PHYTOPHTHORA INFESTANS: CHARACTERISTICS, NUTRITION, SYMPTOMS, CONTROL - BIOLOGY - 2024

Phytophthora infestans is a phytopathogenic oomycete responsible for the disease known as late blight or potato mildew, being one of the diseases that attack this tuber throughout the world. The effects of the plague on crops can be devastating, an example being the Great Irish Famine that it caused in the 19th century.

This oozyme is characterized, among other aspects, by presenting a coenocytic mycelium without septa. In the event that they present it, they are very few. It also has branched conidiophores that resemble a miniature tree. It has a haploid genetic makeup and can reproduce both sexually and asexually.

Phytophthora infestans infesting a potato. Taken and edited from: I. Saček, senior.

During asexual reproduction, it presents deciduous sporangia and oval or ellipsoid shape; while its sexual reproduction is characterized by being of the heterothalic type and produces resistance oospores.

The chemicals used to control the appearance of the disease also affect the potato plant and other vegetables, in addition to contaminating the soil and water, so scientists have tested different forms of biological control using bacteria, fungi. and other organisms.

characteristics

The mycelium of Phytophthora infestans is sparse, fine and has practically no septa, that is, it is coenocytic. It can reproduce both sexually and asexually. The sporangia are apical, deciduous, transparent, with a shape ranging from rounded to ellipsoidal and with a tapered apex. The size of these does not exceed 36 x 22 µm.

The sporangiophores, on the other hand, have a continuous and sympodial growth, their thickness is slightly greater immediately below the sporangium.

The sporangium can germinate directly or give rise to zoospores, which have a single nucleus, kidney-shaped and two flagella (one long and whip-shaped and the other feathery and short).

Taxonomy

From the time of its description to date, Phytophthora infestans has undergone numerous relocations, not only at the genus level, but even at the kingdom level, including the intermediate taxonomic categories.

The genus Phytophthora belongs to the class of the Oomycetes of the kingdom Protista. It is worth mentioning that this class was included for many years within the group of fungi, from which they were excluded based on molecular and biochemical studies.

The family to which it currently belongs is called Perennosporales, which was previously part of the Pythiales. However, according to some authors, this relocation also requires careful reviews.

Phytophthora infestans is the type species of the genus, which was erected by Bary in 1876. The first description of the species was made by Montagne, who named it Botrytis infestans. Subsequently, Caspary had relocated it within the genus Peronospora.

Biological cycle

Phytophthora infestans exhibits both asexual and sexual reproduction mechanisms in its life cycle. The asexual phase is also known as vegetative, while the sexual phase is also called reproductive.

Asexual phase

In the asexual cycle, the production of zoospores occurs. Within 3 to 10 days after a plant has been infected, the oomycete sporangiophores rise to the surface of the plant through the stomata.

Sporangia develop at the apex of sporangiophores, although they can also develop from the germ tube of oospores.

Environmental conditions, mainly temperature, determine the appearance and action of sporangia. The optimum temperature range for the development of Phytophthora infens is between 18 and 22 °. When the environmental temperature is in this range or above it, the sporangia germinate directly.

Oospora of Phytophthora infectns. Taken and edited from: No machine-readable author provided. Fk assumed (based on copyright claims)..

Sub-optimal temperatures stimulate the appearance of zoospores. Each sporangium can produce few zoospores (6-8), but each zoospore has the capacity to produce a lesion and each of these can produce up to 300 thousand sporangia, which explains the explosiveness of seed dispersal.

Another form of dispersal of the pathogen is directly through the sporangium. When it is mature it is deciduous and can be dispersed by the wind up to distances greater than 30 km.

In these cases, if the temperature is higher than 20 ° the sporangium will germinate directly, but at low temperatures it produces between 10 and 20 zoospores. These zoospores are mobile for several hours and if they find a suitable substrate they can germinate. In addition, they can penetrate the potato plant via leaves, stem or tuber.

In the absence of a potato plant, Phytophora infens can survive in other host species, such as tomato and eggplant, as well as some weed species,

Sexual cycle

The sexual reproduction of Phytophthora infestans is of the heterothalic type, that is, each hypha produces only one type of gametangium, either male (antheridium) or female (oogonium). When the mycelia of different organisms grow together, they can each develop a different structure.

During this type of reproduction, the oogonium passes through the antheridium and fertilization occurs, after which the oogonium becomes an oospore, which can germinate directly but under adverse conditions can survive in a dormant state for up to 10 years, until restore favorable conditions.

When this occurs, the oospore can germinate forming a germ tube from which an apical sporangium is formed, which in turn can germinate directly or release zoospores that will initiate the host infection. This type of reproduction guarantees the genetic diversity of the species, in addition to allowing its survival in adverse conditions.

Nutrition

Phytophthora infectns has little resistance in free life, where it acts as a saprophyte, secreting enzymes to carry out extracellular digestion of decomposing organic matter.

As a parasite, it is hemibiotrophic and in the early stages of its life cycle the mycelium invades the host's tissues and absorbs nutrients without killing it, but then causes cell and tissue death.

Symptoms

Symptoms of the disease will vary depending on the place of inoculation.

Leaves

Initial symptoms are irregular spots that are light to dark green in color, moist in appearance and necrotizing, changing from brown to black, sometimes surrounded by a yellowish or light green halo. These lesions, which appear first on the tip and edges of the leaf, are not limited by the veins.

Lesions can progress to cover the entire surface of the leaf and progress towards the petiole. If the lesion on the petiole covers the entire diameter of the petiole, the leaf will fall off. If the humidity conditions are suitable, a downy mildew will appear on the underside of the leaf, which is formed by sporangia and sporangiophores.

Stems and petioles

The lesions are necrotic, of vitreous consistency, generally distributed in the distal third of the sole. The affected areas become fragile and when the lesion reaches the entire diameter of the stem or petiole, it breaks easily. If the humidity is high, sporangia can form in this area.

Tubers

On tubers, the external symptoms are slightly sunken, irregular, moist-appearing areas. The peridermis takes on a reddish hue. Under this are some extensions that advance towards the interior.

As the disease progresses, a dry, granular-looking, light to dark brown rot is observed under the surface of the tuber. These lesions can appear up to a few weeks after the tuber is harvested. Secondary rots may occasionally occur due to other opportunistic fungi.

Phytophthora infectns attacking tomato. Taken and edited from: Rasbak.

Chemical and biological control

Chemical control

For the chemical control of Phytophthora infens, products that inhibit the germination process of the spores are used, as well as fungicides that kill them and products that have the ability to carry out some type of post-infection control. These products are classified as: contact, systemic and translaminar.

Contact fungicides act on the surface of the plant preventing germination and penetration of the pathogen, thus reducing the initial sources of the disease. They are also called protective or residual fungicides. Examples of these products are cupric and dithiocarbamates.

Systemic chemicals act inside the plant after being absorbed by the leaves and roots, inhibiting metabolic processes of the pathogen. The latter can develop resistance to the action of these compounds. This is the case with the systemic ones of the phenylamide family, such as benalaxyl or metalaxyl, for example.

The translaminaries, on the other hand, can move through the leaf, but not between leaves, so that new shoots are defenseless against the attack of the fungus until a new aspersion.

Biologic control

To avoid the undesirable effects of chemical control, such as contamination and toxic effects on organisms other than the pathogen, researchers have evaluated different species that could exert a biological control on the pathogen.

The organisms studied belong mainly to the groups of fungi and bacteria, including actinomycetals. Among the genera evaluated are Trichoderma, Pseudomonas, Methylobacterium, Streptomyces, Serratia, Bacillus and Streptosporangium.

The mechanisms of action of these biological controllers include mycoparasitism, competition, antibiosis, and induction of host resistance against the parasite.

Allelopathic substances produced by different plant species, as well as biosurfactants produced by bacteria, have also been evaluated to verify their biocontrolling effect on Phytophthora infens. Some of these biosurfactants have been effective as biocontrollers of late blight, at least in its attack on tomatoes.

References

1. DC Erwin (1983). Phytophthora: its biology, taxonomy, ecology, and pathology. American Phytopathological Society Press.
2. Phytophthora infestans. On Wikipedia. Recovered from: en.wikipedia.org
3. Phytophthora infestans. In the Argentine National Pest Surveillance and Monitoring System. Recovered from: sinavimo.gov.ar.
4. D. Andrivon (1995). Biology, ecology, and epidemiology of potato late blight pathogen Phytophthora irifestans. Phytopathology.
5. J. Parker & O. Navia (1991). Chemical control strategies for potato late blight (Phytophthora infestans). Latin American Potato Magazine.
6. Phytophthora infestans. Life history & Reproduction. Recovered from: bioweb.uwlax.edu.