AGR-22 MANGANESE TOXICITY IN BURLEY TOBACCO
J. H. Smiley, W. O. Atkinson and Ira E. Massie
Department of Agronomy
Importance and Symptoms
Manganese toxicity is a major disease
of tobacco in Kentucky. This disease causes plants to grow slowly following
setting, turn light green or yellowish between the larger veins, and gradually
develop numerous dead spots, particularly in the older leaves. This disease
is often so severe that the plants are badly stunted or killed, thus reducing
yield and quality. Occasionally, however, plants recover and develop a
normal green color in the younger leaves later in the season. Manganese
toxicity can be easily prevented, but once it occurs in the field, little
can be done for that crop.
Manganese toxicity and soil acidity
go hand in hand. As the soil becomes more acid, greater amounts of manganese
are available, and the tobacco plants take up more than they need for normal
growth. Soil pH 5.0 - 5.5 is the critical level for manganese availability.
When the soil pH is 5.5 or higher, trouble from manganese toxicity seldom
occurs, but at 5.0 or lower, toxicity in tobacco is very likely. Between
pH 5.0 - 5.5, toxicity may or may not cause trouble in any particular year.
When a soil becomes acid, not only
does the amount of available manganese increase with resulting manganese
toxicity, but the availability of other important nutrients such as phosphorus,
nitrogen, and potassium is reduced. Figure 2 shows that a soil with pH
above 5.8 gives the greatest availability of major plant nutrients.