As the season nears an end, many Mississippi County cotton fields are exhibiting areas of plant yellowing and reddening.
Producers and consultants have expressed concerns that their fertility programs may have been inadequate for the crop, and should be adjusted to include higher fertilizer rates next season. Yellowing and reddening of the cotton crop in mid to late August may not necessarily indicate a fertility problem. Producers should be cautious about adjusting crop fertilizer rates for next season.
As they approach maturity, green cotton leaves often turn yellow and then turn red. The color change may be associated with either abiotic (temperature, water, nutrients) or biotic (disease, insect) factors. In an irrigated, well-drained field, the most likely abiotic cause of the yellowing cotton leaves is deficient nitrogen. A frequent knee-jerk reaction to yellowing of leaves is that more nitrogen should have been applied to the field.
To the contrary, if this occurs as bolls are opening or near opening, the yellowing leaves indicate that the correct nitrogen rates were applied and that the leaves are naturally maturing. If leaf maturation and nitrogen level are ideally synchronized, the leaf should essentially run out of nitrogen as it transfers its last available photosynthates into the mature boll. In this case, no additional yield would be realized by a higher nitrogen rate and the maximum efficiency of available nitrogen has been achieved. Throughout the season, fruit retention across the county was generally high. It therefore stands to reason that a robust boll load would serve as a tremendous sink for photosynthates from the plant's canopy, and result in leaf yellowing and reddening. The absence of a color change in the crop's canopy late in the season could indicate that fertilizer amendments were over applied during the season. This natural maturation of leaves can be beneficial to the crop by allowing more sunlight into the plant canopy and reducing boll rots. Additionally, natural leaf maturation and senescence can allow a more efficient defoliation of the crop in preparation for harvest.
Since these mature leaves are stressed, they become more susceptible to many leaf spot pathogens. However, late-season leaf spots rarely or never cause a reduction in yield, and may be beneficial by reducing the leaf area of the plant. Applying higher than optimum rates of nitrogen might reduce these "garbage" late-season leaf spot, but will tend to delay crop maturity, increase boll rots, enhance Verticillium wilt, hinder defoliation, and increase production costs without increasing yields.
The most common biotic factor causing late-season yellowing of leaves is Verticillium wilt. Verticillium wilt is caused by a fungus that occurs in most soils. The fungus grows in the vascular system of an infected plant and will interfere with water and nutrient transfer. In essence, the effects of Verticillium can be described as plugging up the plant's plumbing. As boll load increases, more demands are placed on the plant's plumbing. Plugging of the vascular system by Verticillium will often lead to yellowing and loss of the plant's leaves because they will not be able to obtain sufficient water and nutrients.
The easiest way to avoid Verticillium wilt is reduce plumbing demands of the plant by eliminating bolls! In some cases, lines that are most resistant to Verticillium wilt are also resistant to yield. Incidence of Verticillium wilt is enhanced by susceptible varieties, low plant densities, high nitrogen rates, and relatively cool conditions (which may coincide with summer thunderstorms). None of our varieties are completely resistant to this disease, but some tolerate the disease better than others. Since the Judd Hill location of the Arkansas Cotton Variety Test typically has a high incidence of Verticillium, high relative yields at this location indicate tolerance to the disease. To offset higher seed costs, planting rates are often lower which may lead to lower plant densities and more Verticillium wilt.
Low plant densities along with high nitrogen rates and frequent irrigations set the stage for high incidence of Verticillium wilt. After on-set of the disease, no effective remediation for Verticillium wilt is available. If a field is prone to Verticillium wilt, the producer should consider planting a more tolerant variety and avoid low seeding rates and high nitrogen rates.
Therefore, you should not become alarmed when you see some yellowing and reddening of late-season cotton leaves. It may simply signal natural maturation of the crop and indicate that you are getting maximum efficiency of your nitrogen.
If your field is prone to Verticillium wilt, the complete absence of any yellowing leaves should cause more alarm because it suggests that you have a low boll load, or have over-applied nutrients during the season.
Benson is Mississippi County Extension staff chair and Bourland is Cotton Breeder and Agronomist -- University of Arkansas Division of Agriculture