Phytophthora palmivora: causal agent of peach palm stem base rot in Brazil

ABSTRACT The area planted with peach palm (Bactris gasipaes) for the production of palm heart has increased in the Brazilian states of Bahia, Espírito Santo, Rio de Janeiro, Tocantins, São Paulo, Paraná and Santa Catarina. In these regions, the crops are attacked by stipe basal rot (SBR), a disease associated with the oomycete Phytophthora palmivora. There are few studies on this disease, and some doubts about the species Phytophthora palmivora remain. Thus, the objective of this study was to identify and morphologically, physiologically and molecularly characterize Phytophthora sp. isolates obtained from three peach palm plantation sites in order to elucidate its etiology. The 31 Phytophthora isolates were obtained from peach palm crops in the states of São Paulo (Eldorado, Cajati and Registro), Paraná (Paranaguá and Morretes) and Santa Catarina (Massaranduba, Garuva and Joinville). Mycelial growth of isolates was evaluated at eight temperatures (8, 12, 16, 20, 24, 28, 32 and 36 °C), as well as the aspect of the colony and the production of sporangia, chlamydospores and oospores. Fifty structures of each isolate were measured, as well as molecular analysis based on the regions ITS1 and ITS2 and Cox1 and Cox2. Three colony patterns were found: slightly starry, starry and cottony, and the first pattern was most frequent. There was no growth at 8 °C and 36 °C and the optimal growth temperature was 23.7 °C. The sporangia formed on sympodial ontogeny were papillate, predominantly ellipsoid, showing 21.1 – 84.8 µm length and 17.4 – 41.7 µm width, caducous, with a short pedicel of 0.4 - 6.6 µm and a length-to-width ratio varying from 1.3 to 1.9. The papillae measured between 0.9 – 11.2 µm length and 0.5 – 11.4 µm width. Chlamydospores were globose, terminal and interpolate, showing 20.0 – 53.6 µm diameter and 0.3 – 4.4 µm wall thickness. All isolates were heterothalic of the compatible mating type A1. Oospores were globose and aplerotic with no ornamented walls, measuring 26.0 – 63.6 µm, and showing amphigynous antheridia. Based on these morphological and molecular characteristics, the peach palm isolates were identified as Phytophthora palmivora (Butler) Butler.摘要 用于生产棕榈心的桃棕（Bactris gasipaes）种植面积在巴西巴伊亚州、圣埃斯皮里图州、里约热内卢州、托坎廷斯州、圣保罗州、巴拉那州及圣卡塔琳娜州均呈增长趋势。在上述区域，该作物遭受到茎基腐病（SBR）的侵染，该病害与卵菌棕榈疫霉（Phytophthora palmivora）密切相关。目前针对该病害的研究较为匮乏，关于棕榈疫霉的部分学术疑问仍未得到解答。为此，本研究旨在对从三处桃棕种植基地分离得到的疫霉属（Phytophthora）菌株进行鉴定，并从形态学、生理学及分子生物学层面开展系统的特征分析，以明确其病原学特性。本研究共获取31株疫霉属菌株，均分离自圣保罗州（埃尔多拉多、卡雅蒂与雷吉斯特罗）、巴拉那州（帕拉瓜苏与莫雷蒂斯）以及圣卡塔琳娜州（马萨兰杜巴、加鲁瓦与若因维利）的桃棕种植园。对所有菌株的菌丝生长情况在8个温度梯度（8、12、16、20、24、28、32及36℃）下进行了评估，同时观测了菌落形态、孢子囊、厚垣孢子及卵孢子的产生情况。对每株菌株的50个相关结构进行了尺寸测量，并基于ITS1、ITS2区域以及Cox1、Cox2基因区域开展了分子生物学分析。研究共发现三种菌落形态：轻微星芒状、星芒状与棉絮状，其中第一种形态最为常见。在8℃与36℃条件下菌株均无菌丝生长，最适生长温度为23.7℃。通过合轴个体发育形成的孢子囊具乳突结构，多呈椭圆形，尺寸范围为21.1~84.8μm（长）×17.4~41.7μm（宽），具脱落性，带有0.4~6.6μm的短梗，长宽比介于1.3~1.9之间。乳突尺寸为0.9~11.2μm（长）×0.5~11.4μm（宽）。厚垣孢子呈球形，可顶生或间生，直径为20.0~53.6μm，细胞壁厚度为0.3~4.4μm。所有菌株均为A1亲和交配型的异宗配合菌株。卵孢子呈球形且不充满藏卵器，细胞壁无纹饰，尺寸为26.0~63.6μm，雄器为围生型。基于上述形态学与分子生物学特征，本研究将分离自桃棕的菌株鉴定为棕榈疫霉（Phytophthora palmivora (Butler) Butler）。