Effects of biological agents on Fusarium avenaceum, the causal agent of apple fruit rot

Abstract

The quality of apple fruits can be affected by many factors during storage, and phytopathogenic fungi are among the most important. Fusarium avenaceum and F. graminearum are increasingly detected on stored apple fruits as causal agents of Fusarium apple fruit rot in Serbia. Besides causing economic losses, these species can also contaminate apple fruits and fruit products with mycotoxins. Post-harvest use of synthetic fungicides is not allowed in Serbia and there is a need for effective, liable alternative methods against causal agents of apple fruit rot in storage. In this trial, antifungal activity of Bacillus species complex (commercially available microbiological fertilizer) applied at concentration rates of 1% and 5%, and of metabolites produced by Streptomyces hygroscopicus applied at 5% and 100% concentration rate, was evaluated against F. avenaceum on apple fruits (cv. Golden Delicious). In previous in vitro trials, both tested agents exhibited significant inhibition of F. avenaceum mycelial growth. Metabolites of S. hygroscopicus were produced by cultivation for three days in a liquid medium composed of glucose, soybean meal, CaCO3, NaCl, MgSO4, (NH4)2HPO4 and K2HPO4, at 26 ± 1 °C, in a shaker at 150 rpm, under conditions of spontaneous aeration. After incubation, the culture was centrifuged for 10 minutes at 10,000 g and the obtained supernatant was used in the trial. Apple fruits were surface-sterilized, injured with a cork borer and 2 μl of prepared solutions of biological agent/metabolites was added into wounds. Control fruits were treated with sterile distilled water. After 10 minutes, the fruits were artificially inoculated with 7-days old mycelial plugs (Ø 3 mm) of F. avenaceum. Inoculated fruits were incubated in wet plastic chambers at room temperature for 10 days. After incubation, necrosis development was measured and compared to untreated control. Although Bacillus spp. applied at concentration of 1% significantly inhibited necrosis development (necrosis diameter 19,75 mm) compared to control (32.12 mm), this inhibition was significantly lower compared to the higher applied concentration of the same agent (5%) (9.25 mm), as well as both concentrations of S. hygroscopicus metabolites. S. hygroscopicus metabolites exhibited the same inhibitory effect applied at lower (5%) and higher concentrations (100%), 8.00 and 8.06 mm respectively. The results indicate high potential of S. hygroscopicus metabolites use in control of fusarium apple rot and concentrations below 5% should also be tested to determine minimum effective concentration for this agent. It can be concluded that tested biological agents have potential as biological tools against causal agent of Fusarium fruit rot which could be used post-harvest in order to maintain quality of fruits and prevent losses that can occur during storage.