Verticillium dahliae, Verticillium Wilt of the Strawberry

Verticillium dahliae

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Changed on: 05.11.2018
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Verticillium Wilt of the Strawberry

befallene und gesunde Früchte (12)
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Affected and healthy fruit (12)

Verticillium wilt of the strawberry is caused by the microsclerotia-forming fungus Verticillium dhaliae. Damage caused by this fungus has increased over recent years. Cultivation areas might be lost as a result of this fungus, in addition to losses in yield. The pathogen can survive in the soil without a host, using persistent resisting mycelium structures (microsclerotia)  to survive. Moreover, Verticillium dahlia has a large number of host plants – plants affected include potatoes and legumes, in addition to strawberries. Verticillium dhaliae is spread widely over several climate zones.

More information
befallene und gesunde Früchte (12)
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Affected and healthy fruit (12)

Verticillium wilt of the strawberry is caused by the microsclerotia-forming fungus Verticillium dhaliae. Damage caused by this fungus has increased over recent years. Cultivation areas might be lost as a result of this fungus, in addition to losses in yield. The pathogen can survive in the soil without a host, using persistent resisting mycelium structures (microsclerotia)  to survive. Moreover, Verticillium dahlia has a large number of host plants – plants affected include potatoes and legumes, in addition to strawberries. Verticillium dhaliae is spread widely over several climate zones.

More information

Symptoms

The most typical symptom is the wilting of the plant during drought stress. It starts at the outer leaves, while the younger leaves and core leaves remain green. The petioles develop normally if the plant is only affected slightly and the wilting can be reversed. However, once the infestation has progressed, newly produced leaves show reduced growth and the plant only forms rosettes with greatly shortened petioles and smaller leaf blades (leaf lamina). The outer leaves die gradually until the entire plant is finally dead.

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Healthy plants (6)
[Translate to English:] Gesunde Pflanzen (6)
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First signs of the wilt (7)
[Translate to English:] Erste Anzeichen der Welke (7)

Infestation Course

The images on the right show the chronological sequence of the wilting symptoms in a strawberry field planted with the susceptible variety Elsanta. Image No. 6 shows the plants without symptoms and normally developed foliage. Image No. 7 shows the first signs of the wilt. The older leaves wilt and lie on the floor, the younger leaves are still healthy.

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BILDER: Plants affected (9)
[Translate to English:] Befallene Pflanzen (9)
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Severely affected plants that have already died (10)
[Translate to English:] Stark befallenen Pflanzen die schon abgestorben sind (10)

These symptoms make it possible to distinguish verticillium wilt from other soil-borne diseases: phytopthora species or Rhizoctonia fragariae fungi affect older and younger leaves equally.
Subsequently, the younger leaves will also show symptoms (Image 8), the petioles are shortened and the leaf blades smaller. Now, the older leaves begin to die (Image 9), both plants are dead (Image 10). 

 

 

Symptoms on Fruit

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Affected plant (11)
Erkrankte Pflanze (11)
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Affected and healthy fruit (12)
befallene und gesunde Früchte (12)

Plants that are affected by verticillium produce smaller fruit when they suffer of drought stress during the ripening of the fruit. The fruit flavour remains normal – unlike when affected by Phytophthora cactorum – although it is less juicy and smaller compared to healthy fruit. The stress caused by verticillium also affects the plant’s water supply indirectly. The image depicts ripe fruit from an affected plant (Image 11). Image No. 12 shows smaller, viscous berries from a plant displaying wilt symptoms next to normally developed fruit.

Symptoms Rhizome Section

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Affected rhizome (13)
Erkranktes Rhizom (13)
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Damage on rhizome (cut open) (14)
Schadbild Rhizom (aufgeschnitten) (14)
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Plant already dead (15)
Bereits abgestorbene Pflanze (15)

Verticillium dahlia lives as a parasite in the vascular system of the host. This becomes visible in the brown vascular bundles in the rhizome section once the disease has progressed.
The plants in image 13 and 14 display heavily advanced symptoms of the disease on their leaves. The plant on the right was already dead (Image 15).

 

 

Diagnosis

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Verticillium evidence on petri dishes in a laboratory (1)
Verticillium-Nachweis auf Petrischalen im Labor (1)
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Verticillium colonies in a petri dish (2)
Verticillium-Kolonien auf einer Petrischale (2)
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Verticillium colonies on agar (3)
Verticillum-Kolonien auf Agar (3)
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Microsclerotia of Verticillium (4)
Mikrosclerotien von Verticillium (4)
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Spores and spore carriers of Verticillium sp. (5)
Sporen und Sporenträger von Verticillium sp. (5)

Tests for Verticillium sp. (Wilt Disease)

Infected, but not completely dead plants are ideal for testing. When testing for soil-borne pests, the samples should consist of a mixture of equally distributed, individual samples. To do this, take 25 individual samples / ha using an auger (0-20 cm deep), mix them well and send approximately 1 kg to be examined:

Institute for Sustainable Plant Production
Spargelfeldstrasse 191, 1220 Vienna

Tests on Verticillium sp. (wilt disease) include detection in the plant, a simple microscopic examination and the microbiological detection of V. dahlia / V. albo atrum.

The number of the fungal microsclerotia is used for detecting Verticillium dahliae in the soil.

A soil test helps determine the number of microsclerotia per gram of soil. To do this, the soil goes through a wet sieve and is incubated on a selective medium (PGA) for three weeks (Image 1). Image 2 depicts the verticillium colonies that have developed: the left petri dish shows severely contaminated soil with a large number of colonies (positive control), while the right dish shows uncontaminated test soil. Image 3 depicts two colonies of verticillium dahlia microsclerotia, which develop beginning from two individual microsclerotia in the soil sample and grow into the test soil around the original microsclerotia in a radial manner. Image 4 shows microsclerotia in a variety of shapes and sizes, image 5 the whorls with spore carriers and spores of Verticillium sp., with three to four whorls per node.

The tests take about five weeks as a result of the method used. This examination makes it possible to reach conclusions concerning the infection risk presented by Verticillium dahlia before planting.

Testing for other strawberry diseases

Phytophthora sp. (Pathogen of various types of root rot in plants)

Xanthomonas fragariae (Leaf spot disease )

and other fungal plant pathogens.

Contact

For further information on strawberry diseases and the submission of samples, as well as prices, follow this link: https://www.ages.at/en/service/services-agriculture/plant-health/

 

 

 

Control

Controlling the disease directly with the use of plant protection products is not possible. Thus, the focus is on prophylactic measures.

  1. Testing the soil: soil tests help calculate the number of fungal microsclerotia in the ground. This makes it possible to check the suitability of the part of the field before planting and prevent harvest losses.
  2. Using healthy planting materials. The planting material must be free of any pests that could reduce its quality – also Verticillium dahliae free. It is recommended to have the plants examined before planting them. 
  3. Choosing varieties: the highly susceptible main variety, Elsanta, cannot be used in some cultivation areas given problems with verticillium infestation. More resistant varieties should be planted in areas that have featured verticillium in the past; e.g. Daroyal, Salsa, Queen Elisa… . The Clery variety, often used to replace Elsanta, is less susceptible than Elsanta, but still displays symptoms on farms with serious levels of contamination.
  4. Optimising crop management: balanced water supply, especially at times of drought stress.
  5. Soil improvement measures: attempts to use organic soil fumigation showed that the direct influence on the microsclerotia produced by Verticillium dahliae in the soil is rather low. However, the fumigation treatment affected other pests in the group of the black root rot and various weed seeds. Incorporating biomass during the soil fumigation process raises the soil’s antiphytopathogenic potential, making soil fumigation recommendable as a soil improvement measure.
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