The Tobacco Plant

Developments in Growing Tobacco

E. E. Clayton.

During the early years of tobacco culture in the United States, the established practice was to grow tobacco only on new land. Trees were cut down, forests cleared, and the fields so made were planted to tobacco for a few years. The round was repeated as new lands and settlements opened farther West.

Under the system there was no tendency for disease problems to accumulate. But the supply of new land began to run out just before the turn of the century, and the next 30 years saw a rapid increase in disease problems. Granville wilt, black shank, and blue mold appeared then; under the new system of continuous cultivation in the same areas, growers found out that tobacco as a cultivated crop is subject to a whole series of diseases.

The uncertainty of the crop and the unhealthy conditions of many fields that were cropped repeatedly to tobacco led to the belief that tobacco was "hard on the land." Growers of burley in Kentucky learned that their crops did not grow well unless the land was "rested" a number of years between crops. We now know that the rest period was needed to reduce the amount of black root rot in the soil and to improve the physical condition of the soil not because of exceptional demands on fertility.

The first line of defense against the diseases has been crop rotation. It is only moderately effective. It does most good against nematodes (root knot) and least against black shank and Granville wilt.

Hopes next were pinned to resistant varieties. Now we combat the diseases by using both rotations and disease-resistant sorts a combination that is better than either one alone against such diseases as wilt and black shank. Soil treatments are available for plant beds as supplements to control weeds and diseases. Fungicides can be used to control blue mold. But chemical control measures are not popular with the tobacco farmer, who is typically a small operator, although fungicidal treatments for blue mold are well established in major areas.

BLUE MOLD is commonly a disease in plant beds. It first became established in the commercial tobacco-producing areas of the Southeast in 1931. Before then it occurred on wild Nicotiana (related to our cultivated tobacco) in Texas, California, and other Western States. Before 1931 the disease appeared in northern Florida, but failed to establish itself there.

The fungus that causes the disease produces a durable resting spore that lives over winter in the soil. For that reason old plant beds are primary sources of mold infection each spring. The first occurrence is generally in early February in the southern part of Georgia.

The early symptoms of blue mold vary somewhat with the age of plants at the time they become infected and the severity of attack. Because the effective use of control measures often depends on early detection of the disease, it is important to know the first symptoms.

When infection develops while the plants are very small plants with leaves up to the size of a quarter; the first evidences of blue mold are small patches of seedlings with erect leaves. When plants are a little older leaves up to the size of a dollar the first evidence of blue mold is round, yellow areas. In either case, distinctly cupped leaves grow in the center of each affected area. Some of the cupped leaves have a whitish or violet mold growth over the lower surface. About 2 weeks later one can expect to find the disease throughout the beds. When the infection is limited to the small patches, there is still time to begin a control program with fair chance of success. After the disease has spread through the bed, spray or dust treatments have little value.

The damage caused by blue mold depends on the age of the plants when attacked and on weather conditions at the time that the infection becomes widespread.

Sometimes the disease merely kills irregular areas of leaf tissues, giving the appearance of a burn caused by scalding water. Sometimes all leaf tissue, except the growing tip, is invaded and killed. Actual killing of the plants is most apt to happen if blue mold attacks young plants. The growth of affected plants may be delayed up to 4 or 5 weeks, although delays of 2 weeks or less generally are not serious. Plants that are not killed do recover and produce new foliage.

The new leaves are healthy and are temporarily immune to attack by the fungus. The immunity lasts 3 or 4 weeks, but it gives time for transplanting and the recovered plants grow normally.

It is best not to transplant from a bed infected with blue mold until the recovery is definitely under way. Recovery can be hastened by watering, if the beds are dry, or by fertilizing, if the plants need food. Indiscriminate applications of concentrated fertilizers, such as nitrate of soda, are likely to do more damage than good.

The spores of the fungus spread the disease and are produced in tremendous numbers. Winds can carry them for miles. Each year nearly every plant bed from Florida to Pennsylvania may become infected even beds surrounded by woods. Wisconsin is the only tobacco area in which the disease does not occur otherwise the area in which blue mold occurs each year is the area occupied by the crop.

The question that faces the growers, then, is not whether the disease will occur. It is: Will the attack be severe? In Georgia a destructive epidemic may be expected about 1 year in 3. In South Carolina destructive outbreaks are slightly less frequent. In North Carolina and Virginia the disease has been serious about 1 year in 5, but never so destructive as in Georgia. In Tennessee and Maryland really destructive outbreaks are unknown, but localized damage is not uncommon.

Blue mold illustrates how wasteful can be some attempts to control disease by fungicides. One has to spray or dust before disease occurs. Therefore in many areas and years, when the disease does little damage, the labor and materials used are largely lost. Growers often start to use control measures too late, stop too soon, or decide to take a chance. As a result, thousands of beds are poorly protected or unprotected.

An effective system for predicting the occurrence of blue mold would be helpful. On the basis of experience, if one merely predicted each year that blue mold attack would be moderate to light, however, his average accuracy would be 67 percent in Georgia and South Carolina, 80 percent in North Carolina and Virginia, and 95 percent in Maryland and Tennessee.

Conditions most favorable for the disease prevail in Georgia. What are those conditions? The first is low temperature. Blue mold develops best during maximum daytime temperatures of 60 to 75 F. and minimum night temperatures between 40 and 60 . Disease activity is slowed by minimum temperatures in the 30's and 40's, but ultimate damage then may be most severe because the recovery of affected plants is retarded even more than the disease is. Thus there is no true lower limit to the activity of blue mold. Blue mold ceases to become active when minimum night temperatures rise above 65 . Maximum day-time temperatures above 85 also check the disease.

Low light also is important. When plants are shaded, the disease can flourish at considerably higher temperatures. Shading also greatly increases the percentage of plants killed. That is why blue mold is occasionally destructive in the Connecticut Valley shade-tobacco fields, while Havana and Broadleaf tobacco growing nearby in full sunlight is not seriously attacked.

Heavy rains do not favor the disease, light rains and the foggy, humid periods lasting 2 or 3 days are most favorable. The cycle of blue mold from infection to the production of another crop of spores is only 7 days. Consequently a succession of three or four moist periods, about a week apart, plus cool weather, provides ideal conditions for blue mold.

The age and vigor of plant growth are other factors. In plant beds, blue mold can kill plants from the time they appear above ground until they are about half transplanting size. The smaller they are the more quickly are they killed. After plants are half transplanting size or larger, they may be completely defoliated by blue mold, but the growing tip and the stem usually remain alive. Such plants usually produce new leaves and recover, although transplanting may be delayed. Under field conditions, damage is limited to the killing of localized leaf areas, and rarely are entire leaves destroyed. Vigorous, rapidly growing plants are most susceptible to attack; plants that are retarded for any reason may escape serious damage.