Unfortunately, the two types of fungus Verticillium dahliae and Verticillium albo-atrum do not stop at popular fruit plants, such as strawberries. Effective fungicides have not yet been developed, mainly because the fungi attack the plants from below and clog the water pipes inside. Nevertheless, the Verticillium wilt can be successfully controlled, as the following explanations show.
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The diagnosis
On the surface, the damage caused by Verticillium wilt resembles a growth disorder caused by a lack of water, especially since the fungal disease often occurs in connection with long periods of drought and high temperatures. Therefore, an informed diagnosis is essential in order to take the correct action. In contrast to pure drought stress, in the event of a fungal attack, the oldest leaves of the plant initially wither, whereas the younger parts of the plant remain green for the time being. The further the fungi spread upwards through the water conduction pathways of the plant, the growth of new leaves is reduced, while at the same time the younger, outer parts of the plant wither. In the final stage, the entire plant dies.
- Deciduous trees such as Norway maple, chestnut, robinia, trumpet tree
- Nightshade plants such as potatoes, strawberries, tomatoes, peppers
- Ornamental trees such as rhododendrons, roses, spiraea, hydrangeas
- Fruit trees such as currants, raspberries, pome and stone fruits
The gradual occurrence of wilting phenomena is an essential distinguishing feature from conventional water shortages. The infection only has an indirect effect on the fruits of the plant infected by the Verticillium wilt. The strawberries, for example, are still tolerably edible due to the clogged supply lines, but they are tough and significantly smaller than in a healthy plant. Depending on the vitality of the infected host plant, the pathogens do not colonize all conduction pathways for water and nutrients, so that the infected wood or fruit plant can live with them for several years. This is especially true when the weather is permanently damp, which means that the plants need less water from the soil.
Successfully combating Verticillium wilt as a preventive measure
The fungal infection may affect many hundreds of host plants, but largely uniform procedures apply to successful control. The selection of the optimal location, including a balanced water and nutrient supply, has a prophylactic effect against fungal attack:
- Carefully examine each plant for signs of wilting when purchasing it.
- Do not plant in wet soils with the risk of waterlogging.
- Plant only as deep as absolutely necessary.
- Avoid any damage to the roots when planting.
- Plant trees and woody plants with tree snorkels for root ventilation.
- Refrain from unilateral administration of nitrogenous fertilizers.
- It is better to regularly supply garden compost, horn meal and potash.
- Prefer resistant plant species.
- Prevent drought stress through regular watering.
- Perform thinning and pruning regularly.
- Immediately close cuts and other injuries.
- Do not leave any branch stumps (clothes hooks) when cutting.
- Only cut back with disinfected tools.
If more extensive plantings are planned or if a particularly valuable crop comes into the ground, experienced experts advise you to have a detailed soil analysis carried out. The Verticillium wilt pathogens form permanent spores in the soil, so-called microsclerotia, which survive for more than 10 years even without a host plant. Laboratories specializing in soil analyzes determine for a price under 100 euros whether such fungal zombies are in the potting soil and in what number and are just waiting to strike again. In contrast to a pH test, such a soil analysis cannot be carried out by the hobby gardener himself. The soil samples, however, can be taken by hand, which keeps the costs for the analysis within manageable limits. At least 25 individual samples are taken from drilling depths between 0 cm and 20 cm from the bed and mixed according to a specified grid. About 300 grams of this mixture are packed in a plastic bag, labeled in detail and sent to the laboratory. After an average of 5 weeks, the written result is available and gardening enthusiasts can precisely estimate the risk of their plants being infested by Verticillium wilt.
Reduce permanent spores of the Verticillium fungi by improving the soil
If the soil analysis indicates a risk from Verticillium wilt or if there was an infection at this point years ago, the knowledgeable gardening enthusiast will take the following measures to improve the soil in order to eliminate any permanent spores (micro sclerotia) that may be present. As part of professional composting, known in the technical jargon as biofumigation, active plant substances are incorporated into the soil endangered by the soil-borne fungi. These suppress the pests that are responsible for the Verticillium wilt or eliminate them completely. Above all, the following are possible:
- White mustard (Sinapis alba)
- Ölrettich (Raphanus sativus)
- Sudangras (Sorghum sudanense)
- Braunsenf (Brassica juncea)
- Sareptasenf (Brassica juncea)
- Gelbsenf (Sinapis alba)
and other suitable plants, especially cruciferous vegetables, that are chopped up and incorporated into the ground. Ready-made seed mixtures of suitable materials for biofumigation or biological soil fumigation are available from specialist retailers. It can be sown outdoors from April to September and blooms within a few weeks. Then the biomass is harvested, finely chopped and incorporated into the endangered bed. Ideally, the area is still rolled and lightly watered so that the active plant components can do their job. A period of at least 14 days should then pass before the planned crop is planted. The success of this biological control of the Verticillium wilt has been proven in several tests. It is important to note that the effectiveness of this practice is only given in combination with the other preventive measures mentioned. An isolated method of successfully combating Verticillium wilt is not yet known.
Verticillium resistant plants
When gardening enthusiasts prefer genera, species and varieties that have proven to be resistant to fungal attack when choosing plants, this measure makes an important contribution to prevention.
Among the trees have proven to be resilient:
- Birken (Betula)
- Hainbuchen (Carpinus)
- Rotbuchen (Fagus)
- Plane trees (Platanus)
- Walnut family (Juglandaceae)
- Oak (Quercus)
- Pear trees (Pyrus)
- Willows (Salix)
The following strawberry varieties have proven to be at least tolerant of the fungal disease:
- Honeoye
- Silence
- Fraroma
- Thuriga
- Mieze Nova
- Florence
- Seascape
Good to know that all conifers and conifers are not attacked by Verticillium wilt. They can therefore also be planted in contaminated soil without hesitation.
Plant strengtheners for more resistance
The healthier and stronger the plant, the better it can defend itself against infection by the Verticillium fungi. In addition to the preventive measures mentioned, well-informed gardening enthusiasts give their plants biological tonic to support the immune system.
Organic tonic
Mother nature supplies a wealth of herbal ingredients to make effective means to keep the lovingly cared for plants healthy. This includes in particular manure from field horsetail, nettles, tansy, comfrey and wormwood. Every hobby gardener can easily produce such manure himself, as the following example for field horsetail shows:
- Soak 500 grams of fresh horsetail in 5 liters of water.
- Let it steep for at least 3 hours.
- Then simmer for 30 minutes.
- Filter after cooling.
- Use diluted or undiluted depending on the infestation.
- Spray every 2 weeks for a long time.
In addition, the Federal Office for Consumer Protection and Food Safety lists an abundance of preparations made from algae extracts and essential oils that are beneficial to the resistance of endangered woody plants and plants and are approved for use in the home garden.
Inorganic strengthening agents against fungal attack
As the fungi responsible for Verticillium wilt attack the cells of the susceptible species directly, experts recommend preparations that strengthen the cell wall. These mainly contain inorganic ingredients, the silicates, phosphorus and potassium salts or various carbonates. The BVL’s online database provides more information and a detailed list of currently approved preparations.
Act immediately and limit the damage
Anyone who suspects that it is Verticillium wilt in the face of withering, falling leaves and dying shoots can quickly determine this. The bark of the wood is scratched with a knife. If the ducts running longitudinally there show a brownish discoloration, the fungal disease has established itself. The diseased parts of the plant must immediately be cut back into the healthy wood. In addition, it is imperative to collect all fallen leaves from the ground. All infected material is either incinerated or disposed of with household waste. It has no place on the compost.
So that a tree infected with Verticillium wilt can recover quickly, experts recommend root ventilation. Endangered species, such as the maple, should be provided with a tree snorkel when they are planted. If this measure was neglected, a subsequent root ventilation is also possible. Carrying out this measure by hand with a spade carries the risk of damaging the tree roots, with the result that even more fungi can penetrate the plant. It is better to hire a specialist company that has a special device that loosens and aerates the soil up to a depth of 1 meter with the help of compressed air. On this occasion, of course, an effective fertilizer or tonic can also be administered directly to the roots.
If all the control measures put forward fail to save the infected plant, it is important to dig it up completely and remove it. In addition, the soil should be exchanged over a large area to ensure that permanent spores of the fungi do not remain there and become active again years later.
Fight Verticillium wilt successfully with the right prevention
Verticillium wilt is rightly one of the most feared fungal infections on trees, ornamental trees and fruit plants. Once the fungus species responsible for this have spread through the plants and clogged the supply lines, it is usually too late. Successful control begins much earlier, namely with the planting. A professional soil analysis creates a precise overview of the risk of a fungal attack in advance. Breeders have produced a number of resistant species and varieties. In addition, a profound knowledge of the optimal culture conditions and their observance are of decisive importance. The rule of thumb for successfully combating Verticillium wilt is that an infection can be largely ruled out,
