Fundamentals of Plant Pathology

BIOLOGICAL CONTROL

Biological Control- Concept

• Biological control of plant pathogens refers to the total or partial destruction of pathogen population by other organisms.
• It occurs routinely in nature. For example, several diseases in which the pathogen can not develop in certain areas either because the soil, called suppressive soil, contains microorganisms antagonistic to the pathogen or because the plant that is attacked by a pathogen has also been inoculated naturally with antagonistic microorganisms before or after the pathogen attack.
• Sometimes, the antagonistic microorganisms may consist of avirulent strains of the same pathogen that destroy or inhibit the development of the pathogen, as happens in hypovirulence and cross protection.
• Agriculturalists have increased their efforts to take advantage of such biological antagonisms and to develop strategies by which biological control can be used effectively against several plant diseases.

Suppressive Soils

• Many soil borne pathogens, such as Fusarium oxysporum (causing vascular wilts), Gaeumannomyces graminis (causing take-all of wheat), Pythium spp. (causing damping-off) and Heterodera avenae (oat cyst nematode) develop well and cause severe diseases in some soils, known as conducive soils, whereas they develop much less and cause much milder diseases in other soils, known as suppressive soils.
• The mechanisms by which soils are suppressive to different pathogens may involve biotic and/or abiotic factors and may vary with the pathogen.
• They operate primarily by the presence in such soils of one or several microorganisms antagonistic to the pathogen.
• Many kinds of antagonistic microorganisms have been found to increase in suppressive soils; such as Trichoderma, Penicillium, and Sporidesmium, or bacteria Pseudomonas, Bacillus and Streptomyces.

Reducing Amount of Inoculum through Antagonistic Microorganisms

a) Control of soil borne pathogens

• Several non-plant pathogenic oomycetes and fungi including some chytridiomycetes and hyphomycetes, and some pseudomonad and actinomycetous bacteria infect the resting spores of several plant pathogenic fungi.
• Among the most common mycoparasitic fungi are Trichoderma sp., mainly T. viride and T. harzianum.
• It parasitizes mycelia of Rhizoctonia and Sclerotium and inhibits the growth of many oomycetes such as Pythium, Phythophthora, and other fungi, e.g., Fusarium and Heterobasidion (Fomes).
• Other common mycoparasitic fungi are Laetisaria arvalis (Corticium sp.), a mycoparasite and antagonist of Rhizoctonia and Pythium; Sporidesmium sclerotivorum, Gliocladium virens and Coniothyrium minitans.

b) Control of aerial pathogens

• Many fungi have been shown to antagonize and inhibit numerous fungal pathogens of aerial plant parts.
• Chaetomium globosum and Athelia bombacina suppress Venturia inaequalis ascospore and conidia production in the fallen and growing leaves, respectively.
• Tuberculina maxima parasitizes the white pine blister rust fungus Cronartium ribicola.
• Darluca filum and Verticillium lecanii parasitize several rusts.

Control through Trap Plants

• If a few rows of rye, corn, or other tall plants are planted around a field of beans, peppers, or squash, many of the incoming aphids carrying viruses that attack the beans, peppers, and squash will stop and feed on the peripheral taller rows of rye or corn.
• Trap plants are also used against nematodes which are sedentary endo- or ecto-parasites.
• Crotolaria plants trap the juveniles of root- knot nematodes.

Control through Antagonistic Plants

• Plants such as asparagus and marigold are antagonistic to nematodes
• They release substances in the soil that are toxic to several plant parasitic nematodes.

Use of Resistant Varieties

• Grow varieties that have both vertical (initial inoculum- limiting) and horizontal (rate limiting) resistance and most resistant varieties have both type of resistance.
• Many of them carry only one or a few genes of vertical resistance and an unspecified number of genes of horizontal resistance.
• Such varieties are resistant only to some of the races of pathogen and if the pathogen is air borne, a new race can be brought in easily as happens with cereal rusts, powdery mildews and Phytophthora infestans.
• The new race virulent to the resistant variety may appear and become wide spread in this way.

Use of transgenic biocontrol microorganisms

• Genetic engineering techniques have been used to add new genes or to enhance the genetic make up of the biocontrol organisms so that it may attack the pathogen better.
• Such genes may be of plant or microbe origin that code for toxins, enzymes, and other compounds affecting the pathogen adversely, or regulatory genes that over-express appropriate biocontrol genes already present in that organism.

Direct protection by biological control agents
The most commonly used microorganisms include:

• Gliocladium virens, for the control of seedling diseases of ornamental and bedding plants
• Trichoderma harzianum, for the control of several plant pathogenic fungi
• Trichoderma polysporum, for the control of wood decays
• Agrobacterium radiobacter K-84, for the control of crown gall
• Pseudomonas fluorescens, against Rhizoctonia and Pythium causing damping off and other diseases
• Bacillus subtilis, used as a seed treatment

Biological Control of Postharvest Diseases through Fungal and Bacterial Antagonists

• Post harvest rots of several fruits could be reduced by spraying the fruits with spores of antagonistic fungi and saprophytic yeasts at different stages of fruit development, or by dipping the harvested fruit in their inoculum.
• Yeast treatments reduced post harvest rotting of peach and apple.
• Botrytis rot of strawberries was reduced by several sprays of Trichoderma spores on strawberry blossoms and young fruits.
• Several antagonistic yeasts protected grapes and tomatoes from Botrytis, Penicillium, and Rhizoctonia rots.

Biological Control of Postharvest Diseases through Fungal and Bacterial Antagonists

• In bacterial antagonists, Pseudomonas protected lemons from Penicillium (green mould) and pear from various storage rots.
• Two Pseudomonas syringae strains control the post harvest decay in citrus, apple and pear under the trade name Bio-Save.
• Stone fruits such as peaches, nectarines, apricot and plums when treated with suspensions of the antagonistic bacterium Bacillus subtilis, they remain free from brown rot, caused by the fungus Monilinia fructicola for nine days.
• Bacillis subtilis also protected avocado from storage rots.
• Pseudomonas protected lemons from Penicillium (green mould) and pear from various storage rots.
• Two Pseudomonas syringae strains control the post harvest decay in citrus, apple and pear under the trade name Bio-Save.
• Stone fruits such as peaches, nectarines, apricot and plums when treated with suspensions of the antagonistic bacterium Bacillus subtilis remain free from brown rot, caused by the fungus Monilinia fructicola up to nine days.
• Bacillus subtilis also protects avocado from storage rots.