Final diagnosis


Damping-off, Collar cankers, Leaf spot (Thanatephorus cucumeris, Rhizoctonia solani)

Damping-off of seedlings caused by <i><b><b>Rhizoctonia solani</b></b> </i>(teleomorph: <i><b><b>Thanatephorus cucumeris</b></b></i>). Leaves of affected seedlings wilt, turn yellow and dry and exhibit browning at the collar and/or roots.
Seedling completely girdled at the collar, with petioles and lamina showing water-soaking and necrosis as the infection spreads from the stem. <b><i>Thanatephorus cucumeris</i></b> (<i><b>Rhizoctonia solani</b></i>)
On an older seedling, the fungus can cause suberisation of collar and small cankers on petioles. <b><i>Thanatephorus cucumeris</i></b> (<i><b>Rhizoctonia solani</b></i>)
A dark brown to black canker, located at the soil level girdling the stem of a tobacco plant. <i><b>Thanatephorus cucumeris</b></i> (<i>Rhizoctonia solani</i>, sore shin)
Sometimes a tuft of brown mycelium can be seen on the surface of the canker. <i><b>Thanatephorus cucumeris </b></i>(<i>Rhizoctonia solani</i>, sore shin)
Sore shin caused by <i><b>Rhizoctonia solani</b> </i>(teleomorph: <i><b>Thanatephorus cucumeris</b></i>) on field-grown tobacco. A brown canker girdles the collar at the soil line and the cortex is decaying.
Later the canker expands and develops a hard, dry texture. (<i>Rhizoctonia solani</i>, sore shin)
This young tobacco plant has many brown, superficially corky roots. <i><b>Rhizoctonia solani</i></b>
Superficially corky roots on which the majority of rootlets have disappeared. <i><b>Rhizoctonia solani</b></i>
On larger roots many rootlets have disappeared and the cortex is partly suberous. <i>Thanatephorus cucumeris </i>(<i>Rhizoctonia solani</i>, sore shin)
Translucent and brown spots with concentric rings on young nursery seedlings. <i>Thanatephorus cucumeris </i>(target spot)
Irregular, beige to brown spots with well defined margins and fragile central tissues which dry up, split and disappear. <i><b>Thanatephorus cucumeris</b></i> (<i><b>Rhizoctonia solani</b></i> anam.)
Target spot, caused by <i><b>Thanatephorus cucumeris</b></i>. Following a period of high humidity, lesions with a “"bull's eye”" effect may develop on Virginia type tobacco.
Tissue in lesions cracks and falls out, causing the leaves to be pierced by irregular holes. <i><b>Thanatephorus cucumeris</b></i> (<i><b>Rhizoctonia solani</b></i>, target spot)
Under highly favourable conditions, lesions may reach several centimeters in diameter. <i><b>Thanatephorus cucumeris</b></i>(anam. <i><b>Rhizoctonia solani</b></i>, target spot)
Lesions are typically light brown, round, surrounded by a yellow halo, and have discrete concentric rings. <i><b><b>Thanatephorus cucumeris</b></b></i> (anam. <i><b><b>Rhizoctonia solani</b></b></i>, target spot)
Brown and ramified mycelium of <i><b>Rhizoctonia solani</b></i> is easily observed on this root system.
The mycelium of <i><b>Thanatephorus cucumeris </b></i>(<i><b>Rhizoctonia solani</b></i>, damping-off) turns brown (melanises) with age. Hyphae are septate and show a constriction at the point where 45° branches are formed, which is very characteristic for this fungus.
Dark brown mycelium (m) can be observed on affected roots; and less frequently irregular sclerotia of 2-5 mm (s) are observed. <i><b>Thanatephorus cucumeris </b></i>(<i><b>Rhizoctonia solani</b></i>, damping-off)
Development of a greyish white hymenium on the surface of affected tissues characterises the presence of the fungus. <i><b>Thanatephorus cucumeris</b></i> (damping-off)
Basidia bearing basidiospores of <i>Thanatephorus cucumeris</i> (<i>Rhizoctonia solani</i>)
A few basidia, often with 4 basidiospores may be observed on the white hymenium which partly covers the infected plant organs and the soil. <b><i>Thanatephorus cucumeris</i></b>