Pathogen Diversity and Biological Control of Verticillium Wilt of Tomato

Abstract

<p><i>Verticillium dahliae</i> is an important soil-borne pathogen that attacks a wide range of hosts, resulting in economic crop losses worldwide. With mounting concerns over pesticide use within current consumer markets, there is a need to search for alternative control options. One such approach is the development of biocontrol agents. This was the purpose of this study, which explored the effect of biological control strategies for tomato Verticillium wilt caused by <i>V. dahliae</i> using antagonistic fungi, either alone or in combination with or without salicylic acid (SA).</p> <p>Pathogenicity of four strains of <i>V. dahliae</i> included DAR 33757 from cotton, DAR 31890 from tomato, DAR 44537 from potato and DAR 81260 from olive, and these were assessed on the tomato Grosse Lisse cultivar. Among these strains, <i>V. dahliae</i> DAR31890 from tomato was significantly more virulent on tomato. Therefore, this strain was selected in all subsequent experiments. Potential antagonistic fungi were isolated from cotton roots and soil that adhered to the roots (rhizosphere soil) and evaluated as potential biocontrol agents (BCAs) against <i>V. dahliae</i>. The greatest suppression of fungal activity towards the pathogen was recorded by isolates 2 and 7, which were isolated from rhizosphere soil and subsequently identified as <i>Trichoderma harzianum</i> and an <i>Aspergillus</i> sp.</p> <p><i>In vitro</i>, these antagonists inhibited microsclerotia (MS) germination and mycelial growth of V. dahliae, reduced disease severity and promoted tomato plant growth in the glasshouse experiments. Thus, these two fungi were chosen as BCAs for further experiments in the next experiments. Hoagland nutrient solution plus tomato root exudates significantly stimulated MS germination compared to water agar only, and also increased the fungal weight of the pathogen and antagonistic fungi in liquid culture experiments. Of the root exudate compounds tested, amino acids, organic acids, and sugars significantly stimulated MS germination compared with the water agar treatments.</p> <p>Dipping tomato seedling roots in spore suspensions of <i>T. harzianum</i> or <i>Aspergillus</i> sp. prior to inoculation of stems with <i>V. dahliae</i> significantly reduced disease severity and increased tomato growth parameters. The results also showed that soluble protein concentration was significantly increased in tomato leaves treated with BCAs.</p> <p>SA at concentrations of 0.5, 1 and 2 mM <i>affected</i> mycelial growth of <i>V. dahliae</i>, <i>T. harzianum</i> and <i>Aspergillus</i> sp. Tomato seed germination and seedling vigour were enhanced at 0.5 mM concentration of SA and significantly reduced at 2 mM concentration.</p> <p>The combination of SA and BCAs enhanced activities of these fungi against <i>V. dahliae</i>. The combination of SA and BCAs significantly reduced disease severity and enhanced shoot dry weight of tomato plants more than using either antagonists or SA alone.</p> <p>Overall, this study provides insight into the efficacy of <i>T. harzianum</i> and <i>Aspergillus</i> sp. isolates for biological control of <i>V. dahliae</i>. Also, a combination of BCAs and SA can improve the consistency of biological control against plant pathogens more than using them individually.</p>