**3. Disease cycle and symptoms**

**1. Introduction**

4 Plants for the Future

productivity of these crops.

on Brassica vegetable crops.

The crops in the *Brassicaceae* family are the most economically and nutritionally important for human consumption after cereals [1]. Based on utilities, Brassica species are broadly catego‐ rized into oilseed, vegetable, and sources of condiments. In the Brassica crops, canola/rapeseed is the second largest vegetable oil crop after soybean in the world [2]. Globally, rapeseed/canola has been cultivated in about 36.4 M ha with total production approximately 72.5 M tonnes [2]. Among the Brassica species, *B. napus*, *B. rapa*, *B. juncea*, and *B. carinata* provide about 15% of edible vegetable oil supplies around the world [3]. In addition, Brassica species such as *B. oleracea*, *B. rapa*, and *B. napus* supply nutritionally rich green leafy, stem and root vegetables for human daily diets. Brassica species also possess rich genetic diversity with respect to both speciation and the ample morphotypes [4], which designate important species to be investi‐ gated for genetic evaluation of plant kingdom. The Brassica crops, however, have significant impact by biotic stresses including diseases and pests, which challenge production and

Clubroot is one of the most threatening disease affecting global production and productivity of cruciferous crops including canola/rapeseed and Brassica vegetables. Clubroot disease is caused by the soil-borne obligate biotroph pathogen *P. brassicae* in Brassica crops. Cultivation of different Brassica oilseed and vegetable crops fulfills the host range requirement of the clubroot pathogen, which leads to wide spreading of the pathogen throughout the world. Since the emergence of the clubroot pathogen in vegetable crops of Brassica species, management of the disease has been a big challenge due to the obligate biotroph nature of the pathogen.

The clubroot disease is not a new one in Brassica crops, it has been historically considered as the most important disease [5]. The origin of the clubroot disease is unknown, but it appears as ancient as its host. Earlier literatures reported the existence of clubroot disease in the 13th century in Spain, and later in 17th century, clubroot disease was also observed in England and subsequently it spread in Scotland, France, Germany, Poland, and other European countries. In Russia, clubroot was first reported in Brassica vegetable crops in 1872 [6]. In Japan, the disease was first recorded in 1890s and now it is one of the major constrains in Chinese cabbage and other Brassica vegetable production [7]. Similarly, this disease was first reported in Australia in the early 1890s [8]. Most of the earlier clubroot disease infections were reported

Mustard/rapeseed crops have similar cultivation history like other Brassica vegetables in different parts of the world; however, there was no evidence of clubroot disease in rapeseed/ mustard crop in earlier time. Before three decades, about 2.5% canola/rapeseed crops were reported with clubroot disease in 18 countries [9, 10]. Since then, canola/rapeseed cultivation expanded significantly due to health benefit properties of its oil. This suggests that clubroot

**2. Impact of clubroot disease in Brassica species**

The pathogen *P. brassicae* Wornonin is an obligate biotrophic protist belonging to the class phytomyxea. The pathogen can infect primary and secondary roots at the early stage of plant growth and development that causes significant yield and quality losses. The life cycle of *P.* *brassicae* consists of two phases; in a primary phase, under favorable conditions, resting spores germinate and produce primary zoospores that penetrate in root hairs and mass production of secondary zoospores occurs in the root hairs. The resting spores are about 3 μm in size and subspherical to spherical in shape and the surface of each resting spore is covered with spines [21]. Mass of primary zoospores is released from each resting spore, spindle-shaped or pyriform, 2.8–5.9 μm long, and biflagellate. When the zoospores come in contact with the surface of a root hair, it penetrates in the cell wall and it is also called root hair infection. Secondary phase of life cycle occurs in the root cortex as a result secondary plasmodia and gall formation occurs as a result restriction in water and nutrient uptake by plants (Figure 1) [21, 22]. The life cycle study of *P. brassicae* in *A. thaliana* reported uninucleate and binucleate myxamoeboid structure production within host cytoplasm that caused cell wall burst and production of secondary plasmodia [23]. During pathogen infection, secondary plasmodia proliferate in roots and plant hormone, especially auxin and cytokinin, biosynthesis altered in the root tissues that causes gall formation (Figure 2) [21]. Infected plants become stunted, yellowish in color, and eventually wilt, which causes severe reduction in yield and quality of crops [24]. Mature secondary plasmodia subsequently develop into resting spores that can survive for 20 years or more [25]. Clubroot disease pressure can significantly increase in those fields where crop rotation frequently includes canola/rapeseed or other Brassica crops. Acidic soil with high soil moisture is the most favorable condition for resting spore germination and subsequent secondary infection.

**Figure 1.** Life cycle of *P. brassicae* and club like gall formation on the roots of Brassica host plant.

Understanding the Genetics of Clubroot Resistance for Effectively Controlling this Disease in Brassica Species http://dx.doi.org/10.5772/60936 7

**Figure 2.** Clubroot disease symptoms in highly infected fields. (a) Clubroot-resistant breeding line of *B. rapa* showed no clubroot symptoms on roots, (b) clubroot disease symptoms with large galls on primary and secondary roots in turnip rape line of *B. rapa* under field conditions, (c) clubroot galls on roots of broccoli in *B. oleracea*, and (d) highly infected field of Chinese cabbage in Henan province of China.
