Evaluation of 1997 Late-Season Crop Conditions

Abstract

In the latter part of the 1997 season (August) many fields across Arizona, from Marana to the Mohave Valley, were experiencing premature senescence. In an effort to evaluate the conditions leading to the symptoms and to possibly determine the causes, an extensive series of field examinations were conducted in a number of representative fields in central Arizona. Soil samples were taken from selected fields that ranged in expression of the symptoms from very light to severe. Complete analyses of the soil samples were conducted. Plant tissue samples were also collected and analyzed for plant nutrient levels. No absolute causal agent was identified. However, a factor believed to be of significance was that of low soil K levels, where many of the fields expressing the most severe symptoms also had low soil K levels. It is also important to consider the fact that plants exhibiting K deficiency are very similar in appearance to plants affected by Verticillium wilt, which also appeared to be a primary or contributing factor in many cases.

Introduction

It is not uncommon to experience late-season senescence and stress related crop decline in many cotton producing areas of Arizona. However, it is uncommon to have widespread conditions develop across a broad portion of a region such as Arizona without a readily identifiable trend or cause. In 1997 many cotton fields across Arizona did experience rather rapid senescence and deterioration, beginning in later July and extending into August. The areas affected were quite extensive ranging from Marana across central Arizona to Parker and Mohave Valleys. The lowest elevation areas in Yuma County were not affected as well as the highest elevation areas of southeastern Arizona. Symptoms included wilting of plants, leaf discoloration (yellowing/reddening), and very low crop vigor. Cases ranged from severe in some places to light in others. The possible causes that were being considered included soil salinity/sodicity, micronutrient deficiencies and/or Verticillium wilt.

In an effort to evaluate the conditions leading to the symptoms and to possibly determine the causes, an extensive series of field examinations were conducted in a number of representative fields in central Arizona. This report provides a brief summary of the information collected in August 1997 relative to the late season problem fields (premature senescence, poor vigor, rapid decline, etc.) that were evaluated in Maricopa County, AZ in 1997.

Materials and Methods

Soil samples were taken from selected fields that ranged in expression of the symptoms from very light to severe. Complete analyses of the soil samples were conducted. Plant tissue samples were also collected and analyzed for plant nutrient levels

Soils were sampled from a group of fields that were representative of the range of stress related symptoms and conditions being experienced commonly at the time with the following designations (see Figure 1 also):

Results

Soils were sampled from all fields to a depth of about six to 10 inches according to Figure 2, primarily to address salinity and sodium (Na) gradients. Results from a complete / full spectrum set of analyses are presented in Table 1.

Tissue samples were also collected (leaf blades and petioles) from each field. Results from the complete tissue analyses are presented in Table 2.

Soil samples were also collected from several representative locations in each field and composited by one foot depth increments, to a total depth of four feet (Table 3).

Figure 3 and Table 4 provide outlines of salinity (EC_e ,dS/m) and exchangeable Na percentages (ESP) for the surface samples at Buckeye and Tonopah. Figure 4 and Table 5 offer similar results for the Paloma field, followed by Table 6 with a complete set of soil analyses.

Figures 9, 10, 11, 12, 13, 14, present some of these results in a graphic manner, for selected parameters, by depth.

Summary

There is certainly evidence for some rather saline and/or sodic conditions revealed from these results. However, none of the results indicate anything unusual, particularly in the context of what is "normal" for the areas in question and in terms of relative comparisons. We definitely need to be conscious of these conditions and they can impact several aspects of field / crop management. Soil salinity and / or Na do not appear to be the critical features associated with the crop response patterns experienced. It is interesting to note an increase in soil salinity (ECe) levels at about the three foot depth in the Tonopah soils (Figure 6), that coincide with a gravel layer at that depth and the probable restriction in drainage that would occur under those conditions. All other ECe values indicate good leaching conditions and overall salt management.

Vertical lines are present on many of the nutrient figures (Fe, Zn, K, and P) indicating where current (UA) soil test "critical levels" exist for cotton. The only factor that presents potential concern in the review of this information are the soil K levels (Figures 11 and 12, ammonium acetate extractions). Using a soil test critical level of 150 ppm K, several of the fields are low and some are borderline. It is also interesting to note low soil K values throughout the profile in those cases. This is significant considering cotton root development patterns, and the probability that K uptake is taking place at depths two to four feet below the surface late in the season. However, these results do not offer any clear indication that the problems in question are due strictly or primarily to poor K nutrition. The plant symptoms match K deficiency symptoms to some extent, but they also look a lot like Verticillium wilt.

Potassium nutrition may be contributing to the problems encountered, but Verticillium wilt is probably a factor as well. There are very likely a number of factors contributing to the overall symptoms that were expressed. Verticillium wilt was common in many parts of Arizona in 1997, most of which have soils that are relatively high in extractable K. An important factor to consider relative to these and similar fields, is that poor and/or weak root development is also very likely contributing to the problems. Although this is hard to identify in an absolute manner, symptoms did indicate root system problems.

Speculation could propel a rather lengthy discourse on the problems we encountered late in the 1997 in much of central Arizona. The purpose of this report is to provide an outline of the data obtained from the samples that were collected in a systematic fashion in response to the occurrence of the symptoms and the many questions that were posed as a result. We can offer a good characterization of these fields as a result of the samples and analyses. However, this type of experience demonstrates the fact that field diagnostics are not always simple and clear-cut. Some of the relationships considered in this project have been studied extensively elsewhere, such as the K nutrition/Verticillium wilt interactions. Plant nutrition studies involving K, micronutrients, and possible Verticillium wilt interactions are needed to address the relationships associated with cotton production systems in Arizona and premature late-season senescence.