Oomycetes: Symptoms and Treatment | Gardening On

Downy mildew is an oomycete

Image – Wikimedia Commons / Rude

It is common to confuse oomycetes with true fungi, since the symptoms and damage they cause are practically the same. But in addition, the treatment that is applied for some can also be used for others.

Still, I think it is very important to know the oomycetesorganisms that affect numerous plant species throughout the world.

What are oomycetes?

Oomycetes affect seedsOomycetes affect seeds

Image – Wikimedia Commons / Oliver Ruiz

The oomycetes they are pseudo-fungi (false fungi) that belong to the group of protists Oomycota (or Oomycetes). The species are classified depending on the way they eat. Thus, on the one hand we have saprophytes, which are those that feed on decomposing organic matter, and parasites.

The latter have a special interest in agriculture and gardening, since they are the ones that can end the life of plants if measures are not taken in time.

What are their characteristics?

It is a series of organisms that have a cell wall composed of cellulose. What’s more, throughout their alternate life diploid phases, in which cells present two sets of homologous chromosomes in their cell nuclei, with haploid phases in which cells have a single set of chromosomes.

The haploid phase, in these organisms, is the reproduction phase. This is sexual when it produces gametangia; that is, antheridia and oogonia. In them, meiotic division occurs, which will give rise to a diploid oospore that will have thick cell walls. This will be released, and will end up producing hyphae from which the sporangium will develop.

On the other hand, the asexual phase is the one that occurs when motile asexual spores, called zoospores, have a flagellum that is directed forwards, and another towards the rear. These are found in environments where humidity remains highlike the substrate of a plant.

Why are oomycetes not fungi?

For a long time they were believed to be. In fact, they were classified within the Fungi kingdom. But today oomycetes and fungi are known to have few but important differences:

  • The cell wall of oomycetes is cellulose. Fungi have it from chitin.
  • They are not usually septate organisms. The cells of the fungi, on the other hand, do divide through their internal walls.
  • As they grow up, our protagonists have diploid nucleiand not haploid like mushrooms.

For all this, they are now within the class Heterokonta or estramenopilos, which they share with diatoms for example.

Types of oomycetes

It is estimated that there are about 700 types of oomycetes, among which we distinguish the following:


Mildew affects plantsDowny mildew affects plants

Image – Wikimedia Commons / Rob Hille

El mildew is a very, very common disease in plants, which causes the leaves to be covered with a kind of white dust. Depending on the variety, we find some that seem to have a predilection for a specific type of plant species.

For example, the plasmopara viticola It especially affects the vine, which is why it is known as vine mildew.


Phythium is a parasitic fungusPhythium is a parasitic fungus

Image – Flickr / John Kaminski

The Phythium are a group of oomycetes that affect an even greater number of plants. TO Young plants, such as seedlings, can cause irreversible damage and even death. But when they are adults, and if they are healthy, it is difficult for them to cause serious problems, beyond some mild symptoms such as a few brown spots on the leaves.

Likewise, it is interesting to say that the P. oligandrum species parasitizes other oomycetes, which is why it is used as a biological control agent.


The phytophthora is an oomycetePhytophthora is an oomycete

Image – Wikipedia, the free encyclopedia

It is a genus of oomycetes that attack many, many species of plants. They are quite specific to the species they attack; I mean, that species of Phytophthora they have a preference for a certain type of plant.

For example, P. ramorum especially affects oak trees, causing death; and P. infestans is common in plants such as tomatoes.

What are the symptoms and damage they cause?

It will depend a lot on the species of oomycete that attacks the plants. But in general, the symptoms and damages that we will see are the following:

  • On the sheets: yellowish or brown spots, whitish powder, premature fall.
  • In the trunk: chancres, cracks. Early death of branches.
  • In the fruits: brown or blackish spots, rotting of the fruits. Often the stem that connects them to the branches turns blackish, as in the tomato.

How are they treated?

Although they are not fungi, they can be treated with the same products; that is, with fungicides. But for the result to be what is expected, it is important to identify the disease first and seek a treatment that is designed to treat that particular disease.

One of the most recommended is copper. Cupric fungicides act by contact, and depending on the composition it can be natural and therefore suitable for organic farming. It is especially useful as a preventive in seedlings and young plants, but it is also quite effective as a curative.

The Fosetyl-Al it is a systemic fungicide. The leaves absorb it, and from there it is distributed throughout the plant. It is widely used to combat mildew and Phytopthora. The best known product that has this composition is Aliette, from Bayer, recommended especially for browning of conifers. You can buy it here!.

Can oomycetes be prevented?

Plants can be prevented from oomycetes

Plants can be prevented from oomycetes

As always when talking about pathogenic organisms, it cannot be 100% prevented. What is done is to take a series of measures that will help minimize risks. They are as follows:

  • Buy healthy plants. If they have brown spots, black stems, or ultimately bad appearance, they should not be taken home.
  • Water only when necessary. Excess moisture in the roots weakens the vast majority of plants, except those that are aquatic.
  • Make sure the soil drainage is goodand install systems to improve it in case puddles form that take hours or days to absorb. Learn More.
  • Separate diseased plants from healthy ones as much as possible. The ideal is to have a space enabled where to have them, in which they will be isolated until they improve.
  • For the pots: use suitable substrates for plants, and new. In addition, the pots must be clean and disinfected.

We hope it has been of interest to you.

Oomycetes: Symptoms and Treatment | Gardening On

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