Phytophthora: one name, many diseases

phytophthora on a tomato plant and fruit

Phytophthora is a word used to describe more than just a single disease. The term refers to many different plant pathologies which have more or less severe impacts (crown rot, mildew, ink disease, etc.) It’s more accurate to use the phrase “phytophthora diseases”.

Luckily for us gardeners, symptoms and ways to prevent and treat it are often identical from one disease to the next. Follow our lead to learn more about phytophthora diseases.

In short: Phytophthora causes root rot and, on seedlings, damping off

Key facts about phytophthora diseases

Lifecycle and spread

Phytophthora is a class of fungus that belongs to the Oomycete family. They’re among the parasites that trigger fungal diseases. The etymology of the word itself comes from Greek ” phyto ” which means ” plant ” and ” phtora ” which is best translated by ” destroying “.

Leaf with traces of white and black, part of the phytophthora lifecycle

Contamination and spread of this disease occurs through the “seeds” that this fungus spreads. Such seeds are called spores, and, when they’re mobile, they’re called zoospores. After they reach the ground, they can lay dormant for many years while waiting for a host to appear. Some species such as P. citricola mostly attack the root system. Nonetheless, they will still infect leaves if ever spores or contaminated dirt lands on them.

Which plants get sick?

There are over a hundred species of phytophthora. Some of them are only able to infect a single specific host:

  • Phytophthora phaseoli is responsible for one of the types of bean blight;
  • Phytophthora infestans will, for its part, infect Solanaceae plants such as tomato;
  • Phytophthora fragariae which is commonly called “red stele rot”, only attacks strawberry;
  • etc.

Tomato stem direct infection by phytophthoraHowever, other species have a much wider range of host plants. This is called polyphagy. Examples that often infect ornamental plants include:

  • Phytophthora cryptogea;
  • Phytophthora cinnamomi, which is responsible for sudden oak death and chestnut tree ink disease;
  • Phytophthora citricola attacks citrus and causes citrus foot rot;
  • Phytophthora cactorum, a fungus that causes bitter rot and leather rot on fruits like apples and strawberries;
  • Phytophthora cambivora.

Phytophthora diseases strike all types of plants: vegetables, perennials, shrubs, trees, etc.

Important note: Phytophthora ramorum is a particularly virulent species. For a long time, it remained in its native habitats in the United States and in the United Kingdom. Sadly, it has now appeared in mainland Europe where it keeps spreading since its discovery in the year 2000. The first identified victims were Viburnum and Rhododendron. However, it is now also contaminating larch plantations, to the point that authorities discourage planting Japanese larch.

Conditions that favor appearance

There are many different factors that trigger appearance of plant disease in general and of phytophthora in particular:

  • the plants themselves might already be in stressful situations (lack of water or , conversely, excess water, too much nitrogen, etc.). Such stress weakens plants and disables their natural defenses against attack.
  • Growing plants too close to each other not only leads to fungus appearing, it also speeds its spread. Indeed, with low ventilation and high moisture, even just two leaves touching each other are enough for the fungus to hop across to a new host.
  • Clay soil, often heavy and compact, retain water very well. In some situations, this is a good thing, but when you consider phytophthora, soggy soil leads to nightmarish developments.
  • Young plants, seedlings, specifically, are particularly vulnerable. When they die, it’s called sprout damping.

Symptoms related to phytophthora

Mock-orange hedge dying off because of a phytophthora infectionPhytophthora diseases may attack all parts of the plant, from root to leaf, of course including stems.

Root and crown infection

Generally, roots and the root crown (where the stem meets the ground) are the parts that are most often infected by phytophthora. This results in rot spreading through the root system: roots lose their natural coloration and there are no more tiny rootlets (the small roots that do all the feeding). When the root crown is infected and the plant is already dying, you can even notice the area where the battle is taking place, just at ground level. It’s easy to identify because plant tissues look very healthy on the top part, but the infected bottom part looks brown or brownish red.

Foliage

When symptoms appear on leaves (wilting, drooping), it means the root system is already very infected and that then entire base of the plant is sick. The plant can’t even photosynthesize anymore because leaves lack water and nutrients. Some phytophthora species can infect leaves directly without going through the roots first. The main symptom in this case is appearance of dark brown or black leaf spots that feel moist or wet. The disease spreads across the surface of the leaf, reaches the petiole, and then infects the stem.

Stem

As is the case for leaves, there are a few phytophthora species that infect stems directly. There, they create brown or black lesions. A direct result of this is that any leaves higher up along the branch start wilting away and dying off. However, portions that are beneath the infected wound stay healthy, including roots and the root crown.

Prevent phytophthora from appearing in the first place

To limit risks of a phytophthora disease infecting your plants, just follow these few guidelines:

  • Since zoospores have the capacity to move along wet areas underground, it’s best to avoid growing crops in heavy soil (clay) and in soil that doesn’t drain well. Stagnating water is a clear factor that results in the disease appearing and spreading. If need be, make your soil lighter by mixing river sand into it.
  • Apple with a spot of rot due to phytophthoraSpace rows and crops well. This will lower moisture and increase airflow, which reduces risks of the disease appearing and slows its spread.
  • Water plants at their base and not by wetting the leaves.
  • Disinfect your tools after you work with them, every time you change plants. Cleanliness is important, even in the down-in-the-dirt world of gardening. When you’re reusing pots and trays, you can also disinfect them before repotting other plants or seeds in them.
  • After purchasing new plants, inspect them carefully. Sometimes gardeners have a special quarantine zone where they put new plants for a week or two just to catch eventual outbreaks. Careful observation for two weeks is usually enough, and if nothing untoward appears, you can transfer the plants to your garden or vegetable patch.
  • When propagating your plants (clump division, cuttings, layering, etc.), make sure the mother plant you’re working with is perfectly healthy. When selecting stems for cuttings, choose those that are high up in the air, not those that are touching the soil.
  • In the vegetable patch, diversify your crops and keep vulnerable species far away from each other. Also, work to set up crop rotation every year.

Treatments and treating the disease

Treating against phytophthoraEven if you follow the rules above to the letter, you still might be confronted with a phytophthora infection. If that happens, you must cleanse the infected area, recovering all the infect plant parts (leaves, stems, roots). Burn them. The last step regards treating contaminated plants. For that, several options are available. Which to use depends on what part of the plant is infected.

Foliage

This is the easiest case. Indeed, phytophthora species that attack leaves are typically those responsible for mildew. To treat against them, try to find bordeaux mixture and spray the plants as recommended on the manufacturer’s label.

Soil, roots

Soil-borne phytophthora, those that travel and live underground, are difficult to treat. Best is to prevent their appearance in the first place, and to rotate your crops. Choose species that resist or aren’t too vulnerable to this fungus in the first place.

There are fungicides that work against the disease, garden stores will help you choose the best ones. Some of the best are based on aluminum compounds. Such treatments are both preventive and curative, and they can normally be used on many crops and fruits (vegetables and fruits). They usually belong to the “systemic treatment” category, meaning the compounds enter into the plant to fight the disease off from the inside. They work both upwards (spreading to the leaves) and downwards (towards the roots). Other products with compounds such as mancozeb and dimethomorph are also available.

Sadly, as of today, there are no natural treatments against phytophthora that have yet been discovered. It’s difficult to pursue organic gardening when the disease has set foot in your plot. On the upside, there is ongoing research that seems to hint at the possibility of using biological control agents (BCA) to fight off Oomycetes such as phytophthora.

Safety first

When you use chemical products to treat a phytophthora attack, it is critical to protect yourself and to follow all the instructions on the manufacturer’s label. If need be, check on the Risk Assessment Sheets that manufacturers must provide free of charge on their websites.