Department of Plant Pathology, University of California Davis, Kearney Agricultural Research and Extension Center, Parlier, CA, 93648, USA.
Department of Plant Pathology, University of California Davis, Kearney Agricultural Research and Extension Center, Parlier, CA, 93648, USA.
Fungal Biol. 2022 Apr;126(4):277-289. doi: 10.1016/j.funbio.2022.02.003. Epub 2022 Feb 24.
Alternaria rot has been recently described as an emerging fungal disease of citrus causing significant damage in California groves. A survey was conducted to determine latent infections on fruits, twigs, and leaves and investigate their seasonal patterns during 2019 and 2020. On fruits, latent infections were more associated with the stem end than with the stylar end, except during spring when a significantly high percentage of flowers (86%) had latent infections. Latent infections on twigs varied markedly between years (28% in 2019 and 9.5% in 2020), while Alternaria spp. were also recovered from citrus leaves. Alternaria isolates collected during the survey were identified based on multigene sequence analysis, confirming that Alternaria alternata and Alternaria arborescens are the two species associated with infections of citrus fruits. Of the 23 isolates, 19 were identified as A. alternata and demonstrated the dominance of this species over A. arborescens. Isolates representing populations of these two species were selected as representative isolates for physiological and morphological studies. A. alternata and A. arborescens showed similar conidial dimensions but differed in the number of conidia produced. Growth rates demonstrated that A. alternata grows faster than A. arborescens at all the temperatures evaluated, except at 25 and 35 °C. The growth patterns were similar for both species. The sporulation rate of the Alternaria isolates was influenced differently by temperature. This parameter also influenced conidial germination and appressorium formation, and no significant differences were observed between Alternaria species. Pathogenicity and aggressiveness tests on detached fruit demonstrated the ability of A. alternata and A. arborescens to cause internal lesions and produce fruit drop in the orchards with no quantitative differences between them (disease severity indexes of 58 and 68%, respectively). The fungicide sensitivity tests showed that DMI fungicides are the most effective fungicides in reducing mycelial growth. The SDHI fungicides had intermediate activity against the mycelial growth but also suppressed spore germination. The spore germination assay suggested that some of the isolates included in this study might have some level of resistance to QoI and SDHI fungicides. The findings of this study provide new information about the pathogens associated with the excessive fruit drop recently observed in some California citrus groves.
链格孢菌炭疽病最近被描述为一种新兴的柑橘真菌病害，在加利福尼亚果园造成了严重损失。本研究于 2019 年和 2020 年进行了一项调查，以确定果实、嫩枝和叶片上的潜伏感染情况，并调查其季节性模式。在果实上，潜伏感染与果柄端的关系比与花柱端更为密切，除了在春季，有相当高比例的花朵（86%）存在潜伏感染。2019 年嫩枝上的潜伏感染率为 28%，而 2020 年为 9.5%，差异显著，同时还从柑橘叶片中分离出链格孢菌。根据多基因序列分析，从调查中收集的链格孢菌分离物被鉴定为，确认Alternaria alternata 和 Alternaria arborescens 是与柑橘果实感染相关的两种物种。在 23 个分离物中，有 19 个被鉴定为 A. alternata，并证明了该物种在 A. arborescens 上的优势。选择这两个物种的代表分离物进行生理和形态学研究。A. alternata 和 A. arborescens 的分生孢子尺寸相似，但产生的分生孢子数量不同。生长速率表明，除了在 25 和 35°C 下，A. alternata 在所有评估的温度下的生长速度都快于 A. arborescens。两种物种的生长模式相似。链格孢菌分离物的产孢率受温度的不同影响。该参数还影响分生孢子萌发和附着胞的形成，在链格孢菌物种之间没有观察到显著差异。在离体果实上进行的致病性和侵袭性测试表明，A. alternata 和 A. arborescens 能够引起内部病变，并在果园中导致果实脱落，它们之间没有定量差异（分别为 58%和 68%的严重度指数）。杀菌剂敏感性测试表明，DMI 杀菌剂是抑制菌丝生长最有效的杀菌剂。SDHI 杀菌剂对菌丝生长具有中等活性，但也抑制了孢子萌发。孢子萌发试验表明，本研究中包含的一些分离物可能对 QoI 和 SDHI 杀菌剂具有一定水平的抗性。本研究的结果提供了有关与加利福尼亚一些柑橘果园最近观察到的过度果实脱落相关病原体的新信息。
