Two Organophosphorous Amino Acid Analogues Enhance Sensitivity of Some Plant Pathogenic Fungi to the Folicur Fungicide
The ability of two organophosphorous analogues of natural amino acids, 1-(aminoethyl)phosphinic acid (AEPA) and (aminomethyl)thiophosphonic acid (AMTPA), able to inhibit the biosynthesis of fungal polyketide mycotoxins, to enhance the antifungal action of the Folicur commercial fungicide (a.i. tebuconasole) on some plant pathogenic fungi (Phoma glomerata, Fusarium culmorum, and Alternaria alternata) has been evaluated. The tested compounds and Folicur were added to PDA or minimal medium in sub-fungicidal concentrations providing no or slight suppression of a fungal growth. The fungal growth suppression level was determined by a colony diameter measurement. A possible synergism between Folicur and AEPA or AMTPA resulting in a sensitization of fungi to the fungicide was determined using a Limpel’s criterion: the observed growth-inhibitory effect (Er) of a fungicide co-applied with a putative sensitizer should exceeded the expected additive effect of the treatment with these two substances (Ee) calculated by the Limpel’s formula. In the case of co-application of AEPA (0.01%) with Folicur (0.25 ppm) on PDA medium, Er exceeded Ee for all three fungi (55% vs. 38.94% for P. glomerata, 44% vs. 28% for A. alternata, and 63% vs. 57% for F. culmorum) that confirmed the existing synergistic effect between these compounds. To exclude possible competition between amino acids containing in the medium and their phosphoanalogues, a new experiment was arranged for F. culmorum using minimal medium. The synergistic character of the AEPA co-application with Folicur (0.01 ppm) was confirmed for two concentrations of this compound, 0.0001 and 0.0002%; in the first case, Er (15.9%) > Ee (10.22%), and in the second case, Er (22.4%) > Ee (10.07%). Another putative sensitizer, AMTPA, combined with Folicur (0.01 ppm), also confirmed its synergetic action (Er (29.0%) > Ee (20.83%)). The obtained results showed some prospects in the search for inexpensive commercially available compounds possessing a sensitizing effect toward plant pathogenic fungi, which may be used as a low-cost solution for reduction of effective dosages of commercial fungicides to prevent pesticide contamination of the environment.
Keywords: Chemosensitization, plant pathogenic fungi, fungicide, polyketide biosynthetic pathway, Folicur, organophosphorous analogues of amino acids