Major disease pests of tobacco
From Bugwoodwiki
Black shank (fungus Phytophthora parasitica var. nicotianae)
Symptoms include a yellowing and a sudden wilt of all the leaves. The root system will be black and rotting. The lower portion of the pith of the stem will be blackened and classically the pith area will be divided into disks of tissue. Sometimes only a small portion if the pith will be discolored at the lower extreme of the stem and discing will not be evident. Diseased plants may be randomly distributed in the field or large areas of dying plants may be present. Black shank is more severe in wetter or poorly drained portions of the field.
Nematodes (root-knot Meloidogyne spp.)
Root-knot nematode is the most prevalent tobacco damaging nematode type. These nematodes will cause tobacco plants to be stunted and exhibit symptoms of nutrient deficiencies. Plants with severe damage will wilt during the heat of the day. Knots or galls will develop on the roots and will range from pinhead to golf ball size.
Damping off (fungi Pythium spp.; Rhizoctonia solani)
Damping off is a problem in tobacco plant beds but can be a problem in the field soon after transplanting. Pythiam spp. causes a watery, soft rot of the lower stem and root system and is more common during cool, wet weather. Rhizoctonia causes brown, irregular cankers to develop on the lower stems and injury is more common during warm weather. Lesions can enlarge and girdle the stem while plants that are not severely damaged by Rhizoctonia may recover in the field.
Brown spot (fungus Alternaria alternata)
Leaf spots are brown in color, circular and enlarge to about the size of a nickel. Concentric rings can be seen within the spots and these may coalesce forming irregular areas on the foliage. Disease severity is positively correlated with cloudy, wet weather. The disease occurs primarily on mature leaves often beginning on the lower leaves and progressing to other leaves higher up on the stalk.
Angular leaf spot (bacterium Pseudomonas angulata)
Angular leaf spot is more prevalent during warm, wet weather. Small, dark brown spots will develop over the leaf surface. Spots will be angular in appearance and delineated by the small leaf veins.
Tobacco Mosaic Virus
This virus disease causes varying degrees of damage depending on earliness of infection. Symptoms include a general mottling of the leaves with irregular light and dark green splotches over the leaf surface. Expanding bud leaves may be distorted, narrow and crinkled in appearance. Affected leaves are susceptible to sun scorch and may have a scalded appearance following hot weather.
Bacterial wilt, Granville wilt (bacterium Pseudomonas solanacearum)
Wilt symptoms produced are similar to those caused by black shank. The leaves do not yellow, but wilt suddenly. Discolored tissue will be evident beneath the outer layer of the stem. The central portion of the stem will be darkened but not divided into disks as with black shank.
Blue mold (fungus Peronospora tabacina)
Blue mold is favored by cool, wet weather and is a problem in plant beds and in the field. During periods of cool, wet and humid weather, circular yellow spots develop on the upper surface of the leaf and the blue-gray fungus can be seen sporulating on the underside of these areas. Spots later turn brown, die and may fall out giving a ragged appearance to the leaf. The fungus can grow systemically through the leaf and into the vascular system of the stem, resulting in a stunted plant. A brown discoloration will be evident in the stem. On young plants, new leaves will be crinkled, twisted and the bud may be killed.
Weather fleck
High ozone levels result in the physiological breakdown of leaf tissue and produce weather fleck. Tiny flecks of dead tissue frequently arranged in circular patterns will form over the leaf surface. Flecks will first be brown in color but later will change to white and will be more prevalent on older leaves.
