Native Killer Yeasts as Biocontrol Agents of Postharvest Fungal Diseases in Lemons.

Economic losses caused by postharvest diseases represent one of the main problems of the citrus industry worldwide. The major diseases affecting citrus are the "green mold" and "blue mold", caused by Penicillium digitatum and P. italicum, respectively. To control them, synthetic fungicides are the most commonly used method. However, often the emergence of resistant strains occurs and their use is becoming more restricted because of toxic effects and environmental pollution they generate, combined with trade barriers to international markets. The aim of this work was to isolate indigenous killer yeasts with antagonistic activity against fungal postharvest diseases in lemons, and to determine their control efficiency in in vitro and in vivo assays. Among 437 yeast isolates, 8.5% show to have a killer phenotype. According to molecular identification, based on the 26S rDNA D1/D2 domain sequences analysis, strains were identified belonging to the genera Saccharomyces, Wickerhamomyces, Kazachstania, Pichia, Candida and Clavispora. Killers were challenged with pathogenic molds and strains that caused the maximum in vitro inhibition of P. digitatum were selected for in vivo assays. Two strains of Pichia and one strain of Wickerhamomyces depicted a significant protection (p <0.05) from decay by P. digitatum in assays using wounded lemons. Thus, the native killer yeasts studied in this work showed to be an effective alternative for the biocontrol of postharvest fungal infections of lemons and could be promising agents for the development of commercial products for the biological control industry

Wound protection control at low temperature of 27 strain against <i>P</i>. <i>digitatum</i>.

<p>Error bars indicate standard deviations.</p

Degree of relative inhibition of killer yeast strains against <i>P</i>. <i>digitatum</i>, <i>P</i>. <i>italicum</i> and <i>P</i>. <i>citri</i>.

<p>Degree of relative inhibition of killer yeast strains against <i>P</i>. <i>digitatum</i>, <i>P</i>. <i>italicum</i> and <i>P</i>. <i>citri</i>.</p

Control of <i>P</i>. <i>digitatum</i> by wounds protection with killer yeasts, after 5 days of incubation.

<p>(A) Control lemons, inoculated with pathogen spore suspension. (B), (C) and (D) pretreated lemons with 27, 28 and 56 yeast strains, respectively.</p

<i>In vivo</i> antagonistic effect of killer yeast against P. <i>italicum</i> observed after seven days of incubation.

<p>(A) Control fruit, only inoculated with the plant pathogen. (B), (C), (D), (E) and (F) correspond to pretreated lemons with 27, 28, 56, 120 and 137 yeast strains, respectively.</p

<i>In vitro</i> inhibition of killer yeast 137 against fungi on PDA medium (10 days of incubation at 25°C) under loupe (10X magnification).

<p>To the left is shown the mycelium from <i>P</i>. <i>italicum</i> (A), <i>P</i>. <i>digitatum</i> (B), and <i>P</i>. <i>citri</i> (C) inhibited by killer yeast 137. To the right control mycelia from each fungus are shown.</p

<i>In vitro</i> inhibitory activity of killer strain 27 against <i>P</i>. <i>digitatum</i>, <i>P</i>. <i>italicum</i> and <i>P</i>. <i>citri</i> on PDA medium after 10 days incubation at 25°C.

<p>(A) Control plates inoculated only with plant pathogens. (B) Plates inoculated with killer strain 27 and plant pathogens.</p

Efficiency of 27, 28 and 56 killer yeast strains in the wounds protection control against <i>P</i>. <i>digitatum</i>.

<p>Error bars indicate standard deviations.</p