7 PESTS AND PATHOGENS INTRODUCTION Pests and pathogens of brassicas are considered in this chapter mainly from the perspective of their control. Details of host–pest and host–pathogen interactions, biology and epidemiology are widely available elsewhere in numerous textbooks. As with weeds (Chapter 6), the strategies underlying methods of control are shifting very substantially away from reliance on single ‘magic bullet chemicals’. Previous decades saw an explosion in the use of synthetic chemicals that frequently acted by disrupting specific sites in enzyme-guided metabolic pathways of both pests and pathogens. These chemicals had modes of action which broadly included eradicants, protectants, soil sterilants, seed dressings and systemic compounds. Many were used in Brassica crops destined for human consumption albeit following stringent evaluation and testing to determine the extent of their interaction with the environment, human health and other non-target organisms. Although such chemicals do provide remarkably effective pest and pathogen control, they have many disadvantages. These chemicals exert intense selection pressures on the populations of the target organisms, leading to the rapid appearance of tolerant strains in the well-established ‘boom and bust’ cycle (Dixon, 1981). They adversely affect surrounding populations of non- target organisms, diminishing forms of natural biological control, and may lead to undesirable residues and by-products reaching human and other food chains. Public recognition of these disadvantages has led to social pressures demanding the drastic limitation, and in many cases complete banning, of many pesticides. The use of synthetic chemicals in Brassica crop production is diminishing especially in Europe, and also increasingly elsewhere wherever the quality assurance processes demanded by Western supermarkets are made mandatory. One consequence is that most agrochemical companies are very reluctant to develop new molecules for use specifically in Brassica crops. These are no longer seen as a profitable market sector and, additionally, many traditional pesticides have disappeared because the companies find it © G.R Dixon 2007. Vegetable Brassicas and Related Crucifers (G.R. Dixon) 183 184 Chapter 7 uneconomical to support them as a result of the ever more stringent statutory requirements for data concerning their toxicological and environmental impact. It is predicable that brassicas grown worldwide for human consumption will be produced with only very minimal use, if any, of synthetic pesticides within the next two decades. Consequently, specific references to individual pesticides and their uses for the control of pests and pathogens of Brassica crops are avoided in this book except where there is relevance to a general biological or agronomic principle concerned with control. Specific references to the use of insecticides, fungicides and other agrochemicals are largely limited to the latter parts of this chapter where particular examples of pests and pathogens are provided. Currently, developing strategies for pest and pathogen control utilize combinations of control measures in integrated systems (Theunissen, 1984). A basic component of integrated control systems is the use of forms of genetic resistance. Brassica crops, because of their genetic flexibility, lend themselves admirably to this approach and, even in the era of chemical control, pest- and pathogen-resistant cultivars played a major role in assisting crop production. Examples of the development of resistant cultivars are described for several of the pests and pathogens discussed later in this chapter. The techniques used for pest and pathogen control on brassicas, like other horticultural crops, are typified by their diversity, as advocated in Dixon (1984). These include the use of biological methods of control, a range of cultural techniques that minimize pest and pathogen attack, and certified healthy seed and planting material. The recent emergence of a market for seed grown by organic methods which excludes the use of any synthetic chemicals has added impetus for the development of diversity in the components used in integrated control strategies. The control of pests and pathogens in Brassica crops now demands an open-minded and flexible approach that accepts novel and unusual techniques. This typifies the changing attitude overall to pest and pathogen control that now seeks to alter the edaphic and aerial environments in favour of the crop and to the disadvantage of the pest or pathogen. Successful use of this strategy demands a detailed understanding of host and pest or pathogen biology combined with an understanding of the surrounding physical, chemical and biological environments. A diverse range of factors governs the extent of damage caused by predatory organisms. Factors such as host plant chemistry, the geographical region in which the crop is grown, the availability of alternative hosts and environmental temperature, rainfall and soil structure affect the outcome of encounters between pests, pathogens and their hosts. Brassica crops are vulnerable to very large numbers of pests and pathogens. Regrettably, only a few of those with major significance worldwide can be included here. The selection of damaging pests and pathogens discussed in this chapter solely highlights general principles related to those that are of greatest significance Pests and Pathogens 185 for both ware and seed vegetable Brassica crops (Dixon, 1981, 1984; Jones and Jones, 1984; Finch and Thompson, 1992). FORMS OF CROP LOSS CAUSED BY PESTS AND PATHOGENS Land capital value Land areas devoted to individual vegetable Brassica crops tend to be relatively small, although in specific geographical regions they may appear as substantial concentrations by virtue of favourable soil types, climatic conditions, cultural traditions and, consequently, the availability of skilled labour. Concentration of production especially where several Brassica crops are grown on one area of land in successive seasons almost inevitably leads to rampant infestation with soil-borne pests and pathogens. These diminish crop yield and may well render the land completely unfit for economic production of Brassica crops. In consequence, the longer term asset value of the land drops. Classic examples of pathogen and pest infestations that diminish land value include clubroot disease caused by Plasmodiophora brassicae, white rot caused by Sclerotinia sclerotiorum and the Brassica cyst nematode, Heterodera cruciferae. Direct yield loss Single vegetable Brassica plants have high individual cash values. Pest and pathogen damage to even small percentages of a crop can have devastating effects on yields and economic returns. Indeed, even the presence of only slight blemishes may make produce unacceptable to the market and hence totally valueless. Consequently, the impact of damage is not directly correlated with the incidence and intensity of pathogen or pest population as is often the case with extensive agricultural crops. A range of forms of loss may arise in Brassica crops throughout the supply chain from the field to the point of retail sale, for example: ● failure of growth and reproduction by individual plants in the field; ● damage to the saleable portions of the plant in the field; ● distortion of the harvesting sequence with consequent failure to reach the desired market at an optimal time and maximum value; ● rejection during harvesting, grading or display; ● development of blemishes in transit, during storage or at points of sale; ● downgrading during the marketing chain; ● total rejection at any point from field to retail sale. 186 Chapter 7 The consumer increasingly fails to recognize the connection between demands for reduced use of pesticides in food production and the likelihood of increasing the frequency of blemishes developing. Consumers are not prepared to accept Brassica products where there are signs of pest activity, eggs, larvae or frass, nor areas of disease symptoms caused by pathogen infection. These demands expressed through supermarket buyers, for blemish- and pesticide-free produce, are driving strenuous research and development programmes which aim to find alternative approaches to pest and pathogen control. CROP MONITORING, FORECASTING AND DISEASE MANAGEMENT The philosophy of ‘forewarned is forearmed’ whereby pest and pathogen invasion is anticipated and crops monitored for the first signs of their arrival is not new, but the application of new technologies makes it more efficient and effective. Monitoring Brassica crops helps to ensure that control measures, particularly where chemical methods remain available, are applied swiftly and effectively, and as part of the integrated strategy. This contrasts with the previous practices whereby chemical control measures were either applied at specified and regular intervals during the growing season irrespective of the presence or absence of a particular pest or pathogen or they were used as ‘fire brigade’ tactics once an outbreak had reached visibly damaging proportions. In the former case, pesticides were often used in excess of their requirement, resulting in financially costly, wasted applications and environmental pollution as, for example, with powdery mildew (Erysiphe cruciferarum) on Brussels sprouts where little is known about the threshold values for spray control and chemical application is advocated as soon as symptoms appear in August or early September. With the ‘fire brigade’ approach to control, applications