In my world of all things “disease and nematode management” of Georgia’s cotton crop, August is a very important month for growers. It is important for two very different reasons. First, and most urgent in the minds of cotton farmers, is, “What should I be doing right now to protect my crop from losses to diseases?” Second, and equally important but less obvious is, “What am I seeing in the field between now and harvest that I can’t fix anymore but that is important in planning for the 2020 crop?”
There are generally six diseases that a grower might come across in his field in August. (It is hard to believe that cotton growers now must be concerned with SIX diseases late in season!) Fusarium wilt, characterized by stunted plants, “tiger-striped” foliage, and darkened pith when the lower stem is split, is of increasing importance in Georgia. There is nothing that can be done at this point in the season to protect the crop from Fusarium wilt.
Bacterial blight has been rarely reported this season, a few leaves here, a few leaves there. Characterized by dark, sunken lesions on the bolls, angular, blocky spots on the leaves and a “lightning bolt” appearance on the veins of the leaves, there is nothing a grower can do late-season to battle this disease, other than to avoid over-irrigation.
The third disease is Stemphylium leaf spot, most often observed on potassium-starved cotton leaves with hues of red and yellow which look as if they are preparing for the autumn season. Characterized by small spots over the entire leaf canopy that are bordered by dark brown-purple rings, Stemphylium causes significant defoliation. However, this disease cannot be effectively managed with use of a fungicide because the underlying cause is a potassium deficiency.
The Cotton leaf roll dwarf virus (CLRDV) has been identified in some cotton fields in Georgia. Early-season symptoms appear much like the “bronze wilt” from 20 years ago to include reddening of leaves and petioles, a drooping and wilted appearance, and stunting. These affected plants can be quite obvious, especially as the season progresses. We are now working to identify late-season symptoms as well. Growers are encouraged to watch for this disease in their fields in order to educate themselves about the symptomology; however there is nothing that can be done to manage this disease at this point.
Target spot begins to develop deep within the plant canopy where high humidity and extended periods of leaf-wetness provide an excellent environment for infection and spread of the disease to occur. Management of target spot with a fungicide is warranted if, A) the disease is identified within the first six weeks of bloom, B) environmental conditions are favorable for development and spread of target spot, and C) plant growth throughout much of the field is both conducive for disease spread and for good yield potential.
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Areolate mildew is sometimes found during the latter part of the season and is characterized by a white, powdery fungal growth on the foliage. Unlike Stemphylium leaf spot, which occurs when there is a potassium deficiency, and target spot, which typically develops in fields with excellent growth and yield potential, areolate mildew can occur across a variety of growing conditions. Areolate mildew can be effectively managed with judicious use of fungicides, typically one, or at most, two applications.
If areolate mildew appears within 3-4 weeks of planned defoliation, I would not worry about it and would not spray. If the areolate mildew appears with more than 4 weeks until planned defoliation, I would consider use of a fungicide to protect yield potential.
NOTE: In most cases, I would not recommend spraying “preventatively” for areolate mildew, but would recommend scouting for disease and consulting with your local county agent to determine the status of the disease.
After the furrow is closed at planting, there is very very little that can be done to further protect a cotton crop from plant-parasitic nematodes, from Fusarium wilt, or from bacterial blight. However, the damage caused by nematodes and Fusarium wilt (areas in the field that are severely stunted despite adequate irrigation and soil fertility, and with the interveinal leaf necrosis, “tiger-striping”) becomes increasingly and painfully evident as we move through August.
As growers work in their cotton fields now, whether applying growth-regulators, defoliants, or anything else, it is important for them to identify areas where nematodes and/or Fusarium wilt are/is a problem and to take time and effort to confirm the diagnosis. Understanding that these issues exist in a field growers are better able to develop management plans based upon crop rotation, variety selection and use of nematicides in 2020.