Plant breeding for disease resistance has been underway since plants were first domesticated, but it requires continual effort. This is because pathogen populations are often under natural selection for increased virulence, new pathogens can be introduced to an area, cultivation methods can favor increased disease incidence over time, changes in cultivation practice can favor new diseases, and plant breeding for other traits can disrupt the disease resistance that was present in older plant varieties.
Plant breeders focus a significant part of their effort on selection and development of disease-resistant plant lines. Plant diseases can also be partially controlled by use of pesticides, and by cultivation practices such as crop rotation, tillage, planting density, purchase of disease-free seeds and cleaning of equipment, but plant varieties with inherent (genetically determined) disease resistance are generally the first choice for disease control.
A plant line with acceptable disease resistance against one pathogen may still lack resistance against other pathogens.
Plant breeding for disease resistance typically includes:
Identification of resistant breeding sources (plants that may be less desirable in other ways, but which carry a useful disease resistance trait). Ancient plant varieties and wild relatives are very important to preserve because they are the most common sources of enhanced plant disease resistance.
Crossing of a desirable but disease-susceptible plant variety to another variety that is a source of resistance, to generate plant populations that mix and segregate for the traits of the parents.
Growth of the breeding populations in a disease-conducive setting: This may require artificial inoculation of pathogen onto the plant population. Careful attention must be paid to the types of pathogen isolates that are present, as there can be significant variation the effectiveness of resistance against different isolates of the same pathogen species.
Selection of disease-resistant individuals: It is essential to note that breeders are trying to sustain or improve numerous other plant traits related to plant yield and quality, including other disease resistance traits, while they are breeding for improved resistance to any particular pathogen.
Each of the above steps can be difficult to successfully accomplish, and many highly refined methods in plant breeding and plant pathology are used to increase the effectiveness and reduce the cost of resistance breeding.
Resistance is termed durable if it continues to be effective over multiple years of widespread use, but some resistance “breaks down” as pathogen populations evolve to overcome or escape the resistance.
Resistance that is specific to certain races or strains of a pathogen species is often controlled by single R genes and can be less durable; broad-spectrum resistance against an entire pathogen species is often quantitative and only incompletely effective, but more durable, and is often controlled by many genes that segregate in breeding populations.
However, there are numerous exceptions to the above generalized trends, which were given the names vertical resistance and horizontal resistance, respectively, by J.E. Vanderplank.
Crops such as potato, apple, banana and sugarcane are often propagated by vegetative reproduction to preserve highly desirable plant varieties, because for these species, outcrossing seriously disrupts the preferred plant varieties. See also asexual propagation.
Vegetatively propagated crops may be among the best targets for resistance improvement by the biotechnology method of plant transformation to add individual genes that improve disease resistance without causing large genetic disruption of the preferred plant varieties.
Host Range
There are thousands of species of plant pathogenic microorganisms. Only a small minority of these pathogens have the capacity to infect a broad range of plant species. Most pathogens instead exhibit a high degree of host-specificity.
Non-host plant species are often said to express non-host resistance: The term host resistance is used when a pathogen species can be pathogenic on the host species but certain strains of that plant species resist certain strains of the pathogen species.
There can be overlap in the causes of host resistance and non-host resistance: Pathogen host range can change quite suddenly if, for example, the capacity to synthesize a host-specific toxin or effector is gained by gene shuffling/mutation, or by horizontal gene transfer from a related or relatively unrelated organism.
Epidemics and Population Biology
Plants in native populations are often characterized by substantial genotype diversity and dispersed populations (growth in a mixture with many other plant species). They also have undergone millions of years of plant-pathogen coevolution.
Hence as long as novel pathogens are not introduced from other parts of the globe, natural plant populations generally exhibit only a low incidence of severe disease epidemics.
In agricultural systems, humans often cultivate single plant species at high density, with numerous fields of that species in a region, and with significantly reduced genetic diversity both within fields and between fields.
In addition, rapid travel of people and cargo across large distances increases the risk of introducing pathogens against which the plant has not been selected for resistance. These factors make modern agriculture particularly prone to disease epidemics.
Common solutions to this problem include constant breeding for disease resistance, use of pesticides to suppress recurrent potential epidemics, use of border inspections and plant import restrictions, maintenance of significant genetic diversity within the crop gene pool.
Crop diversity, and constant surveillance for disease problems to facilitate early initiation of appropriate responses. Some pathogen species are known to have a much greater capacity to overcome plant disease resistance than others, often because of their ability to evolve rapidly and to disperse broadly.
Epidemic
In epidemiology, an epidemic (epi – meaning “upon or above” and demic – meaning “people”), occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is “expected,” based on recent experience (the number of new cases in the population during a specified period of time is called the “incidence rate”).
(An epizootic is the analogous circumstance within an animal population.) In recent usages, the disease is not required to be communicable; examples include cancer or heart disease. Another example includes the infamous Black Plague of the Middle Ages.
Classification
Defining an epidemic can be subjective, depending in part on what is “expected”. An epidemic may be restricted to one locale (an outbreak), more general (an “epidemic”) or even global (pandemic).
Because it is based on what is “expected” or thought normal, a few cases of a very rare disease may be classified as an “epidemic,” while many cases of a common disease (such as the common cold) would not.
Syndemics
The term syndemic refers to interacting epidemics that increase the health burden of affected populations.
Social conditions that heighten the health risk of populations (e.g. poverty, discrimination and stigmatization, and marginalization) by increasing stress, malnutrition, interpersonal violence, and the experience of deprivation, increase the clustering of epidemic diseases and the likelihood of their interacting.
Non-Infectious Disease Usage
The term “epidemic” is often used in a sense to refer to widespread and growing societal problems, for example, in discussions of obesity or drug addiction.
It can also be used metaphorically to relate a type of problem like those mentioned above.
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Factors Stimulating New Epidemics
Factors that have been described to stimulate the rise of new epidemics include:
Alterations in agricultural practices and land usage;
Changes in society and human demographics;
Poor population health (e.g., malnutrition, high prevalence of HIV);
Hospitals and medical procedures;
Evolution of the pathogen (e.g., increased virulence, drug resistance);
Contamination of water supplies and food sources;
International travel;
Failure of public health programs;
International trade;
Climate change;
Reduced levels of biodiversity (e.g. through environmental destruction);
Bad urban planning.
Plant Breeding for Pest Resistance
1. Resistance Breeding Before Mendel
Wild relatives of crop plants such as beans, wheat, and maize are not uniformly resistant to insect and disease pests.
This can be demonstrated in simple fashion when selections of these wild populations are set out in plant-rows, some of them are highly susceptible, others are resistant, and some are intermediate in resistance to the common pests of the region.
The first plant breeders, those women and men who domesticated crops such as beans, maize, and wheat, could save only those genotypes that had some level of resistance, i.e., those individual plants that did not succumb to pest depredation.
In effect, therefore, they selected for pest resistance and thus changed the population structure of their crop species in favor of resistance genes.
This change made it possible to grow the crops in monoculture, which was convenient for food production and harvest. It was also convenient for multiplication of disease and insect pests that might not be affected by the limited sample of resistance genes.
Plant breeding thus set the stage for sequential cycles of pest resistance and pest susceptibility of crop plants.
We have no direct record of the consequences of this ancient ecological meddling, but myth and historical accounts tell of disastrous disease epidemics and insect outbreaks, so one can assume that from time to time large plantings of crops that were uniformly susceptible to a new kind of insect or disease fostered increases of that pest to epidemic proportions.
Resistance genes were essential for crop domestication and monoculture but they did not guarantee perfect safety.
We have no record at all and little or no speculation about how the newly domesticated crops might have affected their wild relatives, which no doubt were growing in close proximity to the domesticates.
2. Resistance Breeding After Mendel
Genetics-based plant breeding, launched in the early years of the 20th century, produced new crop varieties with improved resistance to major disease and insect pests. Usually such resistance was developed as a second phase – a rescue operation – after new varieties, selected primarily for high yield, were discovered to be susceptible to a particular insect or disease.
Breeders found early on that they could identify single genes (usually dominant) that conferred essentially complete resistance to the pest in question.
Varieties containing such excellent resistance were developed and released for large-scale farmer use. But breeders then discovered, all too often, that the “perfect” resistance lost its effectiveness after a few seasons.
They soon learnt, with the aid of entomologists and plant pathologists that insect and disease pests are highly diverse genetically, and that almost without fail a rare pest genotype will turn up (or perhaps be created de novo by natural mutation) that is not affected by the newly-deployed resistance gene.
The new pest genotype multiplies and the crop variety’s resistance “breaks down.”
As years went by, breeders found that some kinds of resistance did not fail, and that such resistance often was less than complete; the plants suffered some damage but gave performance overall.
This longer lasting resistance was dubbed “durable” resistance. Further, the breeders discovered that durable resistance usually (but not always) was governed by several genes rather than by one major gene.
The multifactorial kind of resistance has been called “horizontal resistance.” The major- gene resistance has been called “vertical resistance.”
The good news, then, was that breeders could identify and breed for durable resistance.
The bad news was that the breeding was more difficult because several genes had to be transferred at one time, thus requiring larger populations for selection, as well as multiplying the usual problems with “linkage drag” (undesirable genes that are tightly linked to the desired ones).
To this day, breeders use both kinds of resistance in varying proportions, according to the crop and where it is grown.
At first, breeders found and used resistance genes from the adapted, local landrace populations that also were the initial gene pool as a source of resistance genes for their new varieties.
As years went by, these gene pools began to dry up and breeders looked further afield, turning to exotic (unadapted) landraces, and even to wild relatives of their crop.
Sometimes they made extraordinary efforts to hybridize the domestic crop with a very distant wild relative – making a cross that could not succeed under natural conditions.
Embryo rescue and even x-ray treatments were used to make “unnatural” crosses and derive breeding progeny from them. The breeders fooled around with Mother Nature; they moved genes farther than natural processes would allow.
But the breeders as a whole preferred to not breed from exotic varieties or distant and often wild relatives. They used exotic material only when there was no other choice.
This preference was due not only to the difficulty of wide hybridization, but also to the fact that exotic germplasm exacerbates the problem of undesirable linkages.
Few or none of the foreign genes – except the desired resistance genes – were suitable for the needs of high yielding, locally adapted varieties. But often the breeders had no choice; either they got the needed resistance genes from a distant relative, or they got nothing at all.
At about this time, breeders realized that it would be important to conserve remnant seed of landraces from all around the world, but especially from the centers of diversity of their crop.
As farming worldwide grew more commercial, farmers turned more and more to professionally bred varieties that were better suited to commercial production, and in so doing they abandoned their landraces.
If remnant seed of those landraces was not collected and saved in special storage facilities, the genetic base for crop breeding in the future would be drastically narrowed. Seed “banks” were needed.
Through the efforts (especially in the 1960s and 1970s) of a few far- sighted plant breeders, seed banks were established in several countries and in international research centers.
So at the end of the 20th century, plant breeding for pest resistance had laid out the genetic framework of vertical and horizontal resistance, and identified important sources of new resistance genes, i.e., plant germplasm from anywhere in the world.
Sources were limited, however, to the crop species itself or its relatives, either wild or cultivated. All of the introduced genes therefore came from plants.
Plant breeders selected not only for tolerance or resistance to disease and insect pests, they also selected for tolerance to abiotic stresses such as heat and drought, cool temperatures, or nutrient imbalance.
Much of this selection was involuntary; in selecting varieties with top performance over many seasons and many locations the breeders necessarily selected varieties with tolerance to the prevailing abiotic stresses of the diverse seasons and localities.
In selecting for tolerance to environmental stresses, breeders necessarily changed the genetic makeup of the crop species, altering it still further from that of the original wild species, which had been restricted to certain environmental niches.
Witness teosinte (the probable parent of maize), restricted to certain habitats in Mexico as compared to maize that now is grown in nearly every country of the world except Iceland.
Global distribution of crop plants often means that they are grown with no proximity to wild relatives that might intercross with them. Teosinte is not found in Germany or China, nor for that matter in the US Corn Belt.
In other cases, however, wild species with hybridization potential coexist with their cultivated crop relatives, often as weeds.
Canola, sunflower, and grain sorghum are examples of crops with hybridization potential with either a related species (canola with wild mustards) or with a weedy form of the same species (sorghum with shatter cane, cultivated sunflower with wild sunflower).
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