Thielaviopsis basicola

(black root rot)
 

Thielaviopsis basicola (syn. Chalara elegans), causing black root rot, was for decades one of the main factors limiting the production of Virginia and Burley tobacco in France. This situation was caused by monoculture of dark tobacco which is tolerant to the fungus (responsible for large inocula in some soils), the survival ability of the fungus, and often too short rotations. Furthermore, Virginia or Burley tobaccos grown in the 80 and 90 in France, were particularly susceptible to this soil fungus and were highly affected by it. The situation was reversed with the arrival of resistant varieties in both tobacco types. The resistance is essential for T. basicola because the fungus is still present in many soils and in large quantities.
 
Symptoms of T. basicola on the roots are characteristic. As the name of the disease indicates, it results in browning or blackening in various degrees of the roots (figures 1 and 2). On larger roots, damage is more limited in some areas, appearing in the form of sleeves of variable aspect (figures 3 and 4). In all cases, observation with a handlens or a microscope of brown chlamydospores on and in tissues confirms the diagnosis (figure 5).
 
Note that one should be careful not to confuse these symptoms with those caused by migratory endoparasitic nematodes of Pratylenchus genus. A number of tobacco varieties, now resistant to T. basicola, have revealed the presence of these nematodes in certain situations (see the next topic).
 
Further information of this fungus can be found in Thielaviopsis basicola fact sheet.