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

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 on Brassica vegetable crops.

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 disease is relatively new in canola/rapeseed compared to other Brassica vegetable crops in which this disease is known from as early as 13th century.

In Canada, canola/rapeseed is the second most important crop next to the wheat and it is mainly cultivated in the Prairie Provinces. Canola is economically the most important crop for the Canadian agriculture, food, and animal industries. The crop supplies nutritionally healthy edible oil to the food industries [11] along with nutritionally balance seed meal to animal industries. Annually, over 8 M ha canola/rapeseed crop has been grown with greater than 15.6 MT productions in Canada [12]. Canola crop contributes more than \$15 billion each year to the Canadian economy [13]. In Canada, clubroot disease has been a problem on Brassica vegetables in producing areas including Ontario, Quebec, British Columbia, and the Atlantic Provinces. Clubroot has been periodically reported in few cases on Brassica vegetables in Alberta and Manitoba over the past 80 years [14]. This situation, however, entirely changed with the discovery of about 12 infected canola fields in Alberta in 2003. Annual survey carried out in Alberta, Saskatchewan, and Manitoba have revealed that clubroot is a much more widespread and serious disease in Canadian canola because canola is one of the major crops in the Prairie Provinces. In 2011, clubroot disease has been confirmed in over 800 fields distributed in most part of the Alberta [15], and from two fields in Saskatchewan [16]. Clubroot disease also reported in the North Dakota state in a few canola fields having patches of >80% plant mortality [17].

Clubroot disease has caused different degree of yield losses in canola/rapeseed fields depend‐ ing on pressure of the disease and nature of genetic inheritance (susceptible/moderately resistance/ resistance) of canola cultivars planted. Clubroot can cause up to 100% yield loss in heavily infected fields when susceptible canola cultivars are planted [18]. Similarly, about 90% yield loss and 5–6% reduction in oil content was reported in clubroot-infected canola field in Quebec [19]. In a previous publication, Dixon [20] has extensively reviewed clubroot infection in three major Brassica species, *B. oleracea*, *B. rapa*, and *B. napus* based on the survey data [10], and suggested that greater than 10% fields were infected in Australia, Canada, Czechoslovakia, Finland, Germany, Ireland, Netherlands, New Zealand, Norway, Poland, Scotland, United States, and Wales in the early 1980s.

In Asian countries, clubroot disease is widespread in the Brassica species cultivating regions in India, China, Nepal, Bangladesh, Pakistan, Indonesia, and Bhutan. In India, North Eastern part has become widespread due to frequent cultivation of cauliflower and yellow sarson, which are susceptible to clubroot. Similarly, China, Bangladesh, and Nepal are high-risk regions for the clubroot disease, especially for Brassica vegetables, mustard, and rapeseed production.
