Root rot a silent alfalfa killer in China: Distribution, fungal, and oomycete pathogens, impact of climatic factors and its management

Abstract

Alfalfa plays a significant role in the pasture ecosystems of China's north, northeast, and northwest regions. It is an excellent forage for livestock, improves soil structure, prevents soil erosion, and has ecological benefits. Presently root rot is a significant threat to the alfalfa productivity because of the survival of the pathogens as soil-borne and because of lack of microbial competition in the impoverished nutrient-deficient soils and resistant cultivars. Furthermore, these regions' extreme ecological and environmental conditions predispose alfalfa to root rot. Moisture and temperature, in particular, have a considerable impact on the severity of root rot. Pathogens such as Fusarium spp. and Rhizoctonia solani are predominant, frequently isolated, and of major concern. These pathogens work together as disease complexes, so finding a host genotype resistant to disease complexes is challenging. Approaches to root rot control in these regions include mostly fungicides treatments and cultural practices and very few reports on the usage of biological control agents. As seed treatment, fungicides such as carbendazim are frequently used to combat root rot; however, resistance to fungicides has arisen. However, breeding and transgenic approaches could be more efficient and sustainable long-term control strategies, especially if resistance to disease complexes may be identified. Yet, research in China is mainly limited to field investigation of root rot and disease resistance evaluation. In this review, we describe climatic conditions of pastoral regions and the role of alfalfa therein and challenges of root rot, the distribution of root rot in the world and China, and the impact of root rot pathogens on alfalfa in particular R. solani and Fusarium spp., effects of environmental factors on root rot and summarize to date disease management approach.

Keywords: China; alfalfa; climatic factors; distribution; management; root rot.

Figure 1

Geographical distribution and affected region in China. Color depth indicates affected provinces, and dots indicate root rot pathogens based on China Academic Journals full-text database (CNKI), Web of sciences (WoS), and other websites.

Figure 2

Schematic representation of alfalfa root rot and other symptoms and inoculum sources. Inoculum sources include zoospores, oospores, sporangia, conidia, chlamydospores, sclerotia, mycelia, and basidiospores (not reported). Besides root rot, other symptoms include damping-off, seed and hypocotyl rots, discoloration on roots, crown rot, stunting, chlorosis, wilting, and wire stem.

Figure 3

Schematic representation of the presumed disease cycle of Fusarium root rot in China. (A) Alfalfa plant secrete root exudates and in response to exudates Fusarium spp. spores (chlamydospores) germinate and produce infection hypha to penetrate the root epidermis at the root tip. (B) The hypha proliferates in the root cortex and enters into the vascular vessels, i.e., xylem vessels. (C) In the vessels, it grows excessively and causes a blockage; as a result, brown discoloration occurs. (D) First symptoms appear at the base of the stem, and then the symptoms progress upward; as a result, the young leaves withered. (E) Partial chlorosis or complete chlorosis is observed mainly on the mature leaves. (F) Finally, the whole alfalfa plants wilt because of severe root rot followed by death. Fungal spores such as microconidia, macroconidia, and chlamydospores form dead alfalfa plant tissues and remain dispersed in the soil.

Figure 4

Classification of Rhizoctonia like fungi based on Moore (1987).

Figure 5

Schematic representation of the presumed disease cycle of Rhophitulus solani root rot in China. (A), The fungus overwinters in the plant debris and seeds in the form of mycelium and in the soil as sclerotia and mycelium. (B,C) The young hyphae germinate and develop under favorable conditions, sexual fruiting structures basidia and basidiospores are rare. (D) The mycelium penetrates roots near the soil line and colonized in inter and intracellular spaces. (E) The mycelium proliferates further in the cortex ultimately results in necrosis and sclerotia are formed in and on infected tissues and disintegration and acute rotting of roots. (F) Above ground symptoms, include chlorosis, blights, stunting and finally death, the fungus also infects seeds and seedlings and also causing damping-off.