Mid- to Late-Season Stem, Root, and Cotton Boll Diseases

Key Takeaways

• Fusarium and Verticillium wilts are soilborne pathogens that cause similar symptoms, like vascular staining and stunting. Successful management relies on identifying the specific pathogen and planting resistant cotton varieties.
• Pathogens like Fusarium wilt race 4 (FOV 4) and cotton root rot require specialized management strategies, including equipment sanitation and targeted fungicide application.
• Boll rot thrives in hot and humid environments and growers can help reduce the potential for rot by preventing rank growth.

Fusarium and Verticillium wilt Identification and Management

Fusarium and Verticillium wilts are soilborne pathogens that can be difficult to manage. Symptomology of the two diseases is similar, complicating diagnosis. Stem discoloration is typically more evenly distributed with Verticillium wilt. To identify a wilt disease, the bottom stems of cotton plants may be sent to a diagnostic lab. Refer to your local university extension service for sampling procedures.

Proper fertility management, crop rotation to corn or soybean, use of nematicides, and variety selection can help reduce cotton yield losses from wilt diseases. Deltapine® offers cotton varieties with higher levels of resistance to Fusarium and Verticillium wilt as well as nematode resistance to help reduce disease incidence (Table 1).

Table 1. Fusarium and Verticillium ratings for Deltapine® cotton varieties.

Fusarium Wilt Symptoms and Control

Symptoms

Fusarium wilt (caused by Fusarium oxysporum f. sp. vasinfectum) is a fungal disease that can be identified by its diagnostic symptoms of wilted or dying plants often paired with interveinal chlorosis. Stems can be carefully and shallowly sliced open to reveal brown vascular tissue, which indicates damage to the vascular system (Figure 1). This results in poor movement of water and nutrients through the plant and causes the plant to wilt. Fusarium wilt is more likely to be present in fields with root-knot nematodes. Symptomatic plants can have substantial root galling.¹

Diagnostic Notes

Fusarium wilt can be observed across cotton growing areas but tends to occur in specific pockets. This is true in the field as well. Infected plants will be found in clusters instead of throughout the field.¹

The most common race is FOV 1, while FOV 4 can be problematic for growers in Texas and California. Unlike other races of Fusarium wilt, presence of FOV4 in a field is independent of nematodes. FOV4 attacks early in the season (many times on cotton as young as 3 or 4 nodes). Symptom onset is rapid, resulting in root rotting, vascular discoloration, and substantial stand loss.²

Potential Yield Loss and Control

Plants severely infected with Fusarium wilt coupled with damage from root-knot nematodes can lead to plant death and substantial decreases in yield potential. It is recommended to plant root-knot nematode resistant cotton varieties and control nematode populations in the field.¹ To help limit the spread of Fusarium, especially in the case of FOV4, it is important to clean equipment prior to moving to another field and limit tillage in infected fields.²

Verticillium Wilt Symptoms and Control

Symptoms

A plant infected with Verticillium wilt (Verticillium dahliae) may appear wilted and stunted, and may be found next to a healthy plant. As the disease progresses, irregular leaf mottling follows. Some areas of the leaf may turn brown and die, while others can remain green (Figure 2). Infected plants will have fewer fruiting positions. Plant stems may be carefully sliced to reveal brown or black staining in the vascular tissue.³

Diagnostic Notes

Verticillium wilt has been observed across cotton growing areas. This disease spreads quickly in cool, moist, and medium- to fine-textured soils.³ Fields with a history of Verticillium and heavy inoculum levels can have problems in seasons with above-average rainfall or irrigation followed by rainfall and a heavy boll load.

Potential Yield Loss and Control

Infected fields can have substantial yield loss and lower fiber quality. In fields with a history of Verticillium wilt, it is recommended to plant varieties that show resistance to this disease. Infected crop residue should be destroyed by chopping stalks and incorporating plant material into the soil for faster decomposition. Winter weeds should be controlled to limit plant hosts for the disease and crop rotation can be used to help reduce inoculum in the field.³

Cotton Root Rot Identification and Management

Symptoms

Plants infected with cotton root rot (caused by Phymatotrichum omnivorum) will be permanently wilted by the third day of expressing wilt and can rapidly die. Wilted plants can be found directly next to healthy, non-wilted plants. Leaves remain attached and plants can be easily pulled out of the soil due to rotten roots. Roots can have white to brown fungal growth and white, cottony growth can be observed just below the soil surface.⁴

Diagnostic Notes

Cotton root rot is primarily found in Texas and Arizona in areas where the annual minimum mean temperature is 60 °F. The disease tends to be limited to alkaline soils and favors calcareous, montmorillonite clays. Within an affected field, noticeable dead areas can be seen. Affected areas tend to be random throughout the field (Figure 3). Disease incidence is higher in wet conditions.⁴

Potential Yield Loss and Control

A cotton crop infected with cotton root rot can have lower fiber quality and lower harvest efficiency due to rotten plants clogging the harvester. Cultural control has been largely unsuccessful for controlling cotton root rot. Applying flutriafol fungicide to the soil at planting in areas known to be problematic can be an effective and economical way to control cotton root rot.⁴

Cotton Boll Rot Identification and Management