Cotton Diseases Threaten Optimum Yields and Quality

Cotton plants infected with Rhizoctonia (soreshin), which is a perennial fungicide resistance challenge.
Cotton plants infected with Rhizoctonia (soreshin), which is a perennial fungicide resistance challenge.

Diseases in cotton threaten the crop from the seedling stage until late in the season. They can lower yields and quality and, in some cases, wipe out whole stands.

The seedling disease complex represents the most common disease problems found across the Cotton Belt. This complex includes Pythium (damping-off), Rhizoctonia (soreshin), Thielaviopsis (black root rot) and Fusarium wilt/root knot nematode complex. Several organisms are associated with cotton seedling diseases, including soilborne fungi and bacteria. The various phases of seedling diseases include seed rot (the decay of seed before germination), root rot and both pre- and postemergence damping-off.

Seedling diseases and nematodes: dangers to young plants

Pythium. Seed rot and pre-emergence damping-off diseases caused by Pythium can be major problems in cotton regions where soil temperatures at planting are less than 60 degrees F. Symptoms include root decay and death as well as light tan to dark discoloration of rotting roots and stem tissues.

Rhizoctonia. Seedling death resulting shortly after emergence is called postemergence damping-off. Rhizoctonia is usually the cause of soreshin, which refers to situations in which only stem girdling occurs. Disease lesions appear mainly on the seedling’s stem below its cotyledons or seed leaves, but they can also occur on taproots. Lesions have well-defined margins and vary in appearance from water-soaked to light tan, reddish-brown or dark brown. Infection and lesion development normally occur below the soil line, but as the seedling stem grows, lesions can be visible at the soil surface.

Thielaviopsis. Also called black root rot, Thielaviopsis causes blackening of the taproot and rotting of lateral roots, coupled with stunting of cotton seedlings. Development of black root rot is most severe when soil temperatures at planting range from 60–64 degrees F.

Fusarium wilt/root knot nematode complex. The root knot nematode (RKN) is a small, parasitic worm that injures cotton roots, allowing entry for the Fusarium disease. Root knot nematode and Fusarium wilt are grouped as a complex because they are normally found together in sandier soils. There is, however, a Race 4 Fusarium in California that occurs in heavier soils, where it can infect the cotton root system without the help of RKN. This aggressive race of Fusarium can destroy a whole stand of susceptible plants. Pima varieties are most susceptible; however, when inoculum levels in the soil are high enough, upland varieties can also be damaged by this disease. To date, this race of Fusarium has not been reported outside the state of California.

Verticillium wilt: A common disease in the Southwest

Once crops are past the seedling stage, cotton producers still need to manage other diseases. For example, Verticillium wilt, which can be a devastating disease throughout the High Plains and the northern tier of the mid-South, is more prominent in the Southwest and far West. Leaf symptoms can vary according to variety, fungal isolate, developmental stage and environmental conditions. Early leaf symptoms include chlorosis, a yellowing of leaf tissue due to a lack of chlorophyll, between the veins. Verticillium wilt can malform younger bolls, severely stunt plants and cause premature defoliation, yield loss and reduced fiber quality.

Bacterial blight: A widespread problem

Bacterial blight, also called angular leaf spot because of the angular appearance of the resulting lesions, can be found extensively in the Cotton Belt, with hotspots in the mid-South and on the High Plains. Humid weather and heavy field irrigation provide conditions that are conducive for this disease. Symptoms include small, dark green spots, which are first visible on the underside of leaves and later on the upper leaf surface. Lesions can also extend along the main leaf vein. Leaf petioles and stems may become infected and cause premature defoliation. Black cankers may encircle the stem or branches, causing these portions to die above that line. If bolls become infected boll rot can result in rotted seed and discolored lint.

Target leaf spot: Premature defoliation and yield loss

Target spot (Corynespora cassiicola) has been identified in several cotton states, including Georgia, Alabama, Louisiana, South Carolina and North Carolina. Warm, humid conditions favor the development of this disease. Symptoms first develop as red dots that lead to the formation of irregular to circular lesions with tan to light brown centers. Lesions further enlarge into a target-like appearance formed from concentric rings within the spot. Heavy infection can cause premature defoliation, along with yield loss.

Cotton root rot: Rapid wilt and plant death

Cotton root rot, often called Texas root rot or Phymatotrichum root rot, occurs from Texas west to California. This disease often causes the rapid wilt and death of cotton in the late spring, summer and early fall in warm temperatures. Dead and withering leaves remain attached to the plant; however, infected plants may also decline more slowly during cooler weather and in fields with well-managed plants. Dying or declining plants have rotted root systems.


Managing Cotton Diseases

In addition to best management practices for cotton – such as crop rotations – several products can help manage disease resistance and greatly increase success.

Starting off with a good seed treatment is an excellent way to control seedling diseases and protect cotton above and below the ground. The Bayer portfolio of seed treatments includes products offering protection against seedling diseases as well as early-season control of yield robbing insects and nematodes.

For cotton-growing areas with a history of disease pressure and/or conditions that are conducive to seedling disease problems, growers can maximize their stands by planting in optimal soil temperatures and protect their cottonseed investment with a premium fungicide seed treatment such as Trilex® Advanced, which is the preferred fungicide partner for Aeris® insecticide/nematicide seed treatment. Trilex Advanced protects against a broad range of cotton seedling diseases, including those caused by Rhizoctonia and Fusarium, while Aeris offers broad-spectrum protection against insect pests and nematodes.

EverGol® Prime is an innovative seed treatment fungicide that promotes stronger, healthier root growth for faster crop establishment. When applied on cotton, EverGol Prime controls seed and soilborne pathogens, such as Rhizoctonia. EverGol Prime is available with select FiberMax® and Stoneville® cottonseed varieties.

FiberMax and Stoneville varieties also help offset several diseases. For example, Stoneville 4946GLB2 provides good resistance to root knot nematode/Fusarium wilt, while FiberMax varieties have the best tolerance in the industry to Verticillium wilt, for which no chemical controls are available. These options include FM 2322GL, FM 1830GLT and FM 2334GLT. Additionally, Bayer has several FiberMax and Stoneville varieties that are resistant to bacterial blight.

For more information on cotton disease management solutions from Bayer, please contact your local Crop Science US representative.

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