Introduction
Like many countries in the humid tropics, Sierra Leone has a climate particularly conducive to the development and spread of plant diseases. High humidity, rain, and heavy dew are associated with increased activity of disease-causing organisms. As a result, virtually every crop of rice grown in Sierra Leone is subject to at least mild disease infection. In some instances, infection becomes severe, leading to heavy crop losses. This chapter describes several of the most common diseases of rice found in Sierra Leone and touches upon management practices which may help to prevent them.
I. Disease Considerations
The three "ingredients" of plant disease are 1) the host plant, 2) the pathogen, or disease organism, and 3) the environment. Taken together, these comprise the components of the so-called disease triangle.
It is important to recognize that the three components are interdependent. Presence of the host plant and the pathogen does not imply occurrence of the disease. The environment plays a crucial role as well: it must be conducive for the two biological systems (the plant and the pathogen) to interact in a way that infection may take place and disease follow. Consequently, disease control can involve manipulation of any one, or all three, of these components.
Disease infection can be divided into stages: 1) entry of the organism into the plant, and 2) spread of the infection through the plant. Pathogens can enter through weak or injured roots, through mechanically or insect-damaged stems or leaves, through weak outer cell walls, through dead tissue, or through succulent and easily bruised leaves.
II. Diseases Common in Sierra Leone
Listed below are several of the diseases of rice occurring most frequently in Sierra Leone in irrigated crops:
1. Blast (including Neck Blast and Leaf Blast)
Pathogen: Pyricularia oryzae (fungus)
Infection takes place at any stage of growth. Lesions are typically oval or spindle-shaped, with a grayish center and a brown halo. When several lesions join, the infected leaf appears blighted. Mild infections of blast occur regularly in Sierra Leone. Although in most cases the disease fails to kill the affected plants, farmers should realize that lesions on the leaf surface inhibit the plant's ability to photosynthesize and thus reduce yields. Blast is often associated with nutritional imbalances. Note: some varieties (e.g. CCA) are particularly susceptible to neck blast; crop losses of up to 100% have been recorded in some instances.
2. Brown Spot
Pathogen: Drechdleria pryzaae (fungus)
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| Brown Spot |
Brown spot can affect the crop at any stage of growth and often appears as early as the seedling stage. Brown spot starts as small, circular to oval, dark brown lesions with a light yellow halo around their outer edge. (Brown spot can be distinguished from blast by the absence of a grayish center in the lesions.) Infection can spread to cover the entire leaf surface, particularly in stands of upland rice. The disease is often associated with nutritional deficiency and/or drought stress.
3. Narrows Brown Leaf Spot
Pathogen: Sphaerulina oryzae (fungus)
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| Narrows Brown Leaf Spot |
Narrow brown leaf spot resembles brown spot, except -that the lesions are shorter and distinctly more linear. Some lesions have slightly broadened centers. They occur most commonly on the upper leaves and often increase in numbers d ring the latter stages of growth.
4. Sheath Blight
Pathogen: Rhizoctonia solanii (fungus)
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| Sheath Blight |
Initial symptoms of sheath blight infection appear as grayish-green lesions on the leaf sheath between the surface of irrigation water and the leaf blade. Adjoining lesions often merge, weakening the entire stem and causing it to topple and break. The lesions may also extend to the leaf blades, especially among susceptible varieties.
5. False Smut
Pathogen: Ustinaginoidea virens (fungus)
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| False Smut |
Infection by false smut occurs after heading and effects the ripening grains. Infection results in the transformation of the individual grains into greenish spore ball of velvety appearance. The balls are slightly flattened, smooth and yellow, and covered by a membrane. With growth the membrane swells and bursts, exposing the orange content of the ball. Infected grains are rendered inedible.
6. Leaf Scald
Pathogen: rhynehosporium oryaze (fungus)
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| Leaf Scald |
Leaf scald is quite common on mature leaves. The infection forms oblong or diamond shaped blotches. These increase into large ellipsoid olive areas which ultimately dry but and turn gray. Often the entire leaf tip succumbs to the scald, which can be recognized by its characteristic banded pattern of alternating light and dark grays.
III. Management for Disease Control
Because strong plants and a healthy environment can rule out disease even in the presence of pathogens, it is easy to see why management practices play an important role in combating plant diseases. Encourage farmers to adopt as many as possible of the following management practices to decrease the incidence of disease in their crops:
a) Selection of Resistant Varieties
As a method of rice disease control, the selection of resistant varieties is often the most practiced method. The reasons are obvious when one considers the simplicity of the practice. It represents the most economical and sensible approach, and it appeals best to farmers because it is cheap, effective, and within the reach of their means and technical skills. Work with farmers to keep track of which varieties seem most disease-free under local conditions and suggest that they be planted exclusively.
b) 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 evidenced by the clear relationship between diseases such as blast and brown spot and nutritional imbalances. Balance fertilizer applications (i.e. apply fertilizer in several splits instead of all at once) to minimize the risk of nutrition-related disease.
c) Crop Rotation
Crop rotation can disrupt the life cycle 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 the rice plant.
d) During of Crop Residues
Disease organisms do not die when rice is harvested; they either produce spores which linger on in the paddies to infect subsequent crops, or they seek alternate hosts among nearby weeds. Encourage farmers to burn residues left over after the harvest (chaff, stubble, refuse) to kill disease-causing organisms and to destroy alternate hosts.