Red Rot of Sugarcane :
A. OCCURRENCE AND IMPORTANCE
One of the most serious and important disease of sugarcane is the red rot. This disease mainly occurs in the tropical and subtropical regions of the world.
In India, the disease appeared in epiphytotic form in Northern India (U.P. and Bihar) during 1939-1940 and 1946-1947 seasons. The disease attacks standing canes and causes huge losses to the cultivator and millers.
Red Rot of Sugarcane Classification
The form-genus Colletotrichum belongs to the form-family Melanconiaceae, form-order Melanconiales, form-class Coclomycetes, sub-division Deuteromycotina and the division Eumycota.
According to Sutton (1973), there are 11 species of Colletotrichum. Alexopoulos and Mims (1979) reported 1000 form-species of Colletotrichum of which a majority and them seem to be synonyms. According to the latest account of Baxter et e; (1985) there are 21 species of this form-genus.
B. RED ROT OF SUGARCANE SYMPTOMS
Colletotrichum fulcatum is a common species which is responsible for the red rot disease of sugarcane. This fungus mainly attacks stem and leaves of the sugar-cane plant. The leaves of the upper portion of the plant become pale or dark red and which ultimately droop down.
The stem splits and many red coloured longitudinal streaks are formed on it. The red colouring is mainly in the vascular bundles and often in the pith.
At severe infection the stem gets rotten, shrink at internodes and become dull in appearance. The pathogen attacks all the parts above the ground, but more especially the canes (stems) and midribs of leaves.
In early stages disease is not recognisable in the field. First symptoms appear after the rainy season when the growth of plant stops and sucrose formation begins.
Loss of colour and drooping of leaves (3rd or 4th from the top) are the earliest symptoms, gradually the entire tip withers. In later stage canes become shrivelled, the rind sinks and become longitudinally wrinkled.
When the diseased canes (stems) are split open, the tissues of the internodes will be found longitudinally reddened (normally white or yellowish-white) in one or more internodes.
This red colour is interrupted by white patches extending crosswise on the canes. In case of true red rot the red colour extends through many internodes.
Infection originates in the midribs of leaves as a dark-reddish area which elongates rapidly forming blood-red lesions with dark margins.
The cane-extract (juice) often gives bad odour and does not set well on boiling, because of the conversion of sucrose into glucose and alcohols by the action of enzymes of the pathogen.
Late in the season dark-coloured (black), minute, velvety dots (acervuli of the fungus) are formed near about the nodes of the diseased canes and also in the shrunken areas.
C. THE CAUSAL ORGANISM
Red rot of sugarcane is caused by Colletotrichum falcatum Went. This pathogen is a deuteromycete (fungi imperfecti). The perfect stage of this pathogen is Glomerella tucumanensis Arx & Muller or Physalospora tucumanensis Speg. It is an ascomycete.
E. DISEASE CYCLE OF RED ROT OF SUGARCANE
The disease is chiefly seed-borne and spreads to the healthy crop through setts (i.e. sugarcane seeds) taken from the diseased canes.
But, the pathogen can also exist in the soil in the form of conidia and chlamydospore-like structure for a specific period.
The conidia, after falling from the diseased canes may germinate in the soil and form appressoria which often act like chlamydospores.
Sugarcane developing from diseased setts are always diseased. If healthy setts are planted in a field where viable conidia, chlamydospore-like structures, viable mycelia etc. are present in crop-debris, then infection takes place through roots.
Besides, ratooning of a diseased crop is another effective method of survival and recurrence of the disease. Conidia of the fungal pathogen are produced in the rind of diseased canes, on midrib and the leaf blade-surface of the leaf.
On falling to the soil they are disseminated by irrigation water and cause the secondary infection. Dispersal of conidia also takes place by air, rainwater and insects. buds and leaves of healthy canes are infected by those conidia.
- Use of healthy seed setts is an important control measure against the disease. Setts for seed should be selected from healthy canes. For seed purposes, sugarcane should be grown in selected plots under special supervision.
- The disease occurs in epiphytotic form due to the accumulation of inoculum in the soil year after year. Hence field sanitation is necessary for checking this inoculum build-up. As soon as isolated cases of the disease are noticed in the field, the entire plant should be dug out and burnt.
- Crop-rotation after a lapse of 2-3 years is also beneficial.
- Treatment of setts with a 0.25% solution of Aretan-6 or 1-2% solution of Agrosan GN before sowing reduces the incidence of disease.
- The best control measure is the use of resistant varieties. Following varieties have been recommended as resistant to red rot: Co.846; Co.951; Co.1007; Co 1148; Co. S.109; B.O.3; B.O.7; B.O. 32; Co 561; Co.562; Co. K. 32; Co. S. 109; etc.