(EDITOR'S NOTE: In this week's Alabama Cotton Shorts Steve Brown, Alabama Extension cotton specialist, Eddie McGriff, Alabama Extension regional agronomic agent, Scott Graham, Alabama Extension entomologist, and Ron Smith, professor emeritus Alabama Extension entomologist discuss Alabama's cotton conditions, foliar diseases and bollworms. The entire Aug. 10 issue can be read at Alabama Cotton Shorts.)
The crop has advanced rapidly in the last weeks. Many April and early May plantings are showing signs of maturity as the crop blooms in the upper canopy, leaves decline with age and leaf spots, and large bolls begin to speckle and harden. As of August 10, over 75 percent of the crop is rated good to excellent. This week’s forecast has good probabilities for rain in the entire state, so we’re likely to see some much needed relief across Alabama. Had we had a broad inch or so each of the last couple of weeks, we might not could gin all the cotton we’d make. We have a good crop. In places, it’s exceptional; in others, heat and drought have diminished it somewhat.
K Deficiency, Foliar Diseases
Over the past few weeks we’ve observed several cotton fields with Stemphylium leaf spot. The ultimate cause is a depletion of potassium (K) in leaf tissues. Potassium is rapidly exported from leaves and stems to fill developing bolls, and as K becomes scarce in leaves, plants become vulnerable to fungi such as Stemphylium and, less commonly, other secondary pathogens such Alternaria and Cercospera. Potassium adds strength to leaf cells and the lack of potassium in leaf tissues makes them weak and susceptible to these fungal infections. These secondary invaders initially appear as small brown lesions, and as they enlarge, they can lead to massive premature defoliation.
Incidence of Stemphylium is most common in dry land fields under drought stress. Lack of soil moisture reduces uptake of K during vegetative growth and early flowering, and thus the reserves in stem and leaves are insufficient to sustain the leaves during boll fill. Adding to the challenge is that root activity tends to decline as plant resources are directed towards reproduction. Short season varieties can sometimes be more susceptible to Stemphylium since they often have an intense demand for K in a short time. These secondary pathogens can also be a problem under irrigation, particularly in the latter stages of flowering (4th week or later) in the presence of a heavy boll load and if heavy rainfall or watering has promoted K leaching. In recent years there have been a few cases of Stemphylium attributed to high soil magnesium levels competing with K for uptake in plant.
Note, foliar fungicides do not help with these secondary invaders. Potassium deficiency is the issue. Other foliar diseases such as target spot and areolate mildew may respond to timely applications of fungicides, but they are ineffective for Stemphylium and related pathogens.
Thankfully bollworm pressure in south and central Alabama was relatively light this year, and we did not receive any calls on escaped worms in those regions. However, we have received reports from the Tennessee Valley of bollworm egg lays from 5 to 20 percent over the past 7 to 10 days. Currently, we are still recommending allowing the Bt technologies in two-gene cottons the opportunity to control these populations.
In fields that have been treated for other pests, such as plant bugs or stink bugs in the past 2 weeks with hard chemicals (e.g., pyrethroids, organophosphates, or neonicotinoids), we recommend treating when 5 small larvae are found per 100 plants. In fields that have not been sprayed in the last 2 weeks, we recommend treatment when 10 small larvae are found per 100 plants. The reason for the difference is the presence of beneficial insects that should be in fields that haven’t been treated recently. You can watch Eddie McGriff’s presentation on beneficial insects on YouTube.
Spider mites are still building in areas of Alabama. Spider mite infestations are rarely evenly distributed throughout the field and are almost always in clumps either near field edges or scattered randomly throughout the field. Our threshold in the Alabama Cotton IPM Guide says to treat fields when mites are widely distributed and mottling of leaves is common. Determining when to implement controls can be difficult when trying to decide how many ‘hotspots’ suggest mites are “widely distributed” and justify a spray. Spider mites prefer hot, dry conditions and typically can be suppressed by a rainfall event. Fields with spider mite hotspots should continue to be monitored after rain, because populations can build back after several days of hot, dry conditions. Spider mites also tend to build following broad-spectrum insecticide applications for other pests.
Typical spider mite damage on the underside of a leaf. Mites are present but barely visible to the naked eye.
To scout for spider mites, look for leaf stippling or reddening on the top of leaves. If these symptoms are observed, look on the underside of leaves for spider mites, which will be a yellowish color with two black “spots” on each side of its back. Exposing the underside of leaves to the sun may agitate mites, making them easier to see. Also look for mites one or two nodes above the most symptomatic leaves as they may have moved up to fresh leaves.
There are a few miticides labeled for use in cotton that can be found in the Cotton IPM Guide. Abamectin (e.g., Agri-Mek 0.15EC) at 8 to16 ounces per acre is the most economical option but rotating chemistries is necessary if multiple applications are needed. Historically, lower rates of abamectin (8 to 10 oz) have provided adequate control in younger cotton, while higher rates (12 to 16oz) are needed later in the season when plants are larger. (Smith and Graham)