogens move from root up to the seed through xylem vessels. Bacteria are systemic in the vascular system and move into the seeds.
(iii) Pathogen Penetrates the Embryo through the Pistil or Ovary:
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In Bean common mosaic if virus infection of the mother plant occurs earl, the virus ma spread into the ovule. Infection of the embryo may also occur during fertilization b pollen from an infected plant.
(iv) Pathogen Actively and Directly Penetrates the Seed:
In this type the seed is directly invaded by the pathogen.
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(v) Pathogen a Passive Contaminant of Seed:
Spores, vegetative cells, sclerotia, nematode or virus particles of infected plants get on seed in threshing.
Seed Transmission: Successive Phase # 3. Seed to Seed Transfer:
It occurs during threshing, cleaning and planting operations. A pathogen from one lot may contaminate the clean seed of other lot through handling equipment. Pathogen with stick spores may adhere to clean seed during threshing.
Several factors govern the transmission of pathogens from plant to seed and seed to plant which may be:
(i) External based on environment like temperature, humidity, light, soil type, soil fertility etc., and
(ii) Internal like resistance or susceptibility to pathogen based on genes. Cultural practices like date of sowing, plant population, irrigation, fertilizer application, harvesting method, threshing and other post-harvest technology also act as important factors in the transmission of pathogens.
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Seed transmission of pathogen is important for the following reason:
(i) It ensures that the pathogen is present with the host.
(ii) From a properly mixed seed lot seed borne inoculums may be evenly spread throughout the field.
(iii) The presence of a pathogen with the seed favours the earliest possible infection of the seedling.
(iv) Seed transmission may bring into an infested area new race of pathogens more virulent than those already present.
(v) Un-infested soil may be inoculated from infected seed.