A third Phytophthora species causing wild olive decline in Spanish forests

Abstract

Since 2009, a severe decline caused by Phytophthora cryptogea and P. megasperma was observed in a protected wildolive forest (Olea europaea var. sylvestris) of high ecological value (Dehesa de Abajo, Seville, Spain). In this natural forest, two new samplings of roots and soil were done on 25 symptomatic wild‐olives in autumn 2014 and 2015. For each sample, feeder root segments were directly plated on NARPH; and olive leave baits were prepared for soil isolations. Apart from P. cryptogea A1 and P. megasperma, a third Phytopththora species were consistently isolated from roots in 2014 (24% of sampled trees) and 2015 (36% of sampled trees). These isolates were morphologically identified as belonging to the P. citricola complex and confirmed by analysis of their ITS regions. Their morphology fit well to P. plurivora or P. multivora and it will be confirmed according to β‐tubulin and cytochrome oxidase sequences. Temperature growth relationships obtained in CA medium (5 to 35º C) showed no growth at 5º C nor at 30º C, with maximum growth registered at 19.9º C. Pathogenicity was confirmed on 1 year‐old healthy wild olives inoculated by adding oospore suspensions (50 ml per plant, 2.2×104 oospores×ml‐1) to the rootballs. Pathogenicity on wild‐olive was similar for the three Phytophthora species associated, but temperature requirements are quite different. It is hypothesized that they may show their active periods in different seasons depending on the climatic conditions