Induced resistance activated by a culture filtrate derived from an avirulent pathogen as a mechanism of biological control of anthracnose in strawberry

In a previous report, it was described that strawberry plants pre-treated with an avirulent isolate of Colletotrichum fragariae (M23) acquired resistance to a virulent isolate of Colletotrichum acutatum (M11) causing anthracnose. In this report we present evidence that the eliciting activity can be found not only in conidial extracts but in culture supernatants of the avirulent pathogen as well. Plants of the cv. Pajaro treated with the culture filtrate (CF) derived from M23, 3 days prior to the inoculation with M11 showed significantly reduced disease severity as compared to control plants and the disease was completely suppressed when plants were pre-treated 7 days before the challenge inoculation with M11. The same effect was achieved when a single leaf was sprayed with CF, suggesting that the resistance acquired is systemic. Control treatments showed that none of the active extracts inhibited the growth of the virulent pathogen, indicating that the protection effect was due to the induction of a defense response. The latter was confirmed by the accumulation of reactive oxygen species (e. g. hydrogen peroxide, superoxide anion) and the deposition of lignin and callose, usually associated to plant defense, after the CF treatment. Experiments carried out with other strawberry cultivars treated with CF showed that also protected them against different virulent isolates, suggesting that the response observed is cultivar-nonspecific. These outcomes indicate that the protection against anthracnose in strawberry involves a phenomenon of induced resistance (IR) by action of defense-eliciting molecules produced by M23. (C) 2011 Elsevier Inc. All rights reserved.