Diseases make plants function improperly and are recognised by their symptoms and associated visible changes caused by the organisms called pathogens. Many species of bacteria, fungus, nematode, virus and mycoplasma-like organisms are responsible for rice diseases. Disorders or abnormalities may also be caused by abiotic factors such as low or high temperatures beyond the limits for normal growth, deficiency or excessive nutrients …
Diseases make plants function improperly and are recognised by their symptoms and associated visible changes caused by the organisms called pathogens. Many species of bacteria, fungus, nematode, virus and mycoplasma-like organisms are responsible for rice diseases. Disorders or abnormalities may also be caused by abiotic factors such as low or high temperatures beyond the limits for normal growth, deficiency or excessive nutrients in soil and water, pH and other soil conditions affecting the availability and uptake of nutrients, toxic substances such as the H2S produced in soil, water stress and reduced light. In broad sense such disorders and abnormalities are referred to as physiological diseases.
The rice crop is subjected to more than 40 diseases which are the main factors for low yield. The disease may appear at any stage of plant development, attacking the seed, root system, foliage, stalk, leaf sheath, inflorescence and even the grain.
Disease management:
Vigorous plants and a healthy environment can rule out a disease even in the presence of pathogens. It is easy to see why management practices play an important role in combating plant diseases.
Development of resistant varieties:
To control rice disease, the resistant varieties is often selected. It represents the most economical and sensible approach and appeals best to farmers because it is cheap, effective, and within the reach of their means and technical skills. Effective resistance can be achieved by combining into the same cultivar, different race-specific genes and genes conferring quantitative resistance. Another method is by deploying resistance genes in mixed plant populations.
Recent studies indicate that the use of cultivar mixture is an effective tool in disease management. Inter-planting prevents the fungus from continuous build-up of inoculums that may have previously occurred in the monoculture fields. Balanced fertilization: Nutritional imbalances, both deficiencies of nutrients and excesses, render rice plants much more susceptible to attack by pathogens.
Deficiencies result in a general decline in the plant?s health, while excesses can cause a sudden growth spurt leading to a weakening of cell walls. Either condition invites attack as is evident by a clear relationship between the diseases such as blast and brown spot and nutritional imbalances. Balanced fertiliser applications (i.e., apply fertiliser in several splits instead of all at once) minimises the risk of nutrition-related disease. An oversupply of nitrogen will result in luxuriant crop growth that would favour the disease while a deficiency will result in low yield.
Crop rotation:
Crop rotation can disrupt the lifecycle of some pathogens (e.g., fungi) by removing from the environment the necessary host plant. Crop rotation can also discourage multiplication of soil-borne organisms which live among the roots of rice plant.Seed health: Since the pathogen is mainly seed-borne, clean seeds should be used to minimise its occurrence.Salt water can be used to separate light weight, infected seeds from seed lots; this may reduce seed borne inoculums.Biological control: Rice disease management strategies mainly aim at prevention of outbreak or epidemics through the use of host plant resistance and chemical pesticides. The persistent and injudicious use of chemicals has toxic effects on non-target organisms and can cause undesirable changes in the environment.Most of these chemicals are too expensive. Large-scale and long-term use of resistant cultivars is likely to result in significant shifts in the virulence characteristics of pathogens, culminating in resistance breakdown.
Biological control:
It is another potential option for rice disease management. Bio-control assumes special significance being an eco-friendly and cost effective strategy which can be used in integration with other strategies for a greater level of protection with sustained rice yields.
A number of fungus, bacteria, virus, nematode and mycoplasma-like organisms cause disease to rice plants and are considered as a serious constraint for rice production.A diverse group of bio-control agents such as bacteria, fungi, viruses exist in nature.Among them bacterial antagonist are considered ideal candidates because of their rapid growth, easy handling and aggressive colonising character.Bacterial antagonists, pseudomonas, and bacillus in particular are good candidates for biological control.Bacilli are germ-positive endospore-producing bacteria that are tolerant to heat and desiccation; a very good feature required for field application.The pseudomonas are germ-negative rods and have simple nutritional requirements; they are excellent colonisers and widely prevalent in rice rhizosphere.
Potential advantages of biological control: decreased disease intensity lead to higher production; reduced use of chemical fungicides and nematicides; reduced likelihood of undesirable effects (environment pollution, effects on non-target organisms, resistance development against pesticides) from chemical pesticide; provides greater flexibility in rice disease management; can play a key role in integrated management of rice diseases; safe for the users and farming community.
Potential disadvantages of biological control: may have deleterious effects on non-target micro-organisms; pathogens may develop resistance to the bio-control agent; pathogen replacement may follow control of target disease pathogen. Seasonal/weather phenomena can make bio-control agent ineffective.
