Laboratory of Plant Functional Genomics, Center for Genetic Engineering and Biotechnology, P.O. Box 6162, Havana, 10600, Cuba Plant Health Institute, Playa, Havana 11600, Cuba.
Mol Plant Pathol. 2011 Apr;12(3):209-16. doi: 10.1111/j.1364-3703.2010.00664.x. Epub 2010 Oct 1.
Rhizoctonia solani Kühn is a soil-borne fungal pathogen that causes disease in a wide range of plants worldwide. Strains of the fungus are traditionally grouped into genetically isolated anastomosis groups (AGs) based on hyphal anastomosis reactions. This article summarizes aspects related to the infection process, colonization of the host and molecular mechanisms employed by tobacco plants in resistance against R. solani diseases.
Teleomorph: Thanatephorus cucumeris (Frank) Donk; anamorph: Rhizoctonia solani Kühn; Kingdom Fungi; Phylum Basidiomycota; Class Agaricomycetes; Order Cantharellales; Family Ceratobasidiaceae; genus Thanatephorus.
Somatic hyphae in culture and hyphae colonizing a substrate or host are first hyaline, then buff to dark brown in colour when aging. Hyphae tend to form at right angles at branching points that are usually constricted. Cells lack clamp connections, but possess a complex dolipore septum with continuous parenthesomes and are multinucleate. Hyphae are variable in size, ranging from 3 to 17 µm in diameter. Although the fungus does not produce any conidial structure, ellipsoid to globose, barrel-shaped cells, named monilioid cells, 10-20 µm wide, can be produced in chains and can give rise to sclerotia. Sclerotia are irregularly shaped, up to 8-10 mm in diameter and light to dark brown in colour.
Symptoms in tobacco depend on AG as well as on the tissue being colonized. Rhizoctonia solani AG-2-2 and AG-3 infect tobacco seedlings and cause damping off and stem rot. Rhizoctonia solani AG-3 causes 'sore shin' and 'target spot' in mature tobacco plants. In general, water-soaked lesions start on leaves and extend up the stem. Stem lesions vary in colour from brown to black. During late stages, diseased leaves are easily separated from the plant because of severe wilting. In seed beds, disease areas are typically in the form of circular to irregular patches of poorly growing, yellowish and/or stunted seedlings.
Knowledge is scarce regarding the mechanisms associated with resistance to R. solani in tobacco. However, recent evidence suggests a complex response that involves several constitutive factors, as well as induced barriers controlled by multiple defence pathways.
This fungus can survive for many years in soil as mycelium, and also by producing sclerotia, which makes the management of the disease using conventional means very difficult. Integrated pest management has been most successful; it includes timely fungicide applications, crop rotation and attention to soil moisture levels. Recent developments in biocontrol may provide other tools to control R. solani in tobacco.
立枯丝核菌（Rhizoctonia solani Kühn）是一种土传真菌病原体，可导致全球范围内广泛的植物发病。根据菌丝融合反应，真菌菌株传统上被分为遗传上隔离的吻合群（ anastomosis groups，AGs）。本文总结了与烟草植物对立枯丝核菌病害的感染过程、定殖和分子机制相关的方面。
有性型：Thanatephorus cucumeris (Frank) Donk；无性型：Rhizoctonia solani Kühn；真菌界；担子菌门；伞菌纲；鸡油菌目；蜡壳耳科；Thanatephorus 属。
培养中的体细胞菌丝和定殖基质或宿主的菌丝最初是透明的，然后随着老化变成浅黄色至深棕色。菌丝分枝时往往呈直角，分枝点通常收缩。细胞缺乏结合连接，但具有复杂的套孔隔膜，带有连续的副体，多核。菌丝大小变化很大，直径为 3-17 µm。尽管该真菌不产生任何分生孢子结构，但可以产生链状、10-20 µm 宽的椭圆形至球形、桶形细胞，称为 monilioid 细胞，并能形成菌核。菌核形状不规则，直径可达 8-10 mm，颜色从浅棕色到深棕色。
烟草中的症状取决于吻合群以及被定殖的组织。立枯丝核菌吻合群 AG-2-2 和 AG-3 感染烟草幼苗，导致猝倒和茎腐。立枯丝核菌吻合群 AG-3 在成熟烟草植株中引起“胫骨痛”和“靶斑”。一般来说，叶片上开始出现水渍状病斑，并向上蔓延至茎部。茎部病斑颜色从棕色到黑色不等。在后期，由于严重萎蔫，患病叶片很容易从植株上脱落。在苗床中，病害区域通常呈圆形至不规则形状的斑块状，表现为生长不良、发黄和/或矮化的幼苗。
关于烟草对立枯丝核菌的抗性相关机制知之甚少。然而，最近的证据表明，这涉及到一个复杂的反应，包括几个组成性因素，以及由多个防御途径控制的诱导性障碍。
该真菌可以作为菌丝在土壤中存活多年，也可以通过形成菌核来存活，这使得使用常规方法管理病害变得非常困难。综合虫害管理最为成功；它包括及时施药、轮作和注意土壤水分水平。生物防治的最新发展可能为烟草中对立枯丝核菌的防治提供其他工具。
