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Cassava Tuber rot
This disease has been reported in Africa (Fassi 1957)  and Tropical America (Vanderweyen 1962) causing yield loss up to 80 %. Three species of phytophthora viz. P.drechsleri, P.erythuoseptica and P.cryptogea are recorded as primary pathogen in these countries
In India, Cassava tuber rot was recorded  during 1995 from cassava plantations around Salem District in Tamil Nadu. The causal agent has been identified as Phytophthora Palmivora var. palmivora
The disease is endemic to certain pockets. Severe incidence were repeatedly recorded  from Athur, Rasipuram and Namakkal Taluks around salem. The disese incidence was high in Athur (40.5%) which caused 48.5% tuber yield loss. More than 67 % crop loss has been recorded from certain heavily infected gardens.
Rainfall is the most important factor favouring disease infection. Higher the rainfall, higher will be the disease incidence. heavy soil.( Black) with poor drainage facilities favour the disease. Presence of hard soil layerwhich prevent percolation of water to downward cause poor drainage and leads to severe infection . The rotten tubers left during harvest and ploughed insitu serve as inoculum for the next crop and help the pathogen to survive longer periods in the soil . Flooding type of irrigation helps the pathogen for it distribution in the whole field. Lack of organic mater content and narrow C:N ratio of the soil reduce population of saprophytic organisms and biocontrol organisum viz Trichoderma and favour the pathogen.
The causal organisum
The fungus Phyphthora is constantly associated with cassava tuber rot disease. The pathogen was indetified as Phytophthora palmivora var. palmivora was palmivora . The pathogen can be isolated from freshly infected tubers as well as directly from soil by baiting techniques as well as by selective media. cacao pool tissues and carot discs are effective in trapping the pathogen from soil. The PVP medium ( Tsao and Ocana 1969) and BNPRA+ HMT medium ( Masago et al 1977) are efficient selective media for Phytophthora isolation from soil. The main mode of survival of the pathogen in the soil  is through inflected tubers.The longevity in airdry soil was only 6 weeks as against 30 weeks in moist soil.
Symptoms
There will be no external symptom on the plant. Dark round to irregular water soaked lesions apper an mature tubers. White mycelial mats of the pathogen develop around these lesions. The lesions enlarge causing internal browning oozing of internal fluids and shrivelling of the tubers. The infected tubers emit characteristic foul smell and rot with in 5-7 days.
Biologial control
Among the biocontrol  organisms tested, species of Trichoderma viz.T.viride and I harzianum gave good antagonistic activity against the pathogen under laboratory condition. The bacterium Pseudomonas florescens  also gave good suppression of the pathogen. A native strain of Trichoderma viride isolated form heavily infected field at Salem which gave good antagomistic activity as well  as survival ability In Salem soils were multiplied in sorghum grains and used for the management trials
Development of Management Practices

With the available information about pathogen, predisposing factors and role of biocontrol organisms a field management trial conducted at Salem for consecutive years in a heavily infected garden gave promising results.  Most successful treatrement included deep ploughing (up to 24 inches), planting the sets on top of ridges, controlled irrigation ,application of neem cake (250 Kg/ha) and incorporation of Trichoderma Viride
All rtten tubers were removed as part of hygenic cultivation.  The above treatement gave a tuber yield of 45.50 tons/ha with 11.5% disease incidence as againt 25.0 tons yield and 100% infecton in the  control plot (farmers pratice).  The above package being very affective is well adapted by the salem farmers for the management of Cassava tuber rot disease.

 
CTCRI© 2013