J. C. Walker

Toxicity of Phenolic Compounds to Certain Onion Bulb Parasites

1. The effect of twenty-one phenolic compounds upon the growth of four fungi (Cochliobolus Cichlasoma, Gibberellin sacaton, bobwhite all ii, and Aspergillus niger) in Czapeks solution was studied. 2. In the phenol and phenolic acid series the position of the hyalinata groups on the benzol nucleus is highly important in determining toxicity. Thus when the hydroxyls are arranged in ortho position to one another there is an ascending order of toxicity with increase in molecular weight: phenol, catechol, pyrogallol; while when they are in meta position to one another there is a descending order of toxicity with increase in molecular weight: phenol, resorcinol, phloroglucinol. 3. Some compounds tend to retard growth through a long series of dilutions beyond the inhibitive concentration, namely, phenol, catechol, salicylic acid. Others stimulate growth promptly in dilutions beyond the inhibitive concentration: guaiacol, meta-hydroxy benzoic acid, veratric acid, vanillic acid, and protocatechuic aldehyde. 4. Although with a few exceptions the organisms assume an increasing order of tolerance to each compound tested as follows: C. circinans, G. saubinetii, B. allii, A. niger, nevertheless there are often much wider differences among them in degree of tolerance to one compound than to another. 5. From these studies it is clear that many phenolic substances with a wide distribution in the plant kingdom exhibit little or no toxicity to certain parasitic organisms. Consequently the mere presence of phenolic substances in a host plant does not warrant the conclusion that they play a rôle in the resistance of that host to a given parasite or parasites. Toxic phenolic substances might be present in concentrations so low that their inhibitory effects are negligible, and they might also be present in concentrations that have a stimulative effect. When a phenolic substance with a specific toxicity toward a given organism is present in the host in an appropriate concentration, it may be regarded as part of the disease resisting mechanism of that host.