Yoshida - 1981 - Fundamentals of Rice Crop Science

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4.1 . NUTRITIONAL DISORDERS VS PHYSIOLOGICAL DISEASES The terms physiological diseases or physiological disorders were used to refer to nonpathogenic sicknesses of lowland rice in earlier publications (Baba and Harada 1954, Baba et al 1965, Takahashi 1959a, b, 1961). The causes of such physiological diseases were thought to be related to highly reductive soil conditions in paddy fields. A later investigation revealed that disorders of lowland rice in Asia can be attributed to nutrient deficiencies or toxicities to elements or substances (Tanaka and Yoshida 1970). Thus, the term nutritional disorders is preferred for indicating nonpathogenic and nutrient-related disorders. The term nutritional disorders has a narrower and more specific meaning than physiological disorders. The word physiological can imply both nutritional disorders and stresses such as lowand high-temperature stresses. 4.2. OCCURRENCE OF NUTRITIONAL DISORDERS 4.2.1. Cause of nutritional disorders Nutritional disorders are caused by a nutrient deficiency or toxicity to an element or substance. Under certain circumstances, deficiencies of more than two nutrients or toxicities to more than two elements or substances are compounded. The behavior of nutrients and toxic substances in submerged soils in relation to plant response has been discussed in the preceding chapter.
178 FUNDAMENTALS OF RICE CROP SCIENCE This chapter focuses on the diagnosis of nutritional disorders. However, before discussing the standard diagnostic techniques, two problems commonly encountered when we attempt to diagnose nutritional disorders will be examined: visible symptoms and nitrogen response. 4.2.2. Visible symptoms The problem begins when unusual symptoms are observed on the rice plant in the field or when an extension specialist or farmer reports unusual symptoms. The unusual symptoms could be caused by diseases, insects, low or high temperatures, and nutrient deficiencies or toxicities. Some disease symptoms are easily confused with those of nutritional disorders. Blast, Helminthosporium, Cercospora, and grassy stunt virus are common tropical-rice diseases that produce brown spots on the leaves. It is extremely difficult to differentiate between zinc deficiency and grassy stunt virus in the early stages of symptom development. For this reason, it is advisable to become familiar with disease symptoms and to consult pathologists when pathogenic diseases are suspected. Unusual symptoms may attract attention when there is a shift from traditional varieties to improved ones. In the Kala Shah Kaku area near Lahore, Pakistan, there was a rice disorder locally known as Hadda or bronzing because of a brown discoloration of the leaves. This disorder had been known for many years but the symptoms were quite mild in local traditional varieties grown with little or no nitrogen. The symptoms became pronounced after the introduction of improved varieties, which are normally highly fertilized. It was later learned that the symptoms, due to zinc deficiency, were aggravated by increased applications of nitrogen because the rice plant requires more zinc at high nitrogen levels (Yoshida and Tanaka 1969, Yoshida et al 1970). The unusual symptoms could also be caused by wrong management practices done by a farmer himself, such as inadequate application of chemicals. 4.2.3. Nitrogen response Rice yield normally increases when the amount of nitrogen application is increased. If there is little or no yield increase in response to increased nitrogen application, there may be some problem involving variety, soil, or climate. One of the most popularly known examples comes from comparing traditional tall varieties with improved short varieties (Chandler 1968, De Datta et al 1968). Traditional tall varieties fail to yield more in response to increased nitrogen applications mainly because they tend to lodge at high nitrogen levels. Even after the introduction of improved varieties, a rice crop may not respond to increased nitrogen (Table 4.1). At Dokri, Pakistan, the rice yield increased from 5.6 t/ha to 10.3 t/ha with increased nitrogen rates, whereas at Kala Shah Kaku, the yield decreased from 5.7 t/ha to 4.9 t/ha as nitrogen application was increased up to 134 kg N/ha. This negative response was later found to be due to zinc deficiency (Yoshida et al 1970).
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FOREWORD Rice is vital to more than
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CONTENTS Foreword Preface GROWTH AN
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3.2. Adaptation to Submerged Soils
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4.5. Corrective Measures for Nutrit
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1 .1. OUTLINE OF THE LIFE HISTORY T
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GROWTH AND DEVELOPMENT OF THE RICE
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Table 7.6. An example of high yield
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REFERENCES AGRICULTURAL POLICY STUD
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Yoshida - 1981 - Fundamentals of Rice Crop Science

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