Many cotton fields in the High Plains and Rolling Plains of Texas are experiencing browning or bronzing of the foliage (Figure 1), often accompanied by premature defoliation. Several leaf spots are commonly associated with such affected foliage. Different species of fungi have been found to cause these leaf spots.
These are species of the genera Corynespora (Figure 2), Cercospora (Figure 3), as well as Alternaria and Stemphyllium (Figure 4). Some fields have leaf spots associated with only one species, while several species may be present in other fields. However, non-pathogenic stresses are the predominant problem in these fields and the fungal leaf spots are secondary to this.
An example of such a stress is potassium deficiency, which can be confirmed by tissue or soil tests. Often, there are interactions with fluctuating moisture and boll load. Although leaf spots can be found on otherwise normal-looking leaves, they generally are found on senescing leaves or leaves showing the other stress symptoms.
In general, fungi can readily grow on leaf tissue weakened by the other, non-pathogenic stresses. With cotton, the fungi are probably hastening the defoliation, but they are not likely the main factor in defoliation.
In other states like Alabama and Georgia, the fungus Corynespora cassiicola, which causes target spot, can be pathogens by themselves. Extensive leaf spots on a leaf can lead to early defoliation. Disease development is associated with frequent rain and yield losses can be significant. Reduction in defoliation has been possible with the timely application of fungicides.
However, the application of fungicides to control fungal spots will not protect yield where the initial culprit is also a major, non-pathogenic stress, as has been seen in several cotton production areas of Texas, including the High Plains and Rolling Plains.
In 2017 trials in the Upper Coast growing area of Texas, which has a higher rainfall and humidity than areas to the west, fungicide applications did not result in any visible differences in the appearance of cotton foliage.
Similar mid to late season nutrient deficiencies have occurred in years past, and is usually a combination of numerous factors contributing to insufficient nutrient uptake to meet the boll demand, including potassium. These contributing factors include the soil potassium levels, the size of the root system, and boll load.
Adequate soil potassium levels are generally present in most of the High Plains and Rolling Plains soils. However, a small root system caused by compaction, nematodes, or early fruit set can lead to reduced potassium uptake by plants and can lead to insufficient potassium to potassium deficiencies in the plant.
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Potassium, phosphorous, and nitrogen are mobile within plants and are transported to the parts of the plants with the most demand. In a crop with heavy boll load, these mobile nutrients will be translocated from the leaves to the developing bolls and causes nutrient deficiencies in the leaves (Figure 5) and can lead to secondary pathogen infections (Figure 6) and discussed above.
What could or should be done now to remediate this current problem?
On the positive side, these potassium deficiency symptoms usually only occur in the High Plains and Rolling Plains when the cotton yield potential is high. In addition, observations have been made indicating that severity of leaf spots is often linked to crop maturity and seed size.
Development of leaf spot diseases very late in the season in conjunction with these potassium deficiencies should have a minimal impact on cotton yield or quality. In later planted fields that are just beginning to show symptoms, foliar applications of potassium may help elevate some deficiency symptoms, but have a low likelihood of having a positive return on investment for foliar application(s).
The possibility of seeing a positive response is less likely this late in the season. Previous research on foliar applications of potassium has shown inconsistent impacts on yield with early and mid-season applications and require multiple foliar applications to move sufficient levels of potassium into the plant through the leaves.
However, current research is on-going to evaluate the value of foliar potassium applications in high yielding environments. Likewise, the foliar applications of fungicides are not recommended at this time. Information on the efficacy of such applications has been inconsistent.
What should be done to minimize the nutrient deficiency symptoms in the future?
The most important thing is to collect a soil sample and have it analyzed to determine if adequate potassium is plant available in the soil. Once the soil test levels are known (preferably for a 0-6” and deeper samples), then the recommended potassium should be applied and incorporated (or injected) the meet the yield goal.
Also, some varieties are more prone to potassium deficiencies than others, but it is not an easy distinction. Recent research in Texas suggests that the soil K threshold levels are likely on the low side. So, if your soil test levels are between 150-200 ppm (Melich III extraction), then growers should consider adding potassium as part of a maintenance program.
Yield results from broadcast incorporated and injected applications of KCl have increased yields in most years with the injected applications providing more consistent yield increases when soil potassium levels were below 200 ppm. See Figure 7 below.