Review of the 1997 Arizona Cotton Season

In 1997 there were 341,000 acres of cotton in Arizona consisting of 319,000 and 22,000 acres of Upland and Pima cotton, respectively. Statewide average yields in Arizona were 1,234 lbs. lint/acre for Upland and 982 lbs. lint/acre for Pima in 1997 (USDA, Arizona Agricultural Statistics Service). Approximately 60 to 70% of the Upland cotton acreage was planted to varieties containing transgenic, Bt properties. A Bt variety is one that contains some genetic information that was extracted from a naturally-occurring soil bacteria called Bacillus Thuringiensis (Bt) which has insecticidal properties.

The 1997 Arizona cotton season offered a high degree of potential. There were a few rough spots, particularly in the early part of season and again in August and September. However, there were several encouraging features associated with the 1997 season. The weather conditions in June and July were excellent (hot and dry) for cotton production, provided that fields were well-watered and not stressed. The use of many transgenic varieties (those with Bt and herbicide tolerant genes) performed very well in many Arizona production settings. The insect growth regulators (IGRs) used for whitefly control were available once again in 1997 and provided an excellent means of control for whiteflies. Final results for 1997 for many fields seemed to be largely dependent upon how well the conditions in June and July were taken advantage of, and how the various tools at our disposal were put together and utilized in a complete production system.

The 1997 season did not unfold free of problems. Many fields experienced a lot of problems in the spring planting season with the passing of a series of cold fronts through the State. One of the first of these fronts came through in late March, followed by a rather strong (cold and windy) system the first of April. This pattern continued with additional cooling periods in mid-April, the end of April, and even in the later part of May. Throughout this period, even into early June, we experienced rather strong and persistent westerly winds. All together, this pattern of weather made it difficult to establish vigorous stands in many areas. As a result, some areas experienced rather significant amounts of replanting. Most areas had fields that experienced poor early vigor under the cool and windy weather conditions. Low plant vigor early in the season may have continued to affect many fields for weeks or months into the season. By May and early June we had many fields whose stands had been maintained but had poor plant vigor and relatively weak root systems. Problems associated with poor root development became evident in many cases later in the season as well.

Field conditions improved dramatically for most parts of Arizona in the latter part of June in response to warming weather, diminishing winds, and good management. Many fields in Arizona had very strong fruit retention (FR) levels from mid to late June through late July, prior to the onset of the monsoon season. Figure 1 provides a good example (from a field in central Arizona) of what many fields experienced in terms of FR and vigor (height to node ratios, HNR) patterns in 1997. FR was commonly low around the time of first bloom (approx. 1200 heat units accumulated after planting, HUAP) but improved markedly over much of the remaining fruiting cycle, as shown in Figure 1A. This period of June and July, where substantial FR gains were made on many fields, represents a "prime time" for realizing much of the 1997season potential. Also common at this time were many fields low in vigor when developing a strong boll load. This is exemplified very well in Figure 1B which outlines the seasonal HNR pattern that was always below the center baseline for Arizona Upland cotton. Accordingly, any factors that served to reduce or limit plant vigor at this time may have reduced yield potential as well. FR dropped drastically for many fields in the latter part of July and early August, coincident with the beginning of the monsoon season, which marks the end of "prime time". My view is that much of this drop in August FR was weather dependent. Many factors can contribute to reduced FR. However, the relationship of small boll shed (one- to three-day old bolls) to the presence of hot weather and more humid conditions (noted by high night temperatures and dewpoints above 55 oF) is no coincidence. Fields that we were monitoring in 1997 commonly experienced very poor FR levels throughout August and into much of September, which is common under these types of weather conditions.

We also saw a rather high degree of late season crop senescence, somewhat prematurely, which was associated in many cases with Verticillium wilt disease. The incidence of this disease was much greater in 1997 across the state than we see in most years. Though difficult to assess, the absolute impact of this disease on final yields was largely dependent on field conditions prior to and at the time the disease expressed itself, in addition to the severity of the symptoms. Irrespective of the presence or absence of Vert in many of the affected fields, symptoms were commonly indicative of poor root health/vigor, which could be related to a number of additional factors, including perhaps early season problems. Late in the season Arizona was impacted by the affects of Hurricanes Linda and Nora, primarily affecting the western-most parts of Arizona