Yoshida - 1981 - Fundamentals of Rice Crop Science

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).