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Significant in vitro antagonism of the laurel wilt pathogen by endophytic fungi from avocado does not predict their ability to control the disease

Randy Ploetz: Tropical Research & Education Center, University of Florida

<div><em>Raffaelea lauricola</em> (Ascomycota, Ophiostomatales) causes laurel wilt, a lethal vascular disease of avocado, <em>Persea americana</em>. We examined biological control of the disease with endophytic fungi from avocado. Xylem (the infection court for<em> R. lauricola</em>) of 112 trees (seven commercial cultivars) was sampled, and 64 operational taxonomic units (OTUs) were identified with partial sequences of ITS rDNA. Thirty-two OTUs were evaluated against <em>R. lauricola</em> with <em>in vitro</em> dual culture assays, and nine OTUs that strongly antagonized the pathogen <em>in vitro</em> were tested <em>in planta</em> against laurel wilt. In three greenhouse experiments, grafted plants of ‘Simmonds’ or ‘Russell’, cultivars that are susceptible to laurel wilt, were inoculated with endophytes and, after 10-16 days, inoculated with<em> R. lauricola</em>. As expected, laurel wilt developed in plants that were not treated with endophytes within 14 days of inoculation with the pathogen (positive controls), but did not develop in mock (water)-inoculated plants (negative controls). However, laurel wilt also developed in endophyte-treated plants to the extent observed in the positive controls (P=0.05). The pathogen colonized plants rapidly and systemically, but the endophytes generally did not colonize xylem more than 2 cm above the point at which plants were inoculated. We will discuss the management of laurel wilt and the value of <em>in vitro</em> data when developing biological control measures for it and other vascular wilt diseases.</div>