Competition for endogenous and exogenous nutrients between Sporobolomyces roseus and Cochliobolus sativus

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Abstract
Sporobolomyces roseus reduced conidial germination of Cochliobolus sativus on glass slides covered with a thin layer (0.1 mm) of water agar or Czapek Dox agar by ca. 60 and 99%, respectively. On wheat flag leaves S. roseus reduced conidial germination by ca. 15% but reduced germ tube length by ca. 48%. However, cell-free aqueous diffusates collected from water agar slides and leaves with and without S. roseus showed no differential effect on Co. sativus when bioassayed on water agar slides. Diffusates from Czapek Dox agar with S. roseus reduced conidial germination by 46% when compared with the nutrient-rich diffusate from Czapek Dox agar alone. Although this demonstrated the ability of the yeasts to reduce the amount of exogenous nutrients, this did not account for the 99% reduction found in the presence of S. roseus. When Co. sativus was separated from S. roseus on Czapek Dox agar by two layers of cellophane, the reduction was the same and maintained for at least 3 days. If Co. sativus was removed from S. roseus by transferring the upper cellophane sheet to a glass slide, germination was restored, indicating that the yeasts formed also a continuous drain of endogenous nutrients from the conidia. A steady supply of amino acids and (or) glucose to Czapek Dox agar slides with S. roseus and Co. sativus could only partially overcome the antagonistic interaction.