By Bill Robertson, Extension Cotton Agronomist
There are times during the season that initial impressions of the crop are not good. We are at one of those times now. Plants are showing signs of a good boll load and maturing from being near the end of a long, hard race. Add target spot and a week of wet and cloudy weather to get to where we are now. It was scary to part the crop back this week and see 50% to 60% of the leaves defoliated when a week ago these fields sitting at 350 HU beyond cutout in Northeast Arkansas had less than 10% defoliation associated with target spot. During the last week we have also seen ascochyta blight (wet weather blight) explode on young leaves, bracts, and bolls as a result of the extended wet and cloudy weather. The cloudy weather has added additional concerns as significant boll shed has occurred in the last week regardless of the presence of target spot. This shedding is primarily associated with the plants inability to produce the energy needed to keep these small bolls because of the extended heavy cloud cover. However, in most fields these bolls were produced after the last effective boll population had been established are often referred to as phantom bolls. In fields where target spot is not present, I’ve received reports of 5 to 7 bolls per plant with boll rot. All these issues contribute towards the crop not passing the eye test right now. There are still many reasons to be optimistic about this crop. We desperately need the wet weather to pass for a while and have favorable temperatures to finish this crop.
Proper identification of leaf spots in the field is critical. Diagnosis and Management of Foliar Diseases of Cotton in the United States (http://www.cottoninc.com/fiber/AgriculturalDisciplines/Plant-Pathology/Diagnosis-Management-Foliar-Diseases/Diagnosis-Management-Foliar-Diseases.pdf) is an excellent publication. It has a very useful key on the back page.
Target spot has been the topic of most of my calls lately. We have been in close contact with our counterparts in the Southeastern U.S. to get a better handle on problems associated with target spot. They consistently state that when the occurrence of target spot is late-season like we are experiencing that we should let it go. They have a different message when it occurs during the first few weeks of flowering. One common point they make is that target spot coming in late will open rank canopies and have a positive impact on reducing the occurrence of boll rot.
Research conducted by Dr. Derrick Oosterhuis and his students included in a Cotton Physiology Today Newsletter (http://www.cotton.org/tech/physiology/cpt/plantphysiology/upload/CPT-May90-REPOP-144.pdf looked at leaf productivity and leaf age and how that relates to boll filling. A better understanding of this makes it easier to understand where we are now and what we need to do.
An individual cotton leaf is most productive at 16 to 18 days after unfurling (80% expanded). This aligns closely to the uppermost fully expanded mainstem leaf and a subtending leaf at flower. The productivity of an individual leaf drops significantly as it approaches 50 to 60 days of age. This aligns closely to a full-sized boll. Small leaves are a sink as they require more energy than they produce as they unfurl and expand. Young full-sized leaves are strong sources of energy as they are young, productive, and generally located where little shading occurs. Older leaves are a declining source of energy through a combination of age and shading.
At first-square just under 40% of the total leaf area is a strong source. This value drops by approximately 50% at first flower and drops another 50% at the point of the season we are now in. Right now only about 10% to 12% of the total leaf area of the plant is a strong source.
Most of my experience with disease is in wheat. As the wheat plant is filling grain, our goal is to protect the flag leaf. We can lose all other leaves and not impact yield potential. I equate the younger fully expanded leaves on the cotton plant to the flag leaf on wheat. These leaves are still on the plant now. This line of reasoning makes it easier to understand why we can let late occurrence of target spot go.
High defoliation rates can still be a major point of concern. We cannot relate defoliation from target spot to defoliation from harvest aids. We know the yield penalty from early defoliation with harvest aids. Harvest aids remove the flag leaf equivalent leaves of the cotton plant. While it is scary to see the high levels of defoliation with target spot, remember these are the older less productive leaves that have served their primary purpose. We can lose these leaves and not impact yield potential. We will finish this crop with the newer leaves. The opening of the canopy is providing the bottom defoliation we often want to slow boll rot.
Data in Arkansas is limited for target spot. Our pathologists have field studies in place to address this need. We should continue to make well-informed decisions as we finish this crop. Using what we know about the cotton plant and the pest will help us better understand the potential economic injury and probability of achieving benefits from treatments. There is no room in the budget for feel good treatments.