Fundamentals of Plant Pathology

RESISTANCE

Introduction

• Use of resistant varieties for crop cultivation provides the most cost-effective, the easiest, and the safest of all the methods used for disease control.
• Both from economic point of view, and the possible health hazards involved in some of other methods of disease control, this can probably be termed as the “painless method”. This approach costs little to the farmer and is, therefore, suitable for the developing countries like India.
• Cultivation of resistant varieties provides probably the only means of producing acceptable yields without using toxic compounds for many diseases like the vascular wilts, viral diseases, cereal rusts, powdery mildews, and root rots, etc.
• Several other kinds of fungal diseases and also many others caused by bacteria, nematodes, and viruses are best controlled by this approach.
• However, resistant varieties could be effectively used only in limited number of cases against the diseases of forest and fruit trees, e.g., blister rust of white pine (Cronartium ribicola), fusiform rust of pine (C. quercuum f. sp. fusiforme), and apple scab (Venturia inaequalis).
• It is always preferable to use resistant host varieties that have both vertical and horizontal resistance.
• Most resistant varieties have only one or few (2 - 3) genes for vertical resistance (mono- or oligogenic resistance, respectively) and an unspecified number of genes for horizontal resistance (polygenic resistance).
• Where inoculum production is rapid and its buildup high, and it is air borne, new races of the pathogen may appear quite often and soon become widespread. Such examples are cereal rusts, powdery and downy mildews and late blight of potato.

Resistance break down

• As the new race takes over, resistance of the old variety is no longer effective.
• Depending on the genetic plasticity of the pathogen and the particular gene or combination of genes involved in host resistance, resistant varieties with only vertical resistance, need to be replaced periodically.
• This means that breeding programmes for new resistant varieties has to continue so that some new varieties can be kept in readiness for the replacement of the old ones in case of any eventuality.
• It is hoped that genetic engineering techniques would come to the aid of such breeding programmes and make it possible for a quick transfer of individual genes or a combination of such genes to preferred susceptible host varieties in a much shorter time.

Maintenance of resistance

• Disease management strategies, such as sanitation, seed treatment or use of fungicide reduce the exposure of resistant variety to large pathogen population.
• For pathogens with low inoculum production and slow dispersal rate, resistance of the host variety usually lasts longer.
• The use of varietal mixtures has been widely used in a variety of crops as a possible measure in disease control in cereals, legumes and potatoes.
• A cultivar mixture is simply compounded by mixing seeds of cultivars on the basis of their predicted performance.
• Diversification of resistance naturally presents the pathogen with a difficult target than in the traditional monoculture.

Multiline varieties

• Jenson (1952) first proposed the idea of multiline varieties that is a composite of various isogenic lines sharing most agronomic characters, but carrying different genes for vertical resistance in one or a few of its constituents of the multiline variety.
• Use of multiline variety results in overall reduction of pathogen for a disease, which consequently reduces the rate of disease and also the inoculum presence on each of the component varieties.
• The most fully developed multiline programme involved wheat rusts and crown rust of oats.
• Multilines can delay the onset of disease and also reduce the rate of an epidemic.
• If a constituent variety loses its resistance to a new race of the pathogen, it can be replaced by a suitable alternative line.
• There are, however, certain limitations on the use of multiline varieties.
• The components must be distinct from each other, have different race-specific genes, and also ripen simultaneously.

Reliable resistance

• It is now accepted that crop resistance based on single or few vertical resistant genes is liable to become nonfunctional soon, mostly within 4 years.
• In the long run, the production of varieties with many additional genes for horizontal resistance may perhaps provide the only answer.

Breeding of resistant varieties

• Quite early in the twentieth century it became evident that breeding of resistant plant varieties was possible, and this provided the most desirable approach to plant disease control.
• The environment pollution in chemical control further highlighted the importance of such breeding.
• Plant breeding represents the most significant form of biological control of plant diseases.
• Genetic diversity can be regularly introduced into the plant genome through such breeding programme.
• Cultivated crop plants that we see today represent the results of natural selection or selection and breeding of different lines that evolved naturally in different regions over many thousands of years.
• It has been a very slow process.
• Many of them still exist as wild types at the place of their origin and have survived over such long periods in attack of various pathogens, because of many resistance genes they carried and also gradually acquired through natural crossing within the plant population.
• Weak and susceptible ones were eliminated in course of time.
• The survivors had sets of major and minor genes for resistance and much genetic diversity, adapted to the local health environment and suited to the needs of local population.
• Numerous varieties of each crop plant are cultivated throughout the world and they represent a non-uniform population.
• Widespread systematic efforts of plant breeders all over the world have further increased this diversity.
• Now, biotechnology has come in a big way with techniques aimed at further increasing this.
• The first step in breeding for disease resistance is mostly to decide on type and level of resistance required and whether the pathogen is seed-, soil- or air-borne.
• The decision will depend on the availability of a suitable source of resistance and whether or not it can be manipulated in a breeding programme.
• Many plant diseases cannot yet be properly controlled by host resistance, for example, powdery mildews of cereals, as this is complicated by pathogenic specialization and a complex resistance pattern.

Source of resistance

• Search for resistance is initially restricted to crop cultivars currently in use locally.
• Search has to be widened to include varieties grown in the adjacent regions, wild plant relatives, and species growing in the area where the disease is severe, or where the disease is originated.
• Plant breeders often take recourse to creation of new resistant genotypes for this purpose by inducing mutation or approach gene banks maintained in different countries.
• Larger public collections are maintained in different countries.
• United States Department of Agriculture (USDA) for many crop species at Beltsville, Maryland, USA
• CIMMYT at Londres, Mexico, for maize and wheat
• International Rice Research Institute (IRRI) at Los Bagos, Phillippines, for rice
• International Crop Research Institute for Semi Arid Tropics (ICRISAT) at Hyderabad, India, for legumes and small grain cereals.