On some cultural characters of tobacco black root rot fungus, Thielaviopsis basicola (Berk. et Br.) Ferraris.

Abstract

The effects of nitrogen source, temperature and pH upon growth and spore formation of Thielaviopsis basicola in culture were studied. Three isolates from diseased tobacco roots, different in cultural characters, namely No. 1, No. 11 and No. 13, were used, throughout the experiment.
None of the three isolates used did utilize nitrate as nitrogen source. Asparagine and amino acids tested had a better effect on the mycelial growth than ammonium salts. Formation of chlamydospores and endoconidia of the fungus was influenced greatly by the nitrogen source. For example, (NH₄)₂SO₄ blocked endoconidial formation of No. 11 and 1-leucine blocked chlamydospore formation of No. 13. When inorganic salts were used, No. 13 produced many chlamydospores and few endoconidia, but organic ammonium salts affected the spore formation in the reverse. Among the compounds tested, d, 1-alanine, glutamic acid and aspararagine were considered to be the best nitrogen sources. CaCO₃ and Na₂ HPO₄ increased the mycelial weight in liquid culture, but the latter had apparently antagonistic effect on the former. The growth in plate cultures was good at 18-26°C. The optimum temperature for mycelial development of No. 11 was 26°C at pH 5.5-7.0, and 22°C at pH 5.0 and 7.5. Growth rate of the mycelium decreased rapidly at 28°C., and qt 30°C and 32°C it became very slow; at 8°C the mycelium could not develop. At 37°C the fungus was unable to survive. Although pH of solid medium affected the mycelial growth less than temperature, endoconidial formation was controlled by pH as well as temperature. It was often observed that pH of liquid media changed towards the acid side at first and then towards the alkaline side, which was accompanied by an increase of mycelial weight. The variation of pH was greater in liquid potato media than in synthetic media used, and when culturing temperature was between 8-26°C, variation was increasing with the temperature.