Cassava mosaic disease

A severel mosaic disease was recognized as a serious threat to cassava cultivation in India as early as 1942. The first report on this disease was made by Alagianagalingam and Ramakrishnan (1966) African cassava mosaic disease reported by Walburg in 1984 occurs in African countries ad West Pacific islands ( Lozano and Booth 1976). There are six different viruses recorded causing mosaic symptoms on cassava.

Symptomatology

The first symptom appear on young leaves as chlorotic speck. Grandually they enlarge and intermix with green tissue to provide a mosaic pattern The pale discoloration may be intensified to yellow colour depending on the varieties. The leaf area is reduced and in extreme cases leaf distortion and shoe string appearance are observed. Intensity of symptoms varies with season. During humid cool and raining stage. the symptoms eepression reduced. There may be symptom variation in the same plant. In serve case of infection, the growth of the plant is affected which ultimately leads to crop loss

Disease index

The severity of the disease is assessed by using the following  scale Grade 1- Only specks < 100 sq.mm Grade 2.- wide areas of mosaic, no distortion Grade 3 –Distortion, leaf area reduction 25 % Grade 4-Distortion, leaf area reduction 25-75% Grade 5-Distortion leaf area redction >75 % Maximum intensity of the disease is observed during August –October

Causal organism

Cassava mosaic disease is caused by a single stranded DNA virus belongs to Gemini group. The Indian Cassava mosaic virus ( ICMV) is serologically different  from African cassava Mosaic Virus (ACMV). Two biotypes of ICMV viz Indian Strain and Srilankan strain are recorded.

Transmission

The disease is easily transmitted by grafting, dodder ( Cassytha filiformis) though whitefly Bamisia tabaci from cassava to cassava. Sap transmission of the virus could be achieved only from cassava to Nicotiana. No seed transmission of the virus could be achieved. It has been recorded that a particular biotype of the on white fly B.tabaci is moe virulent in the transmission of ICMV.

Serology

ICMV obtained from Nicotiana leaves purified and used as antigen. The poly clonal  antibodies abtained from rabbit serum is an efficie tool for the detection of virus infected plant and is used as a tool for large scale section of virus free planting material.

Spread of the disease

The primary spread of the disease is manly through indescrimiate use of infected planting material. The secondary spread in the field from disease plant to healthy is thought the whitefly vector Bemisia tabaci. Complete of planting material though secondary  spread occur with in a perod of 4 years.

Variete reaction

Though all available cassava  varieties takeup CMD infection, symptom expression vary with different varieties several highly susceptible local  varieties viz kalikala , Arimaniyan etc are totally extinct due to heavy infection. The hybid varieties from CTCRI viz H-165 H-226 etc though susceptible to ICMV provide sufficient tuber yield Tripoids 4-2 and 5-3 show good field tolenrace to ICMV. Recently introduced cassava line from IITA Nigeria   Nmga-1  have high degee of field to lerance to ICMV

Biochemical changes  caused by ICMV infection

Infection  reduce HCN content  in both leaves and tuber . Reduction of total phenols flavanoids ,ascorbic acid & calcium content of  leaves were also reduce due to infection. Increase in phosphorous, potash & aminoacid  contents  are recorded due to infection.

Yield loss

Tuber yield of diseased plants vary with different varieties as well as time of infection . Upto 80%crop loss has been  recorded in highly susceptible varies as against 18% in field tolerant varieties. Crop loss will be maximum when the disease appeare at the time of planting .No reduction in yield has been recorded when the disease   occurred after 5months of planting .

Tissue culture

Cassava meristem tip is free from the viruses .Hence meristem tip culture is an efficient tool for the elimination of  CMD and regeneration of disease free planting material .The synthetic Murashige &Scoog Medium is very efficient for the sucssesful cassava meristem culture .Trials showed that meristem  derired plants gave higher tuber yield than normal plants .

Transgenic plants

Resistant cassava plants could be produced against  African cassava mosaic virus through coat proterin mediated resiant genes (CPMR).  Similar attempts are made at CTCRI agaist ICMV . The resistant gene could be successfully introduced into cassava genome  through bacterial vectors.

Management of the disease

CMD can be effectively managed though the adoption of the following practices. Use of disease free planting materials Use of field toleranct cultivars-viz H-165, Sree Vijaya, 4-2,5-3, MNga-1,MVD-1, C 0-3,C 0-4 etc Timely roguing of infected plants Use of trap crop viz Redgram, maize etc Use of meristem derived plants Use of true seed plants Nursery techniques Transgenics