Root associated organisms modify the effectiveness of chemically induced resistance in barley

Induced resistance (IR) offers a lasting defense against a broad spectrum of diseases in various plants, but practical applicability is impaired by unpredictable effectiveness. Here we present the results of two greenhouse experiments on the modification of the effectiveness of BION (R) (acibenzolar-S-methyl (BTH)) induced resistance against powdery mildew in barley by different levels of root infection with mycorrhizal fungi (Glomus etunicatum) and parasitic nematodes (Heterodera avenae), respectively. Root infection by mycorrhizal fungi and nematodes both had a negative effect on plant growth. Adverse effects of G. etunicatum suggest that parasitic effects of mycorrhizal fungi outweighed positive effects under the optimal growth conditions provided by the experiment: Bion (R) either did not affect or decreased the number of lesions,caused by powdery mildew at low and medium levels of infection by G. etunicatum or H. avenae, but increased it at high levels of root infection. This clearly proves the important role of root-associated biota in modulating the effectiveness of IR. However, plant accumulation of the resistance related protein BCI-4 was not impaired by G. etunicatum or H. avenae. Soil biota thus appear to impact later steps of the defense pathway, rather than preventing the onset of resistance. (c) 2005 Elsevier Ltd. All rights reserved.