Soil and compost humic fractions regulate the response of Sclerotinia sclerotiorum to exogenously added allelochemical compounds

Allelochemical compounds released by plants to signal their presence and needs interact in soils with very important macromolecules, such as humic acids (HAs), which are able to modulate the ultimate effects on target organisms. Most of the available studies on plants and microorganisms report the effects of allelochemicals or those of humic fractions, separately. In this study, we investigated the combined activity of these two types of compounds on the soil-resident fungus Sclerotinia sclerotiorum. Thus, ferulic acid (FA), caffeic acid (CA), benzoic acid (BA), salicylic acid (SA), gallic acid (GA) and phthalic acid (PA), exogenously applied to the fungal growth medium, were tested both alone and in combination with a soil HA (SHA) and a compost HA (CHA). The two HAs were also tested alone on the fungus. When the allelochemicals were applied alone, only FA, BA and SA evidenced a significant inhibition of mycelial growth, whereas FA, BA and CA increased the number of sclerotia formed. The two HAs alone reduced the early growth of the fungus and markedly stimulated sclerotia formation. A significant attenuation or, in some cases, suppression of the allelochemical effect on mycelial growth was caused by the coexistence in the medium of the allelochemical and each HA, especially CHA. Moreover, in general, the combinations of HA-allelochemical significantly stimulated sclerotia formation, with respect to the sole allelochemical, but decreased it with respect to HA alone. Thus, investigations on the response of fungi to plant-released allelochemicals should not exclude interactive aspects of these compounds with ubiquitous coexisting humic macromolecules.