14.1 Fungal diseases and loss of world agricultural production
Standing out among the examples of how damaging a crop disease can be is the Irish famine of 1845/46, which was caused by the failure of the potato crop in Europe because of just one plant disease, the Potato Late Blight (caused by a filamentous fungus-like member of the Oomycota, Phytophthora infestans). This is an astonishing story of how a crop disease affected the structure of our civilisation and our understanding of nature, while causing the deaths of one in eight of the Irish population. It is a story which goes far beyond statistics of number of deaths due to starvation, number of people emigrating, or crop losses and reduction in agricultural yield, and you can read that story in more detail in Chapter 2 of the book Slayers, Saviors, Servants and Sex. An exposé of Kingdom Fungi by David Moore (2000). But it is a piece of our history which we must read about in the knowledge that even today world agriculture suffers significant losses due to plant disease, despite all our scientific advances of the past 150 years. Hopefully, in that time we have learned enough at least to avoid massive calamities like the Irish famine, and today’s losses can be reported in terms of monetary losses. But behind each such statistic there must be personal tragedies in which the lives of individuals and families are changed dramatically.
Although weeds are the major cause of crop loss on a global scale, major losses are suffered by agricultural crops today due to insect damage and plant diseases (Table 1). Crop protection measures include weed control, which can be managed mechanically or chemically, and the control of animal pests or diseases, which relies heavily on synthetic chemicals. Despite large increases in pesticide use, crop losses have not significantly decreased during the last 40 years. However, pesticide use has enabled farmers to modify production systems to increase crop productivity while still maintaining some measure of control over losses likely to occur from susceptibility to the damaging effect of pests (Oerke, 2006).
Of course, it is not only fungi that cause plant disease (Fig. 1). There are bacteria, viruses, nematode worms (eel worms), aphids and insects as well as fungi. Serious plant diseases are caused by all these other pests, but fungi probably cause the most severe losses due to disease around the world. For one thing there are more plant pathogenic fungi than there are plant pathogenic bacteria or viruses. One survey made several years ago in the American State of Ohio came up with the estimate that the State had one thousand diseases of plants caused by fungi, one hundred caused by viruses and fifty due to bacteria.
Crop losses are caused by both biological and physical aspects of the environment that lead to a lower actual yield than the site can be expected to attain (Figs 1 and 2). The attainable yield is the realistic technical maximum under the best achievable growth conditions. It is generally much less than the yield potential, which is the theoretical maximum that cannot be reached under practical growth conditions in the field. Crop losses are best expressed as a proportion of attainable yield but sometimes the proportion of the actual yield is calculated. Pests reduce crop productivity in various ways, for example by:
- reducing the stand (= the population) of plants (pathogens that kill the host [necrotrophs], like damping-off pathogens that kill seedlings, are examples);
- reducing photosynthetic rate (fungal, bacterial, virus diseases);
- accelerating plant senescence (most pathogens);
- shading and ‘stealing’ light (weeds, some pathogens);
- depleting assimilate (nematodes, pathogens, sucking arthropods);
- consuming tissue (chewing animals, necrotrophic pathogens);
- competition for inorganic nutrients (weeds).
Crop losses can be quantitative and/or qualitative, and expressed in absolute terms (kg ha-1, or financial loss ha-1, for example) or in relative terms (% loss in production tonnage, for example):
- quantitative losses result from reduced productivity giving a lesser yield per unit area;
- qualitative losses from pests can result from:
- reduced content of a normal ingredient(s) of the crop,
- reduced market quality (for example miss-shaped, blemished fruit and vegetables),
- reduced storage quality,
- contamination of the harvested product with pests, parts of pests or toxic products of the pests (for example, mycotoxins).
|Fig. 1. Biological and physical aspects of the environment that lead to a lower actual yield than the site can be expected to attain under ideal circumstances. Modified from Oerke (2006).|
Agricultural survey statistics make it clear that crop losses directly attributable to fungi are very considerable. Of course, it’s changing all the time because, at least in part, losses depend on the weather, but it appears that world agriculture sustains average losses (in terms of monetary value) of around 16% annually as a result of plant diseases (Table 1). This overall average conceals instances of good news; with disease loss in the 1 to 2% percent range as well as bad news of a season of unusually heavy pest incidence which might involve losses in the 30 to 40% region. Among crops, the total global potential loss due to all pests varied from about 50% in wheat to more than 80% in cotton production. Other estimated actual losses are 26-29% for soybean, wheat and cotton, and 31, 37 and 40% for maize, rice and potatoes, respectively (Table 2). Overall, weeds produced the highest potential loss (34%), with animal pests and pathogens each causing about half that loss (Table 1) (Oerke, 2006).
|Fig. 2. Typical crop losses and yield levels estimated with and without various protection regimes. The value of crop protection practices (shown at left as ‘current situation’) can be calculated as the percentage of potential losses prevented by all the crop protection measures that are employed (compare with centre panel). In contrast, the impact of pesticide use on crop productivity (right hand panel) takes into account consequential changes in the agricultural system (for example, use of alternative varieties of the crop, modified crop rotation, reduced fertiliser use), which are provoked by the abandonment of pesticides and which are often accompanied by reduced attainable yield. Redrawn after Oerke, 2006.|
This is ‘21st century agriculture’ we’re talking about, not some primitive agriculture of the distant past. Today, at this very moment, one in every eight crop plants, on average, will fail to yield because of fungal disease and this includes the positive effect of crop protection policies. Without protection of crops in the developed world, loss of crops would range from 50 to 100% (Fig. 2; Table 1).
Table 1. Estimated annual crop loss world-wide, shown as an overall average %age of production tonnage lost to the indicated pest in the period 2001-2003.
|Microbial diseases (70-80% caused by fungi)||
Data from Oerke (2006).
Table 2. Estimated actual losses in the period 2001-2003 to particular crops world-wide shown as an average %age of production tonnage lost due to all pests and diseases.
Data from Oerke (2006). Data about world agricultural production is collected by the Food and Agriculture Organisation of the United Nations (visit: http://faostat.fao.org/) and in The Crop Protection Compendium, developed by CAB International in partnership with many other world agriculture organisations (visit: http://www.cabi.org/compendia/cpc/).
All groups of fungi and fungus-like organisms can cause serious plant diseases and we will expand on some specific examples, below. Here, just to indicate the range, we will give you the example of rusts and smuts, which are diseases caused by members of the group of fungi which is the most advanced in evolutionary terms, the Basidiomycota; while late blight of potatoes and downy mildew of grapes are diseases caused by the most ancient of fungal-like organisms, belonging to the Oomycota in kingdom Chromista. Diseases such as chestnut blight, peach leaf curl, Dutch elm disease, net blotch of barley, beet leaf spot, apple blotch, maple leaf spot and thousands of others are caused by all those fungi in between these extremes. There are an enormous number of plant disease fungi, so many that there’s a rumour about a monastery somewhere in the Himalayas where the monks are listing all the names of plant diseases. When they’ve entered the last one in their list, the Universe ends and we start all over again. But next time mushrooms rule, OK?
Crop losses due to weeds, pathogens and animal pests can be very substantial (Tables 1 & 2) and crop protection measures are needed to prevent, or reduce, potential losses. We lose more than mere money, too. A disease of the native American chestnut, Chestnut Blight (caused by an introduced parasite), effectively eliminated a valuable timber and nut-crop tree from the United States. A similar loss happened in England when large elm trees were killed by Dutch Elm disease (also caused by an introduced parasite but this time the introduction was from the US and into Europe), although this loss is more difficult to quantify because it is a loss of amenity as much as commercial value. Before the development of fungicides, fungal disease periodically caused massive devastation of crops and consequently mass starvation. The Irish potato famine in the mid-19th century is the prime example; but it was caused by Potato Late Blight (Phytophthora infestans) (see Resources Box). So far in this text we have refused to include these Chromistan fungus-like organisms as true fungi at all (see The untrue fungi section of Chapter 3; CLICK HERE to view the page), but every crop we grow suffers at least one Phytophthora disease (Lucas et al., 1991) so we cannot ignore these organisms in a Chapter about plant diseases. Fungi cause the majority of plant diseases, but they figure in only a minority of animal diseases; these will be discussed in Chapter 16.
The Potato Murrain
The Irish potato famine in the mid-19th century was caused by Potato Late Blight (Phytophthora infestans).
Updated December 17, 2016