Phytophthora infestans is one of the most feared fungal pathogens. Here's how to identify and combat the infestation.
Suddenly your plants start to wither, although at first glance you can't see anything unusual? Strange spots form on the leaves and only rotten tubers are found at harvest? This could be caused by a member of the genus Phytophthora.
Phytophthora: Properties and Origin
The egg fungus Phytophthora infestans originally comes from Central America and caused epidemics in Europe as early as the 19th century. A well-known example of the devastation caused by this fungus is the Great Famine in Ireland between 1845 and 1852. At that time the potato was a staple food and there were only two potato varieties grown in Ireland at the time, which were very susceptible to this ovary. Coupled with the damp climate in Ireland, Phytophthora infestans found optimal conditions and the resulting crop failures cost the lives of around one million people. This probably proves what kind of damage this egg mushroom can cause - especially in damp weather. Incidentally, egg agarics are not “real” fungi, but a separate group of organisms that are more closely related to algae than to fungi, for example.
Temperatures of 15 to 23 °C and high humidity are ideal for the egg fungus to start growing. We find these conditions outdoors mostly in June. If the egg fungus, which has overwintered on crop residues, has moisture for ten hours at a time, the permanent spores begin to grow and germinate. The spores can be spread by the wind or find their way into your garden through the patter of rain on your plants. When the spores hit the plant, they need moisture to germinate and penetrate the plant. There they form spores and infect the entire plant from there.Potato tubers become infected with the blight either this way or through spores that have already been in the soil.
Phytophthora is extremely adaptable and can even withstand longer periods of drought without any problems. It can also adapt quickly to control methods, which means that resistance to certain crop protection agents that are used over and over again develops in no time at all.
Phytophthora: Host-specific species
The Phytophthora fungi are a very large group of fungi. Most species specialize in certain plants or plant families. The egg mushrooms not only attack our vegetable crops in the garden, but also trees and shrubs.
A few examples:
- Phytophthora infestans: Late blight on potatoes; Late blight on tomatoes
- Phytophthora cactorum: Leatherberry disease on strawberries; twig dieback on rhododendron; Collar rot on apple
- Phytophthora cinnamomi: Erikadie
- Phytophthora fragariae: Red root rot on strawberries
- Phytophthora sojae: Crown and sprout rot on asparagus
- Phytophthora ramorum: dieback of rhododendrons; . Oak dieback
- Phytophthora syringae: lilac wilt
- Phytophthora rubi: Root death of raspberries
Phytophthora infestans: Late blight
This species of egg fungus has a narrow range of host plants: it only affects nightshade plants (Solanaceae). The infestation usually causes yield losses of 8 to 10%. In rainy years, the fungus can spread like an epidemic and destroy up to 70% of your crop.
Potato late blight
Potatoes (Solanum tuberosum) infected with late blight will show browning of the petioles and stalks. In addition, yellow-green to later brown spots can appear on the leaves. These spots appear water-soaked and start at the edges of the leaves. When the humidity is high, a white fungal coating also forms on the underside of the potato leaves. Eventually the leaves will dry up and rot. The symptoms of the tubers are superficially sunken, gray spots and brown discoloration inside the tubers, which cannot be clearly differentiated from the he althy tissue. The tubers also turn brown and rot at the end.
Tomato late blight
If your tomatoes (Solanum lycopersicum) are infected with late blight, it is quite possible that the infection started in an infected potato patch. The egg fungus usually grows first on the potatoes and the spores then often infect your tomatoes as well. The first signs are wilting leaves, which turn grey-green and die off. You can also see a white fungal coating on the leaves. The stalks can show dark spots, but often the egg fungus can only be recognized when the tomatoes are ripe. The tomato fruits become dull and hard spots form that begin to wrinkle. The tomatoes then rot and are inedible.
PhytophthoraDetect
If you notice the above symptoms on your plants, you can be sure that Phytophthora is in your garden. However, there is also a possibility of confusion:
- If your potato tubers rot, it can also cause early blight. However, the rotten spots on the tubers are clearly separated from the he althy tissue and show rings. In Phytophthora the putrid tissue is not clearly demarcated from the he althy tissue. Spray spots appear on the leaves and these can fall out.
- In tomatoes, Phytophthora can be confused with blossom end rot. This rot is caused by a lack of calcium and always starts at the point where the calyx has separated from the ovary. There is only a large, brownish, sunken rotten spot on the fruit.
Phytophthora successfully prevent
The most important thing in preventing Phytophthora is to use he althy seedlings and less susceptible varieties. Unfortunately, there are not many varieties of it on the market at the moment. Waxy varieties of potatoes are less at risk.
As already mentioned, moisture is very important for the egg mushroom. This means that plants that form dense stands, such as bush tomatoes, are more at risk. Denser stocks dry more poorly - this promotes fungal infestation.
It is also important to follow a crop rotation in the garden. If Phytophtora has ever appeared in your bed, it is advisable not to plant any nightshade plants (Solanaceae at this point (and in the immediate vicinity) for a few years. ) to grow more.
Since the spores are spread by rain and spray water, care must be taken to water the plants carefully. If you work with irrigation (for example in your greenhouse) you should rely on drip irrigation instead of sprinkling. Since moisture and splashing water are the biggest problems when treating egg fungus, it's a good idea to put up a small canopy for your tomatoes or place the plants in a rain-protected, sunny spot. A terrace or the wall of a building is suitable for this, in order to grow the tomatoes in a protected manner.
More prevention tips for Phytophthora:
- Don't overdo it with the fertiliser:
Don't use too much nitrogen fertiliser, over-fertilising can encourage the egg fungus to grow. A potassium-rich slow-release fertilizer like our Plantura organic tomato fertilizer is a very good choice for prevention. - Avoid waterlogging:
Drain your garden or work in some gravel or sand to loosen the soil. This promotes the drainage of water and improves the water and air balance in your garden. - Be careful when harvesting:
Be careful not to damage anything when harvesting and also check your stored tubers for infestation. - Dispose of harvest waste:
Since the egg fungus overwinters in infected plant parts, such harvest waste and plant parts should be removed as a precaution to prevent infestation next year.
Phytophthora Fight efficiently
If, despite all caution, Phytophthora occurs in your area, it is important to remove the infected, diseased plants. Don't dispose of infested plants in your compost - these diseased plants belong in the household waste, where they can no longer infect other plants. Since this egg fungus overwinters on the crop residue, these parts should not end up in the compost or be left in the garden.
Also thoroughly clean and disinfect any equipment that has come into contact with the diseased plants. If the infestation is very severe, you can also replace the contaminated soil with fresh one. Be careful not to spread the infection further.
Fungicides are another way of combating them. However, most of these plant protection products have a preventive effect and should be used in damp weatherbe used.
When using pesticides, careful handling is the most important thing. Always read the instructions for use before use and follow the instructions.
Another fungus that troubles many plants is the egg fungus Pythium. We have information for you on how to recognize and combat Pythium.