Verticillium wilt was first reported in 1914 in Virginia, USA (Carpenter 1914). Since then, this disease was found not only throughout all cotton-producing regions of the USA but also through other cotton-producing countries in the world. Verticillium wilt is caused by a soil-inhabiting fungus, Verticillium dahliae Kleb, and in cotton, it is now considered to be the most important disease. This pathogen can be divided into two types: defoliating and nondefoliating, according to their virulence (Chang et al. 2008). While the nondefoliating pathotype is widespread and only causes mild wilt and no defoliation (Pérez-Artés et al. 2000), the defoliating pathotype develops earlier, faster and induces severe yield and quality losses compared with the nondefoliating one. Verticillium wilt caused by the defoliating pathotype has spread to a number of countries, including the USA, China, Spain, Turkey, Israel, mid-Asia countries, etc. (Li and Yang 2007; Korolev et al. 2008). Moreover, the disease has progressively increased in many regions since 1990 and has become a serious obstacle for cotton production in China. Selection of resistant cultivars is considered the most effective and economical method of disease control. However, because of the lack of immune or highly resistant upland cotton germplasm against the defoliating pathotype, little progress has been made towards this potential solution (Chang et al. 2008). Xiao et al. (1998) found that crop rotation could be a successful practice for managing cotton verticillium wilt, but it was not widely adopted in practice. In addition, no fungicides are currently registered for controlling this disease in cotton (Göre et al. 2009). Therefore, it is necessary to develop alternative methods to manage this disease. The use of biological control agents has been increasing worldwide and is a promising alternative for controlling soil-borne diseases in sustainable and organic agriculture. In the past, rhizosphere bacteria, such as Pseudomonas spp. and Serratia plymutica, have been shown to be effective antagonists towards verticillium wilt (Mercado-Blanco et al., 2004; López-Escudero and Mercado-Blanco, 2011; Erdogan and Benlioglu 2010). More recent studies have indicated that endophytic bacteria colonize the internal tissues of plants can even improve plant growth and plant health (Schulz et al. 2006; Tiwari et al., 2010; Jalgaonwala et al. 2011). Internal plant tissues provide a protective environment for endophytic bacteria, which colonize an ecological niche similar to that of phytopathogens. Therefore, endophytic bacteria are suitable as biocontrol agents to control plant pathogens (Hallmann et al. 1997; Berg and Hallmann 2006). In some cases, endophytic bacteria can significantly improve seed germination and plant growth under adverse conditions (Berg and Hallmann 2006). In addition, endophytic bacteria constitute an environmentally sound alternative to protect plants against the attack of fungal pathogens (Bloemberg and Lugtenberg 2001; Verma et al. 2004). Recently, we systematically studied endophytic bacteria against important fungal pathogens, V. dahliae Kleb and Fusarium oxysporum, in cotton. We found that a considerable population of antagonistic endophytic bacteria is present in cotton roots and that populations fluctuate depending on pathogen genotypes, cotton genotypes and growth stages. The species of antagonistic endophytic bacteria isolates towards both V. dahliae Kleb and F. oxysporum pathogens are diverse, and some have growth-promoting potential (Li et al. 2010). However, these antagonistic endophytic bacterial as biocontrol agents still need to be identified by further tests.
Here, we explore biocontrol potential of the antagonistic endophytic bacteria that inhibit the defoliating pathotype with greenhouse and field trials. Similarly, we investigated the colonization of HA02 with more efficient biocontrol potential in cotton and other hosts in vivo to understand the interaction between HA02 and cotton.