Plant Health Management: Fungicides and Antibiotics
A.J. Leadbeater, in Encyclopedia of Agriculture and Food Systems, 2014
Fungicide Resistance and Its Management
Fungicide resistance is the naturally occurring, inheritable adjustment in the ability of individuals in a population to survive a plant-protection product treatment that would normally give effective control (OEPP/EPPO, 1999). Although resistance can often be demonstrated in the laboratory (and is, indeed, an important part of resistance-risk assessment for new fungicides), this does not necessarily mean that disease control in the field is reduced. ‘Practical resistance’ is the term used for the loss of field control due to a shift in the pathogen′s sensitivity to a fungicide. When it occurs in the field, fungicide resistance affects all those concerned with crop health; the growers, advisors, and the industry that provides these with the advice and products necessary to ensure a healthy, productive crop. Without successful resistance management, the effectiveness and eventually the number of modern fungicides available to the farmer and grower will diminish rapidly, leading to poor yields and reduced crop quality. Such a scenario could quickly lead to overuse of affected fungicides as users strive to get products to work (e.g., higher dose rates being used, or an increase in the frequency of application), leading in turn to increased and undesirable loading on the environment.
Growers and the agrochemical industry have lived with resistance problems for many years. Resistance was not considered to be a real problem in the early 1960s – fungicides had been used over many years, quite intensively in several crops, with no obvious signs of problems. At that time, the fungicide market consisted largely of nonsite-specific, nonsystemic, and moderately effective fungicides. There were some reports of resistance to diphenyl and sodium-o-phenylphenate in Penicillium digitatum in citrus at that time (Harding, 1962), and a failure of hexachlorbenzene to control Tilletia foetida in Australia (Kuiper, 1965); in addition, some isolates of Pyrenophora avenae in oats in Scotland were found to be resistant to organomercurial seed treatment (Noble et al., 1966), but these were a small number of cases and were not considered to be very economically significant. Moreover, the fungicides had been in use for many years and so were not considered to have important consequences in terms of relevance to other fungicide and disease combinations.
However, the cases of practical resistance to the benzimidazole fungicide benomyl after only 2 years of commercial use in the USA on cucurbit powdery mildew caused far more concern (Schroeder and Provvidenti, 1969). Other reports of resistance to benomyl and other related fungicides followed quite quickly afterward. Reports of resistance to fungicides became more frequent following these early cases and, in the 1970s, reports were published documenting resistance to important fungicides such as dodine, kasugamycin, and the phenyltins. Since the 1970s, cases of resistance to fungicides have increased in number, significance, and geographical spread, with the phenylamides, dicarboximides, SMIs (e.g., triazoles), QoIs (e.g., strobilurins), and, more recently, the SDHIs all suffering from to shifts in sensitivity or full practical resistance in key crops and diseases.
From the above, it can be easily realized that fungicide resistance is a real threat to the effectiveness of many fungicide groups and must be managed, to ensure good product performance in the field for the grower and also to justify sustainable investment by the agrochemical industry into new fungicides. Fortunately, despite a general widespread occurrence of resistance to several fungicides, effective management strategies have limited the impact of this on crop protection and production. Fungicide resistance has been much researched and with the expert knowledge that such research has provided, effective resistance management strategies have been developed and implemented. Because the risk of the impact of resistance being high is directly related to the degree of exposure of the plant pathogen to a fungicide (or a group of fungicides belonging to the same cross-resistance group as defined by FRAC), most resistance management strategies involve limiting the number of applications of the ‘at-risk’ fungicide in a disease-control program. The other fungicides used in the disease control program must have different modes of action to the fungicide in consideration. Other fundamentals of resistance management include the use of mixtures of fungicides from different cross-resistance groups to avoid reliance on a single mode of action, starting the fungicide spray program early in disease epidemics (to reduce the probability that a chance mutation conferring resistance has happened in the fungal population) and to avoid long persistence of a single ‘at-risk’ fungicide in a crop such as might be experienced by a soil application targeted to control foliar disease in the crop. It is clear from these basic principles of fungicide resistance management that there is a high requirement for many fungicides with different modes of action to be available for the grower to choose from, which includes the preservation in the market place of the older modes of action such as multisites, which have a low resistance risk (Leadbeater et al., 2008; Leadbeater, 2011). The importance of fungicide resistance management becomes more clear when the fungicide market is considered. In 2011, considering the highest selling fungicides and their classes, 62% of the world fungicide market (by value) consisted of only six single-site modes of action (Table 3). These six mode-of-action groups are the QoIs, the azoles, benzimidazoles, SDHIs, phenylamides, and amines. Of these six, five are classified by FRAC as high risk or between medium and high risk of resistance occurring. Only 18% fall into the category of low resistance risk (23% if ‘resistance not known’ fungicides are included; Table 4). These statistics do not give the complete picture with regard to treated areas with each mode of action of course, but show clearly what fungicides growers depend on most to provide disease control.
Resistance risk classification | Number of fungicide groups (FRAC code list) | Number of fungicides | Worldwide sales 2011a (US$×1000)b |
---|---|---|---|
High | 4 | 30 | 4 041 |
High to medium | 4 | 24 | 836 |
Medium | 8 | 51 | 3 868 |
Medium to low | 11 | 36 | 1 269 |
Low | 11 | 23 | 2 348 |
Not known | 19 | 32 | 718 |
Others (bactericides, etc.) | 225 | ||
Total | 57 | 196 | 13 305 |
- a
- Excluding biologicals.
- b
- Calculated values based on data from AMIS., 2012. Crop Protection and Seeds Database. Midlothian, UK: AMIS and Phillips McDougall (2012) (www.Phillipsmcdougall.com).
In recognition of the challenge set by the possibility of fungicide resistance developing., and with the experiences of the benzimidazoles and phenylamides, FRAC was formed following a course in resistance in 1980 and a subsequent seminar in Brussels in 1981; FRAC is still extremely active today (Leadbeater, 2012b, www.FRAC.info). The purpose of FRAC is to provide fungicide resistance management guidelines to prolong the effectiveness of ‘at-risk’ fungicides and to limit crop losses should resistance occur. FRAC members are recognized industry experts in the field of fungicide resistance and are actively engaged in scientific work and discussions and are frequent contributors at educational events such as scientific symposia and symposia. Important resources have been produced by FRAC and are available on the FRAC website, including educational monographs, sensitivity testing methods, classification tables of fungicide mode of action grouping, summaries of records of fungicide resistance cases, and much more. Importantly, the updated situation with regard to resistance to the most important groups of ‘at-risk’ fungicides is available on the website for consultation and downloading. FRAC does not work in isolation, of course, and the key is cooperation and consultation between FRAC members and researchers, consultants, advisors, and officials in public and private organizations in order to share scientific information and enable all to come to common, supported conclusions, practical advice, and recommendations.