SUGAR CANE (Saccharum officinarum) is grown all over the country as one of the important cash crops. Apart from being the main source of sugar, the crop provides essential ingredients for chipboard, paper, chemicals, plastics, paints, synthetics, fibre, insecticides and detergent industries.
Harvesting, transporting and issues of sugar cane and sugar prices as well as export and import of the commodity are some of the typical technical issues, which play a major role in the politics of our country. But apart from these issues, cane diseases are the hidden realities that result some time in 100 per cent crop losses. Many farmers are totally unaware of these diseases. The Ministry of Food, Agriculture and Livestock (Minfal) has decided to bring over 1.0395 million hectares under sugar cane cultivation with an aim to meet the domestic needs of sugar during the current financial year. According to official source, a bumper crop of sugar cane was harvested during the last financial year by bringing about 1.03 million hectares under sugar cane cultivation and producing 54.8 million tons of the crop. The ministry has decided to increase per hectare output of the crop by using quality seeds and providing financial and technical assistance to the growers. But how it could be possible to achieve the target without proper knowledge of the causes, identification and integrated management of diseases damaging sugarcane crop?
Sugar cane suffers a lot from a number of diseases caused by different organisms (fungi, bacteria, viruses and nematodes) and factors such as environmental and physiological disorders and nutritional deficiencies. The diseases damage the crop from the very first stage of growth and development till harvesting.
American phyto-pathological Society (APS) has reported 64 different diseases of sugar cane the world over, which are: five bacterial and 40 fungal diseases, eight are those which occur due to viruses and mycoplasma-like organisms (MLO); three are caused by plant parasitic nematodes and eight are listed as undetermined, miscellaneous diseases and disorders. In our country, 37 diseases of sugarcane, caused by 55 different fungi, bacteria, nematodes, viruses and nutritional deficiency have been reported. According to another survey only eight diseases have been recognised as major and four as minor abnormalities, and 48 species of nematodes associated with sugar cane have been pointed out. Among them, five genera were observed dominant, but no detailed studies have been carried out till now. Whereas another surveyor described five important diseases, 15 were mentioned as insignificant and nematode problems were ignored.
However, altogether 41 diseases of sugar cane have been reported in the country.crop. It is essential for sugar cane growers, environmental protection agencies and pesticide-dealing personals, to diagnose the diseases and adopt successful management of the crop.
Whip smut (Ustilago scitaminea): The affected canes produce long, whip-like and coiled or curved, black shoots, covered with a thin silvery membrane, containing masses of chlamydospores of the fungus. The smutted shoots may arise from the top of the cane or from lateral buds. Later that membrane ruptures and releases a multitude of spores which contaminate soil and the standing crop. In certain cases, the infected plants remain stunted in growth with increased tillering of little value.
Stem canker or wilting of canes (Cytospora sacchari): The affected canes show dryness of leaves from top to bottom. The cane stems are shriveled with considerable reduction in quantity and quality of juice. Small, black, dot like bodies of disease causing fungus may develop on bud sheaths and hollow portions of canes. Some times only a few internodes are affected, but whole stool or only a few canes in a stool may be affected. Red rot (Colletotrichum falcatum): The disease first appears as red bright lesions on the mid rib of leaves and shows itself as drooping and changing of colour of upper leaves. Withering of leaves proceed from top to bottom. Usually third or fourth leaf from the top is affected and shows dryness at the tip. The pith becomes red and later on brown. In severe cases, complete destruction of the stools is brought about. When the infected canes are spilt open they show reddened areas and give out an alcoholic smell due to fermentation.
Leaf spot (Helminthosporium spp): The disease is characterised on leaves as small lesions, which gradually enlarge along the mid rib and assure dark red to brown colour. In severe infection, leaves become dry affecting photosynthesis. Pokkah boeng or distorted top (Fusarium moniliforme): The disease appears in different stages representing development of chlorotic areas at the basal parts of the lower leaves, development of irregular reddish specks or stripes and appearance of top rot followed by total killing. Young leaves may also show pronounced wrinkling, twisting and shortening, depending upon the varieties and climatic conditions. In tolerant varieties there may be recovery of growth, in improved conditions.
Rust (Puccinia melanocephala): Initially light green to yellow coloured, small, elongated spots appear on the lower leaf surface. These spots enlarge and turn orange to reddish-brown lesions. The reddish spores come off easily as a fine powder on touching the lesions.
Red stripe or top rot (Xanthomonas rubriieans): Initially water-soaked yellowish stripes occur on leaves, later on these become dark red. In severe infection, lateral buds show reddening, terminal buds and spindle leaves may die causing top rot, and the vascular bundles exude foul smelling yellowish gum on cut.
Mosaic virus: Mottling of young crown leaves show a pattern of alternating dark and light green coloured patches of varying size and run parallel to the midrib of leaf.
Nutritional deficiency: Sugar cane is considered good indicator plant for potassium deficiency, in which necrotic spots develop on leaf blades, marginal necrosis or leaf scorch also found, while terminal and lateral buds show die back symptoms, confusing with red rot and top rot diseases. Soils with low organic matter produces chlorosis due to nitrogen deficiency, and the leaves usually become red, especially along vein, in case of phosphorus deficiency.
Minor diseases: Other minor diseases of sugar cane are: chlorotic streak virus, ratoon stunting, yellow spot and genetic variegation of leaf and sheath.
Perpetuation and transmission: Smut, stem canker or wilting of canes, red rot, leaf spot and red stripe or top rot diseases are carried over from year to year by ratooning or planting sets taken from smutted shoots of cane. Soil-borne infection may also take place. Wind disseminates all these diseases including rust, while infected sets and aphids transmit mosaic virus, and maize and sorghum are its alternate hosts. However, it can not be asked how the pokkah boeng disease can survive.
Disease management: Resistance is the most easy, economical and less health hazardous precautionary and control measure against all such diseases. Therefore, available disease-resistant varieties should be planted. BL-4 is resistant to red rot, rust and smut. Triton is resistant to rust and susceptible to red rot and borers (vectors of red rot). Co.L-54 is resistant to red rot and susceptible to rust, smut and borers. BF-162 is highly susceptible to red stripe and rust diseases.
• Most of the diseases causing pathogens survive from season to season in the soil or on seed sets and other crop debris in soil and build up to damaging levels with repeated cropping (ratoon and continuous cultivation). Therefore, 3-4 years suitable crop rotation with non-host crops is recommended.
•Many of insect pests and pathogens are killed in direct sun heating and drought; therefore, fallow lands must be plowed at short intervals, with soil converting deep ploughs and the planting should be done in healthy soil.
•Dry sowing of the crop should be carried out, where smut is prevalent.
•The planting time sometimes favours or disfavours multiplication of disease-inciting organisms. Therefore, change in planting time is recommended in various cases. For example, autumn planting should be avoided, especially with reference to smut.
•Disease-free sets of eight-month-old sugarcane nursery may be preferred for planting. Most of the diseases are introduced into fields through infected seed sets; therefore, efforts should be made to obtain disease-free sets. Or seed-sets should be disinfected either in 0.1 per cent mercuric chloride or formalin solution for five minutes followed by two hours covering under a moist cloth. The other effective chemicals available in market may also be used.
•Hot water treatment of sets at 52 degrees C for 18 minutes can help eliminate the internal infection of smut causing fungi.
•Mostly weeds serve as alternate hosts or sources of infection for pathogenic diseases. Therefore, effective weed control practices should be adopted.
•The crop should be irrigated properly, because, excessive or frequent irrigation as well as shortage of irrigation favours the spread and development of many pests and diseases.
•Proper use of fertilisers is also recommended, because, heavy doses of nitrogenous fertilisers may maximise but judicious use of potassic fertiliser minimises the susceptibility of plants.
•Many pathogens survive in and on the plant debris or pruned parts of plants, therefore phyto-sanitary precautions including collection and burning of diseased plant (or parts) from standing crop help minimise spread of the disease. Just after harvesting, these should be incorporated into the soil by ploughing or disking to hasten decomposition.
•Insects hit directly and are major source of transmission of virus and other diseases. Therefore, insects must be controlled through IPM.
•Monitoring or scouting allows for early detection of all pests and diseases for timely implementation of management practices. T therefore, the crop should be monitored or scouted weekly for assessment of the effectiveness of implemented management programmes.
•No doubt, pesticides are needed for effective management of insect pests, diseases and weeds, but with special reference to sugarcane diseases, no severe and serious disease damages are recorded. Therefore, no need of pesticide has been felt till now.
•Ratooning of the diseased crop should be discouraged.