Senescence:  the highly ordered, genetically programmed processes that lead to plant death. It is generally accepted that there are three phases leading to natural plant death. 1) Initiation phase: shutdown of cell maintenance functions and increase in degradative enzymes. 2) Degeneration: disassembly of metabolic processes. 3) Homeostatic Imbalance: loss of internal controls, cell membrane integrity and ultimately, cell death.

Mature, well ‘cut-out’ cotton is beginning the process defined above. While most of us would like to see the cotton continue to fruit for another two weeks, high temperatures and lack of wide-spread rainfall have most of the dry-land cotton crop rapidly headed to defoliation. Most questions over the last five days have had to do with changes in plant color since temperatures have moderated. Cotton changes color in response to environment and nutrient status. In some fields the outside turn rows are greener than the middle

This can be due to doubling up of fertilizer or a lower plant population due to turning across the ends allowing each plant more ‘soil resources’ to maintain color. Irrigated or late planted cotton is still dark green due to increased moisture status in the irrigated crop and fewer set bolls in the later planted crop.

For the majority of the dry-land crop, a heavy boll load and reduced soil moisture has lead to potassium deficiency symptoms appearing. Potassium deficiency symptoms in the plant are not always an indicator of potassium deficiency in the soil. The speed at which a variety fruits (earliness), high boll retention, and environment (drought) can induce potassium deficiency symptoms. 

These symptoms (Figure 1) start out as an increase in leaf spot disease complexes, marginal leaf yellowing progressing to the entire leaf (and many times all leaves) turning bright yellow, followed by leaf margin necrosis (drying out) and finally leaf abscission.

Additionally, some small bolls in the top of the plant may have cavitated under heat and drought stress and have now become highly visible. Cavitation of small bolls can occur in any cotton variety (Figure 2), but is more often seen in columnar plants that fruit close to the stalk (Figure 3). Cavitation is believed to be caused by disruption of vascular tissue (water and nutrient transport system) leading to the boll. If these fruiting forms would have abscised normally (fallen from the plant) we would call it physiological shed. However, a cavitated boll hanging on the plant catches everyone’s attention and results in many questions.

Severe potassium deficency (Figure 4) can result in premature defioliation. In this picture the plot area on the left showed deficency in soil tests. Most of the cotton that is currently showing potassium deficency is on soil with adequate potassium levels and this level of defoliation and premature maturity is not expected. The addition of foliar potassium at this late stage of crop development is not recommended.