Geoffrey Richard Dixon

The Occurrence and Economic Impact of Plasmodiophora brassicae and Clubroot Disease

The significance of Plasmodiophora brassicae Woronin and clubroot disease which it incites in members of the family Brassicaceae is reviewed as the focus for this special edition of the Journal of Plant Growth Regulation. This is a monographic treatment of recent research into the pathogen and disease; previous similar treatments are now well over half a century old. Vernacular nomenclature of the disease indicates that it had a well-established importance in agriculture and horticulture from at least the Middle Ages onward in Europe and probably earlier. Subsequently, the pathogen probably spread worldwide as a result of transfer on and in fodder taken by colonists as livestock feed. It is a moot point, however, whether there was much earlier spread by P. brassicae into China and subsequently Japan as Brassica rapa (Chinese cabbage and many variants) colonized those lands in archaeological time. Symptoms, worldwide distribution, and economic impact are briefly described here to provide a basis for understanding subsequent papers. Clubroot disease devastates both infected field and protected vegetable and agricultural Brassica crops. Particular importance is placed on recent reports of crop losses in tropical countries, albeit where the crops are grown in cooler altitudes, and in the Canadian prairie land canola crops. The latter is of enormous importance because this crop is the single most important and essential source of vegetable oils used in human foodstuffs and in industrial lubricants where mineral oils are inappropriate.

Plasmodiophora brassicae in its Environment

Plasmodiophora brassicae Wor. is viewed in this article from the standpoint of a highly evolved and successful organism, well fitted for the ecological niche that it occupies. Physical, chemical, and biological components of the soil environment are discussed in relation to their effects on the survival, growth, and reproduction of this microbe. It is evident that P. brassicae is well equipped by virtue of its robust resting spores for survival through many seasonal cycles. Germination is probably triggered as a result of signals initiated by root exudates. The resultant motile zoospore moves rapidly to the root hair surface and penetration and colonization follow. The short period between germination and penetration is one of greatest vulnerability for P. brassicae. In this phase survival is affected at the very least by soil texture and structure; its moisture; pH; calcium, boron, and nitrogen content; and the presence of active microbial antagonists. These factors influence the inoculum potential (sensu Garrett, 1956) and its viability and invasive capacity. There is evidence that these effects may also influence differentially the survival of some physiologic races of P. brassicae. Considering the interaction of P. brassicae with the soil environment from the perspective of its biological fitness is an unusual approach; most authors consider only the opportunities to destroy this organism. The approach adopted here is borne of several decades spent studying P. brassicae and the respect that has been engendered for it as a biological entity. This review stops at the point of penetration, although some of the implications of the environment for successful colonization are included because they form a continuum. Interactions with the molecular and biochemical cellular environment are considered in other sections in this special edition.

Glycoalkaloid Concentration of Potato Tubers Following Exposure to Daylight

The total glycoalkaloid concentrations in white- (cv Pentland Hawk), pink- (cv Kerrs Pink) and red- (cv Desiree) skinned potato tubers were measured following 21 days exposure to daylight using high-performance liquid chromatography. Average daytime irradiance during this period was 232 μmol m -2 s -1 . Regardless of cultivar, glycoalkaloid concentrations were increased after light exposure compared with initial concentrations. Glycoalkaloid concentrations fluctuated with time and continuous accumulation of glycoalkaloids with time was not demonstrated. Glycoalkaloid synthesis was maximal in the sequence cv Kerrs Pink < cv Pentland Hawk < cv Desiree. Exposure to daylight altered the ratio of α-chaconine : α-solanine in tubers of cv Desiree but not those in cv Pentland Hawk and Kerrs Pink. Glycoalkaloid concentrations in all cultivars were higher than the recommended food safety level, this was reached after 8 days in cv Kerrs Pink and Desiree and at 13 days in Pentland Hawk. The implications of these results in terms of food safety are discussed.

Clubroot (Plasmodiophora brassicae Woronin) – an agricultural and biological challenge worldwide

Abstract Clubroot disease and the causal microbe Plasmodiophora brassicae offer abundant challenges to agriculturists and biological scientists. This microbe is well fitted for the environments which it inhabits. Plasmodiophora brassicae exists in soil as microscopic well-protected resting spores and then grows actively and reproduces while shielded inside the roots of host plants. The pathogen is active outside the host for only short periods. Consequently, scientific studies are made challenging by the biological context of the host and pathogen and the technology required to investigate and understand that relationship. Controlling clubroot disease is a challenge for farmers, crop consultants and plant pathology practitioners because of the limited options which are available. Full symptom expression happens solely in members of the Brassicaceae family. Currently, only a few genes expressing strong resistance to P. brassicae are known and readily available. Agrochemical control is similarly limited by difficulties in molecule formulation which combines efficacy with environmental acceptability. Manipulation of husbandry encouraging improvements in soil structure, texture, nutrient composition and moisture content can reduce populations of P. brassicae. Integrating such strategies with rotation and crop management will reduce but not eliminate this disease. There are indications that forms of biological competition may be mobilized as additions to integrated control strategies. This review charts key themes in the development of scientific biological understanding of this host-pathogen relationship by offering signposts to grapple with clubroot disease which devastates crops and their profitability. Particular attention is given to the link between soil and nutrient chemistry.

Climate change – impact on crop growth and food production, and plant pathogens

Abstract Climates are changing worldwide at rates not seen previously in geological time. This affects food production itself and the growth and reproduction of plant pathogens which reduce crop yield and quality. There is a need to develop an understanding of the implications and impacts of climate change on natural biodiversity, artificial landscapes as well as production agriculture (defined here as a generalization embracing all of the primary uses of land for agriculture, horticulture and forestry), since these form parts of an integrated continuum. Currently, 20–25% of harvested crops worldwide are lost to pre- and post-harvest diseases and climatic change is expected to increase these losses. Climatic change results in increasing variability and altered scales of temperature, rainfall and wind velocity and periodicity. These changes affect the activities and vigour of aerial and soilborne pathogens. Some pathogens capable of devastating crops and harvested produce have become more active and damaging because their geographical ranges expand as a consequence of climate change. Human populations are increasing rapidly, resulting in greater demands on all natural resources which far outstrip supplies. Natural biodiversity is being damaged, frequently beyond repair and not infrequently with little or no knowledge of the characteristics of the plant genotypes being lost. Only very recently have analyses of ecosystem services begun revealing the intricate and delicate webs of unappreciated natural assets which are vital for human sustainable (used here to describe processes and actions which balance resource-use with resource-availability and conserve economic, environmental and social welfare) survival. The combination of climatic change, expanding human demands for food, resources and space and the increased activities of plant pathogens presents pathologists with immense challenges. Previously, visionary plant pathologists have contributed hugely to solving humanitys problems. Current challenges compounded by climate change offer opportunities for the exploitation of our professions capacities for working with ‘one foot in the furrow (and) one hand on the bench’ with increasing relevance for society at large.

Glycoalkaloid concentrations in aerial tubers of potato (Solanum tuberosum L)

The total glycoalkaloid concentrations in aerial and subterranean tubers of 14 potato genotypes were measured using high-performance liquid chromatography immediately after harvest. Post-harvest, aerial tubers from all genotypes were exposed to 144 h continuous fluorescent light ; additionally three genotypes (cvs Home Guard, Kerrs Pink and Desiree) were subjected to mechanical wounding prior to glycoalkaloid analysis. Variations in glycoalkaloid concentrations between aerial tubers taken from a single genotype (cv Kerrs Pink) were determined by analysis of eight aerial tubers formed in the second leaf axil, selected from separate individual plants. Irrespective of genotype, total and individual glycoalkaloid concentrations were higher in aerial than in subterranean tubers. The highest glycoalkaloid concentrations were found in aerial tubers of cv Kerrs Pink (1343.0 mg kg -1 FW) and lowest in cv Lindsey (301.0 mg kg -1 FW). Ratios of α-chaconine : α-solanine in aerial tubers differed significantly (P < 0.05) from those in subterranean tubers of cv Cara, Golden Wonder, Home Guard, Lindsey, Maris Piper, Record and 8859 indicating that exposure to light during aerial tuber growth enhanced the synthesis of one glycoalkaloid to a greater degree than the other. In all cultivars except cv Maris Piper, exposure of aerial tubers to 144 h continuous fluorescent light post-harvest increased total and individual glycoalkaloids compared with dark-treated controls. However, the ratios of α-chaconine : α-solanine in all genotypes except cv Golden Wonder (decreased α-chaconine : α-solanine, P < 0.05) were not significantly altered in comparison with dark controls, indicating that light exposure of aerial tubers post-harvest fails to enhance selectively synthesis of individual glycoalkaloids in the majority of cultivars. Regardless of cultivar, total glycoalkaloid, α-solanine and α-chaconine concentrations were higher in wounded than unwounded aerial tubers. Wounding stimulated synthesis of α-solanine more than α-chaconine (P < 0.05) in cv Home Guard and Desiree. Glycoalkaloid concentrations in aerial tubers varied widely from a minimum of 1010 mg kg -1 to a maximum of 2520 mg kg -1 FW when harvested from individual plants of cv Kerrs Pink but selected from equivalent positions on the plant. Throughout the experiments large, non-significant increases in total and individual glycoalkaloid concentrations were recorded following light and wounding treatments. The scientific implications of aerial tuber formation are discussed.

Comparative composition of aerial and subterranean potato tubers (Solanum tuberosum L)

The composition of aerial and subterranean tubers of two potato genotypes cvs Desiree (red skin) and Epicure (white skin) was compared immediately after harvest using a range of analytical methods. Irrespective of genotype, aspartic acid, arginine, glycine, glutamic acid, histidine, leucine, serine, threonine and total amino acid concentrations were significantly higher (P 0·05), but not significantly different between aerial and subterranean tubers of cv Epicure. The potential applications of aerial tubers for industrial purposes are discussed.

Clubroot (Plasmodiophora brassicae) on canola and other Brassica species – disease development, epidemiology and management

Crops in the family Brassicaceae offer mankind a diversity of useful and valuable products which are agronomically and economically second only in importance to the cereals. Collectively, the brassicas deliver: leaf, stem, flower and root vegetables which are eaten fresh, cooked and processed; they provide sources of health-giving oils for human consumption which are low in saturated fats and industrial oils, lubricants, biofuels and illuminants; sources of fodder and forage which contribute especially to overwintering supplies of meat and milk products; forms of condiments such as mustards, herbs and other flavourings; soil conditioners used as green manure, biofumigants and composts and a range of ornamentals which decorate pleasure gardens. The tiny rock cress Arabidopsis thaliana has become the power-tool of molecular biology comparable with the zoologists’ fruit fly (Drosophila melanogaster) which together with the rapid cycling brassicas (Wisconsin Fast PlantsTM) provide essential organisms for education and molecular biological research. Consumption of the brassicas is recognized as an important element in sustaining human health and well-being and combating long-term risks from cancer, coronary and other diseases of affluence. Productivity in all brassica crops is challenged by a wide range of pests, pathogens and physiological disorders. Those which have soilborne life cycles are the most intractable, and of these, the protist microbe Plasmodiophora brassicae, the cause of clubroot disease, is biologically highly intriguing and agriculturally the most destructive. Clubroot disease has been recognized as a substantial and persistent cause of crop losses in the brassicas at least since Roman times. The biology of the pathogen P. brassicae began being unravelled over 150 years ago but because of its well-evolved and highly protected lifestyle much remains unexplained. For reasons elucidated in this Special Issue of the Canadian Journal of Plant Pathology, control and the elimination of clubroot disease is fraught with difficulties and the pathogen P. brassicae is capable of extended and wellprotected perennation in the soil. The damage caused by clubroot disease is well recognized and reported in vegetable crop brassicas. Its impact in broadacre oil seed crops has come to the fore since the 1960s as they assumed increasing importance in the farming economies of Australia, China, North America and Western Europe. Clubroot emerged as a major concern in Canada after its identification on the canola crop in the Province of Alberta. Canola, which can include Brassica napus L., B. rapa L. and B. juncea L., was developed in Canada as a high-quality brand and produces one of the healthiest edible oils on the market. It is very important to the national economy, contributing an estimated Cdn

Plant pathogens and their control in horticulture.

15.4 billion annually from 2007–08 to 2009–10. Given this economic significance, the discovery of clubroot on

The Impact of Climate and Global Change on Crop Production

Plant pathogens and their control in horticulture , Plant pathogens and their control in horticulture , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی