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When supplied via roots to tobacco host plants, aqueous solutions of allopurinol, a specific inhibitor of xanthine oxidase and dehydrogenase, markedly reduced or completely prevented the visible development of tobacco powdery mildew fungus Erysiphe cichoracearum(E. orontii) at concentrations as low as 25–50 μ M administered for periods as short as 2 days. The duration of this allopurinol-dependent protection from the pathogen was of at least 20 days after the last administration. If supplied as foliar sprays at concentrations up to 1600 μ M , allopurinol was not absorbed by the leaf tissues and did not affect the fungal infection. Histological observations revealed that on tobacco plants treated with allopurinol via roots, a sharp reduction inE. cichoracearum mycelial development started only 48 h after inoculation (after formation of the first haustoria) and was accompanied by a strong delay and reduction of sporulation. The proportion of haustoria partially or entirely embedded in thick translucent sheaths (possibly callosic in nature) increased significantly in allopurinol irrigated plants reaching about 41% of the total haustoria (compared to 9% in controls) 7 days post-inoculation. No effects of allopurinol supply was observed on Blumeria graminis and Erysiphe trifolii development on wheat and clover plants, respectively. The present results are consistent with the view that allopurinol affects pathogen growth and sporulation by inhibition of xanthine dehydrogenase in the plant, the pathogen or both organisms. |
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