Inside the beta issue
- Viral transmission
- Cultivation of boro rice
- Moments of college
This article is about two types of virus transmission
Viruses transmit by two main methods –
- Artificial method
- Sap of mechanical transmission: The transmission of a virus from infected plant to healthy tissue is a procedure that is fundamental in the study of virus disease. In laboratory, this is accomplished by grinding the leaf of a diseased plant and rubbing the infectious sap onto the leaf of a healthy plant. The procedure is referred to as mechanical transmission.
- Grafting: This method was particularly useful for viruses that would not mechanically self-transmit and for which no other natural transmission method are known. Virus transmission by grafting occurs most readily when a good union is established between the scion and stock. Various methods of grafting (wedge grafting, sliced approach graft, tongue graft) are used. But wedge grafting is mostly used in plant virology.
- Dodder : The various dodder species have different ranges of host plants and in nature Cuscuta compestris and Cuscuta subinculsa have wide host ranges and have been used to transmit experimentally a number of plant viruses . Dodder transmission is similar to graft transmission, but whereas graft is limited to closely related species dodder transmission is generally low specific.
Procedure: First we have to raise the dodder plant on the diseased plant then put one or two branches to a healthy plant. They spread by sucking the sap through their haustoria. It can isolate the infected viruses. It requires more time than vector and sap transmission.
- Natural method
- Transmission through contact: Worker’s hand contaminated with the juice of the diseased plants are capable of infecting healthy leaves. Potato viruses are transmitted through cutting knives or by touching infected tuber. TMV was first shown to be transmitted through contact between diseased and healthy plant.
- Vegetative propagation method: Many viruses are transmitted from the parent to the offspring at the time of vegetative propagation by tubers, bulb, offsets and cuttings etc. Mosaic and the leaf role of potato and sugar cane virus are common examples of this method.
- Insect transmission: It is the most important and common method of transmission of plant viruses. Important insets are beetles, leaf hoppers and the aphids. The aphid transmitted viruses are first divided into two groups.
Persistent and non-persistent
In the former the viruses are lost by the vector usually after a short period of feeding which in case of persistent, they retain in the vector for long period. The insect obtain viruses through their mouth part at the time of feeding of the diseased plant. It is then ino
culated in the healthy plants.
In some cases the vectors can’t infect the healthy plant immediately after it has fed on the diseased plant. There is thus a delay in development of infective power in the vector. This period of infectivity for the virus within the vector is called as incubation period. The duration of incubation period with different viruses ranges from few hours to few days.
Leaf hopper transmitted viruses –
- Rice dwarf – Nephotettin migropictus
- Rice tungro – Nephotettin virescens
- Rice grassy stant – Nilaparvata lugans
- Potato yellow dwarf – Agallia constrica
Trips transmitted viruses –
- Groundnut bud necrosis – Trips tubaci
- Tomato spotted wilt viruses – Frunlelinella fusca
Mite transmitted viruses –
- Wheat streak mosaic and wheat spot mosaic – Acoria tulipae
- Pegion pea sterity mosaic – Aceria cajani
- Southern bean mosaic and cowpea mosaic – Cartma trifurcuta
Mealybug transmitted virus –
- Coco swollen shoot virus – Planococcoidas njalensis.
- Soil transmission: Most of the viruses transmit through soil, are carried either by nematode or fungi and the process is largely biological. Wheat yellow mosaic virus, oat mosaic, tobacco mosaic, diseases are transmitted through soil born fungi (Chitrids).
- Pollen transmission: It is probable that pollen born viruses enter the ovule along with the male gamete by passing through the pollen tube as it goes into the embryo sac. e.g – Bean common mosaic, Alfalfa mosaic, cherry leaf roll etc.
- Fungal transmission: There are two diseases transmitted by fungi which is soil inhabiting. There are two orders –
- Nematode transmission
- Seed transmission: Mc Clintock (1916) first reported the seed borne virus (plant) in case of cucumber mosaic virus. After 3 years in 1919, Reddicles and stwart confirmed the results of Mc. Clintock. Variation in rate of seed transmission is due to the stage at which the plants are infected. It is of two type – External seed borne viruses (They are systemic in nature. These viruses are adhering on the seed coat. They are extremely carried by raw seed) and internal seed born viruses ( These viruses multiply during seed development and accumulate on the endosperm or embryo)
Time of sowing – November-December
Recommended varieties –
Land selection: Low land area with irrigation facility is suitable.
Seed selection: Recommended varieties of seeds are selected.
Seedbed preparation: Seed beds are prepared by plaughing and leveling. It is recommended to sow the seeds in beds of widths – 2m and desired length of at least 10 meter.
Seed rate is 40-45 kg/ha
Application of manure in the bed:
20-30 kg – Cowdung
80g – Urea
80g – SSP
40g – MOP
Sowing in nursery: Seeds are sown by broadcasting in the prepared field.
Low polytunnel technique: To prevent low temperature injuries, seedlings are covered under a polythene shed from December to middle of January. The height of the polythene shed should be 75 cm.
Preparation of the main field: The main field is prepared by three to four times plaughing, lathering and leveling.
Fertilizer and nitrogen requirement:
In low lying area where water is logged, fertilizers are not applied. But in irrigated condition fertilizers are required as the following amount –
N – 60
P – 60
K – 30
Fertilizers are applied in the following procedure –
Full dose N, P, K is applied as basal dose if applied in rain fed condition. But in case of irrigated condition,
Nitrogenous fertilizers are 3 parted and applied 1/3rd as basal dose, 1/3rd at tillering stage and 1/3rd at panicle initiation stage.
Spacing should be 25 x 20 cm (in tall variety) and 20 x 20 cm in case of dwarf variety.
Depth of planting should be 5 cm and number of seedling per hill should be 2-3. Dead hills are replaced by new hills after 7-10 days of transplantation. This process is called as gap filling.
Irrigation water is supplied to a height of 3 cm 3 days after disappearance of ponded water. Otherwise water is supplied at certain stages of tillering, rooting when water is deficit.
Weeding is done 2 times at 20-40 days after transplanting. Weeding is generally done manually, but if not possible herbicide is applied.
Application of herbicide:
- Pre-emergence herbicide – Butachlore @ 0.75 kg active ingredient/hactor, Pretilachlore @ 0.75 kg/ha mixed with 50 kg of sand and applied 3 days after transplanting.
Post-emergence herbicide – 2,4-D-sodium salt @ 1.25 kg/Ha in high volume sprayer on 15th day of transplantation.