Cotton
Verticillium Wilt
Verticillium spp.
Fungus
In a Nutshell
- Yellowing of leaves - start from margins.
- Main leaf veins remain green.
- Black stripes on the stem.
- Wilting of the plant.
Cotton
Symptoms
Symptoms vary greatly between the different crops. Usually, yellowing appears first on the margins of older leaves. As the yellowing extends to the rest of the tissue, the leaf takes a wilted aspect, often only on one side. This particular trait is called sectoral chlorosis or "one-sided wilt". On the stem black stripes develop and extend from the base upwards, leading to the wilting of stems. In trees, poor growth, early leaf senescence, stunting and death of whole branches are the main symptoms. Staining of woody tissues in the form of circular rings or streaks may be additional symptoms. Sometimes, at closer inspection with a lens, small black dots are visible in the dying tissue or on living tissue as well.
Recommendations
Organic Control
Biofungicides containing Streptomyces lydicus break the life cycle of the fungus and can help to control the progression of the disease.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatments if available. When the trees are affected by the disease it is very difficult to get rid of it. Application of soil fumigants is an effective, but expensive control tactic. The effectiveness depends on the chemical used, the rate, and the environmental conditions at the time of application. Treatment of affected plant parts can also be envisaged.
What caused it?
The symptoms are caused by a series of soil-borne fungi, among others V. dahliae, which can survive on crop debris in the soil when no host is available. It enters the vascular tissue of the plant through the rootlets or wounds in the bark. Once inside the plant or the tree, it grows rapidly and blocks the transport of water and nutrients, resulting in the wilting and decay of the aerial parts (leaves and stems). This is intensified by warm sunny weather. At later stages of the disease, the fungus colonizes the dying tissue and forms dark structures that can be observed with a magnifying lens. The fungus can survive several years at a location.
Preventive Measures
- Use resistant or tolerant plant varieties.
- Do not use susceptible intercrops that could spread the disease.
- Avoid nitrogen-rich fertilizers and excess water.
- Use plant fortifier to strengthen the plants against an infection.
- Prune, remove and burn infected plant material.
- Clean all tools and equipment after handling infected plants.
- Do not work in fields when the foliage is wet.
- Take care not to damage roots while working in the field.
- Expose the soil to solar radiation for a period of time (solarization).
- Remove and incorporate plant debris deep down in the soil or burn them.
- Rotate crops with non-host plants.
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