Scouting Tobacco in North Carolina

Scouting Common Tobacco Diseases

The primary purpose of disease scouting is to determine what diseases are present and at what levels. This information should be recorded and used to make management decisions for the next year (rotation, resistant varieties, etc.). At least one disease survey should be made during the first half of the harvest period. In addition, you should always be alert to the development of disease. Make a note the first time any disease is found in a field during the season. Disease sampling may be done at the same stops used for insect counts.

Soil-Borne Diseases (Granville Wilt, Black Shank, Root-knot Nematode, Fusarium Wilt)
Look for signs of these diseases in the visible area around each scouting stop. Simply record whether or not the disease is present in the area. After scouting the field, determine the percent of stops at which each disease was seen. Following are descriptions of the symptoms of these diseases.

Granville Wilt
The first symptom of Granville wilt above ground on succulent plants is the drooping of one or two leaves. Drooping leaves may recover at night. Often only half of a leaf wilts initially. In addition, early infection is marked by wilting of leaves on only one side of the plant (Figure 20). The leaves on the other side of the plant appear normal. This unilateral characteristic is a predominant symptom.



Figure 20. Wilting of leaves on one side of the
plant is a symptom of early infection by
Granville wilt.

Affected leaves become light green to yellow, eventually wilt and occasionally are scalded. Early in the course of the disease a few roots will be decayed. As the disease progresses, the entire root system usually decays and becomes blackened. If adequate water is present, infected roots become soft and slimy.

A characteristic symptom of Granville wilt is a tan or brown discoloration that develops in the vascular tissue just beneath the bark of infected plants . As the pith and cortex are invaded, they too become discolored. Narrow, brown lines in the xylem tissue nearest the pith will be seen in sections of the affected tobacco stalk cut lengthwise.

Cut a cross section of a plant stalk, and a slimy substance that looks like brown, shiny beads can often be seen where the stem was cut. This disease can also be demonstrated by placing a longitudinal plant segment containing discolored vascular tissue in a container of clear water. Soon milky strands will begin streaming from the edge of the tissue.

Black Shank
Black shank may attack tobacco plants of all ages. The roots and basal parts of the stem are primarily affected. The stems of young tobacco seedlings infected by this fungus will decay near the soil level. Infected plants usually wilt during the hottest part of the day and may not recover at night. The root system is often partly or completely black; a dark lesion may extend several centimeters up the stalk (Figure 22). Older plants infected with this disease will wilt suddenly or the leaves will droop.

Leaves of affected plants gradually turn yellow and hang down the stalk (Figure 22). A lengthwise cut on an infected plant will reveal brown to black discoloration of the pith, which is often separated into disks (Figure 23). Disking may not be noticeable in resistant varieties. Root systems are completely or partially decayed. Soon the leaves on infected plants become brown and, in a few days, the plant is dead. Infection late in the season often results in plants with stalks blackened most of the way to the top. There may be a few small, green leaves in the top of such plants, but they are usually not usable. This disease frequently appears first in low, wet spots in plant beds or fields. Disease development in cool weather is slower than when temperatures are high.



Figure 22. Leaves of plants
affected by black shank
gradually turn yellow and
hang down the stalk.

Figure 23. A lengthwise cut of
a black shank-infected plant
reveals brown to black
discoloration of the pith,
which is often separated
into discs.

Root-knot Nematode
Plants infected with root-knot nematode are often stunted, slightly yellowed, and tend to wilt on hot, dry days. Wilting often results in firing of the leaves. Damage usually is not seen uniformly across a field but occurs irregularly. Galls or swollen areas on the roots, which vary from the size of a pin head to several times the thickness of the root, are characteristic of root-knot nematode infection. Galls may be irregularly shaped, spindle shaped, or spherical (Figure 24).



Figure 24. Galls -- or swollen areas on the roots --
vary in size and are characteristic of root-knot
nematode infection.

Fusarium Wilt
The predominant initial symptoms of Fusarium wilt are slow yellowing and drying of the leaves on one side of the plant. The leaves on the affected side of the plant are usually slow to develop and the mid ribs may curl, resulting in distortion. The top of the plant may be drawn toward the diseased side, and wilting eventually occurs (Figure 25). Newly infected plants may not wilt at first. The wood beneath the outer bark of the stem on the affected side of the plant will be chocolate brown. Such discoloration can also be seen in the mid ribs of affected leaves and in roots (Figure 26). Portions of the root systems on the affected side are usually blackened and killed. The symptoms of Fusarium wilt and Granville wilt are similar and may be confused. The bacterial streaming characteristic of Granville wilt is not seen in plants infected with Fusarium wilt and is a reliable way to distinguish between the two diseases. Fusarium wilt development is frequently accompanied by root-knot nematode infection.



Figure 25. Slow yellowing and
drying of the leaves are
common with Fusarium wilt.

Figure 26. The choclate brown
color of the wood beneath the
outer bark on the affected side
of the plant indicates damage
from Fusarium wilt.

Foliar Diseases (Mosaic, Brown Spot, Blue Mold)
Record during regular scouting trips to the field the number of plants that show evidence of mosaic, brown spot, or blue mold infection. Calculate the percentage of plants infected with each disease. NOTE: Always be alert for the presence of blue mold infection. Take note of this disease wherever it is found in the field. Remedial control of most tobacco diseases is not possible, so thresholds have not been developed. Treatment to control blue mold, however, should begin when any infected plants are seen.

Mosaic
Mosaic is characterized by the development of a mosaic, or mottled, pattern of light and dark green areas on the leaves. This pattern develops quickly and is most pronounced on younger leaves. The first symptom is a clearing of the veins of young leaves; the tissue surrounding the veins fades from its normal color to light green. This early symptom is followed quickly by the mottled, mosaic pattern (Figure 27). If infection occurs early in the season, plants may be substantially stunted.



Figure 27. The mottled pattern of light and dark
green areas on leaves is typical of mosiac.

Leaves containing the mosaic virus are subject to mosaic burn, especially during periods of hot and dry weather. Large dead areas develop in the leaf lamina. This is one of the most destructive phases of tobacco mosaic infection. Mosaic infected leaves may be mildly distorted.

Brown Spot
Brown spot appears first on older leaves as circular, brown lesions or spots (I/` to 1 1/4 inches in diameter) with sharply defined margins. Concentric circles (rings within the spot) are often noticeable. As the leaves approach maturity, a yellowish halo of varying width often forms around the lesions. Lesions may merge, forming large areas of dead leaf tissue (Figure 28). During humid weather, sporulation (the formation of spores) of the fungus may be abundant, giving the lesions a black, dusty appearance. Sporulation progresses >from the lower to the upper leaves as the leaves mature. The rate and amount of spread depends on the weather, the overall health of the plant, varietal tolerance, and harvest rate.



Figure 28. Yellowish halos form around brown
spot lesions on mature leaves. Lesions may
merge, forming large areas of dead leaf tissue.

Blue Mold
Blue mold can occur in the plant bed or field. Circular yellow spots up to 1 inch in diameter develop on the leaves (Figure 29). Blue-gray fungus growth is found on the underside of these spots when the fungus is active (Figure 30). Spots become light brown when the leaf tissue dies. The disease first develops on lower leaves. It rapidly spreads to upper leaves during wet, cool weather.

Infection may spread to the stalk. Such systemic infection of the stalk can develop first as dark brown vein sections in the leaves. Systemically infected stalks are stunted. The leaves are mottled and distorted. When a leaf is removed, discolored vascular tissue is visible where the leaf was attached to the stem. The terminal bud may die, and the central portion of the stem in the terminal bud region will be deeply discolored. Excessive ground suckers may develop.



Figure 29. Circular yellow spots up to 1 inch in
diameter develop on leaves stricken by blue
mold.

Figure 30. A blue-grey fungus growth is found
on the underside of blue mold spots.

Flowers and Suckers            Nematode and Fertility Sampling

Scouting Tobacco in North Carolina