In this article we will discuss about the red rot of sugarcane caused by deuteromycetes.
Host: Saccharum officinarum L.
Pathogen: Colletotrichum Falcatum
Introduction to the Red Rot of Sugarcane:
It is a serious and destructive disease of sugarcane. It occurs in tropical and subtropical regions of the world where sugarcane is cultivated extensively. In India, it occurs in most of the sugarcane growing states particularly Bihar, Uttar Pradesh, Madhya Pradesh, Haryana and Punjab.
Symptoms of Red Rot Disease:
The symptoms of the disease become apparent after the rainy season on the stems (B) and leaves (C). The earliest symptoms are the yellowing and drooping of the upper leaves (near the tip).
In the later stages the stems shrivel, the rind loses its bright colour, becomes dull in appearance and longitudinally wrinkled. If the diseased stems are split open lengthwise the white pith will be found reddened with patches of white running transversely across.
At the advanced stage of disease blood red lesions with dark margins develop on the mid ribs of the host leaves (B). With the formation of asexual fruit bodies (acervuli), these lesions become covered with powdery masses of conidia which serve to spread the disease.
Late in the season, acervuli appear as dark, velvety dotlike structures in the shrivelled areas on the internodes and near the nodes of diseased canes.
Effects of Red Rot Disease:
The red rot disease attacks the standing sugarcane crop and thus causes enormous losses. In an epidemic form, it causes heavy reduction in stands and even threatens to wipe out the entire crop in the field.
Due to enzymatic action of the pathogen, there is conversion of sucrose in the cane juice into glucose and alcohol. This loss in sucrose may be as high as 33%. Consequently there is reduction in the recoverable sugar in the factory.
Causal Organism:
The causal agent of the red rot disease of sugarcane is the form-species Colletotrichum falcatum Went, of form-class Fungi Imperfecti. The perfect or sexual stage of the fungus has also been found by Arx and Muller. It is described in the Ascomycetcs as Glomerella tucumanensis.
Mycelium of Red Rot Disease:
The fungus mycelium is found within and in the intercellular spaces between the pith cells of the host. The hyphae constituting the mycelium are thus both inter- and intracellular. They are slender, colourless, branched and septate.
The cells contain droplets of oil. The hyphae produce terminal or intercalary, thick-walled greenish black chlamydospores in the pith cells. The spherical chlamydospores remain dormant in the soil for a long period.
Asexual Reproduction:
Finally the mycelial cells begin to collect beneath the epidermis to form stromata consisting of densely packed cells. From the upper surface of each stroma arise the conidiophores and bristles called setae (C).
The setae are rigid, long hair-like structures measuring 100-200 µ long and 4µ broad. They are septate, each with 4 septa. They either form a ring around the stroma or are intermixed with the conidiophores in the acervulus.
As the setae grow and push their way up, the overlying epidermis ruptures and exposes the conidia borne on small conidiophores in the acervulus (C).
The conidiophores are small, unscptate structures measuring 20 µ long and 8 µ broad (C). The conidia are one-celled and usually sickle-shaped (falcate) in form (D). They are hyaline, measure 16 to 48 µ long and 4 to 8µ broad.
Each conidium has a large oil globule in the centre. The conidia are readily detached and dispersed by air currents, rain, rain drop splashes and insects. They are short-lived and thus germinate immediately in the presence of moisture.
On germination, each conidium produces a germ tube which develops either into a mycelium or produces an appressorium on coming in contact with a hard surface (E). The appressorium is a thick-walled structure which functions like a chlamydospore.
Chona and Srivastava (1952) found the perithecial stage of the pathogen for the first time in India in cultures at the I.A.R.I. New Delhi. A year later, Chona and Bajaj (1953) reported its occurrence in nature on sugarcane leaves.
Disease Cycle:
The incitant lives from one growing season to the next on the debris of the diseased plants. The conidia are short-lived and thus play no role in the perennation of the pathogen, the thick-walled chlamydospores and perithecia are considered as probable means of survival.
Chona (1950) and later Chona and Nariani (1952) reported that the fungus is capable of growing and producing acervuli in the soil. It thus survives in the active stage for 3 or 4 months. The survival of the mycelium for this limited period is sufficient to provide easy catching of the succeeding crop because sugarcane practically has no dead season.
There is evidence to show that the disease is borne in the seed sets which serve as chief means of survival and spread of the disease. The diseased sets sown in the soil, sprout into infected shoots which soon produce conidia in the acervuli.
The conidia serve as a secondary means of infection and spread of the disease. They get detached and are dispersed through the agency of wind, water and insects. On reaching the surface of the healthy sugarcane plants, they germinate immediately in the moisture retained in the enclosing sheaths.
Penetration into the host takes place at the nodes through the wounds left by the tearing of leaves or caused by insects.
Control Measures of Red Rot Disease:
Field sanitation is an important measure to prevent the build source of primary inoculum. It consists in the collection and burning of sugarcane trash in the field. The other equally important preventive measure is the use of sound and healthy seed sets.
Long rotation of crops minimizes soil borne infection. The use of resistant varieties is the most effective method of controlling the disease. The following CO and BO varieties have been recommended to be resistant to red rot disease, Co. 846, Co. 951, Co. 1148, Co. 561, B.O.3, B.O. 7 and B.O. 32.
