UGA Cotton Newsletter August 25, 2000
|Cotton Situation 1
Got Wilt? 1
"Red" Cotton Likely Due to Weather 2
Roundup Ultra Being Phased Out by Roundup UltraMAX 3
Silverleaf Whitefly 4
When Is a Boll Relatively Safe from Insect Damage 5
Harvest Aid Formulation Changes New Generic Products 6
Cotton Situation. (Brown) August projections from USDA estimate the Georgia crop at 1.68 million bales harvested from 1.3 million acres, which is an average of 620 lb/A. Because of persisting drought and high temperatures, harvested acres could fall to 1.2 million acres.
Prospects are highly varied across the state. Much of the irrigated crop is at least two weeks early. Some irrigated fields are spectacular, others are mediocre, and there are hundreds of dryland fields that are disastrous. Defoliation has begun. Heat continues to shorten the crop here and in other areas of the South.
Thousands of acres have "crashed" over the past weeks. Severe leaf reddening and even leaf drop have developed rapidly over wide areas, and in many fields. This rapid occurrence of cutout has been initiated by heat/moisture stress, nutrient depletion, and early boll load. Whiteflies are building in southern extremes, posing a threat to late cotton.
"Got Wilt?" (Kemerait and Brown) Bronze wilt, the plant malady that affected thousands of acres in 1998, has been quite limited in the 2000 crop. While there has been a report of severe bronze wilt in one field of Paymaster 1218 B/RR in south Georgia, occurrence in other "suspect" varieties has been infrequent. Scattered bronze wilt has been observed in SG 125 B/RR. Across the state, wilted cotton plants that are reddish in color are common and are of concern to many growers. A number of growers fear that bronze wilt has taken a heavy toll on their crop; however, they should understand that nearly identical symptoms are caused by other common factors in the field.
Though growers have had to deal with bronze wilt for several years, there is still considerable controversy over not only as to the cause of the problem but also the diagnosis of the condition in the field. A search on the Internet or cotton literature for "bronze wilt" exposes the reader to a number of conflicting reports about this malady. Bronze wilt is a wilt related to restricted water transport in the plant. It manifests itself with numerous symptoms, including reddish bronze discoloration in the upper canopy, elevated leaf temperatures, extreme reddening of plant stems, and/or loss of fruit and foliage. In young plants, initial pale leaf coloration can progress to total plant death. Bronze wilt is more severe in later plantings and is always randomly scattered in a given field. [In other words, if a pattern of symptoms is detected, the observed problem is probably NOT bronze wilt.] Bronze wilt is more easily diagnosed in young plants that begin to redden and wilt than it is in mature plants with heavy fruit loads. Older plants with heavy boll loads are more susceptible to stresses that produce wilting and reddening.
Given this list of symptoms, some may wonder why there is such confusion in the identification of plants with bronze wilt. This is because the symptoms associated with bronze wilt are also associated with several other important conditions. These include premature cutout, normal plant maturation and senescence, nutrient depletion, Fusarium and Verticillium wilts, and other environmental stresses on the plants. Since diagnosis of bronze wilt does not include the isolation of a specific pathogen as is commonly done for many plant diseases, it is particularly difficult to diagnose. Many times, the diagnosis of bronze wilt is based upon ruling out other possible causes of plant decline.
A USDA research scientist from Texas has given a great deal of attention to the bacterium Agrobacterium tumefaciens as the causal agent, though this is vigorously contested around the country. In Georgia, it is believed that the role of A. tumefaciens in bronze wilt is at best unclear and that interaction between the genetics of the cotton variety and hot weather is the primary factors involved.
The exact stress that triggers the response is not clearly understood. In a current field study,
Dr. Craig Bednarz, Research Physiologist in Tifton, is comparing two varieties over two planting dates and two N rates. The susceptible variety planted in June is severely affected by bronze wilt while incidence has been much less in the early planting. N rate has had no effect on occurrence.
We appear to have dealt with and successfully "solved" this issue by AVOIDANCE; that is, by reducing acreage of varieties with the genetic background known to have potential for the problem. Bronze wilt has thus far been limited to varieties which have TAMCOT SP-37 in their pedigree. These include Stoneville 132 and 373, the Paymaster 1200 series, and a few transgenic lines in which a Paymaster 1200 parent was used as the donor of Bollgard and Roundup Ready genes.
"Red" Cotton Likely Due to Weather. (Harris and Brown) It's that time of year when most May-planted cotton progresses past cutout and starts to show it's "fall colors." Normally, this occurs gradually with leaves turning yellow, red and a splotchy purple on green. Generally this occurs on bottom leaves first and progresses to the top of the plant. However, there have been numerous observations this year of earlier-planted cotton turning red from top to bottom very rapidly ("almost overnight").
In most cases, this phenomenon is simply rapid maturation or accelerated cutout due to the hot, dry weather -- and in some cases additional stress. Excellent early-season fruit retention, especially with short season varieties, has also played a role. Even irrigated fields have "crashed" much more rapidly than normal. This may be due to lack of good subsoil moisture and the fact that it is very difficult to produce high yields with irrigation alone.
Inadequate fertilization, particularly potassium, has been blamed for some of this unusual rapid cutout. However, the dry weather or lack of water to move nutrients such as potassium into the plant is most likely the primary problem, with potassium deficiency being secondary. Many fields are showing rapid cutout in weak areas such as terraces and drought-stress eroded areas. Conversely, water logging, where heavy rains cause water to temporarily stand in low, poorly drained areas, is also evident in few fields in southeast Georgia. Coupled with the heat and drought stress, several additional stresses including nutrient deficiencies and nematode injury can contribute to rapid cutout.
Unfortunately, there is little that can be done to slow the process of rapid cutout. In most cases it is too late to try to rescue and prolong the fruiting period on May-planted cotton by foliar feeding. Foliar feeding nutrients such as nitrogen, potassium, and boron is most effective when done during the peak bloom period (first 4 weeks of bloom). As with Pix applications, foliar feeding is more effective when applied earlier and on time versus later in the fruiting cycle. At this point, late-planted (June) cotton that is still in early bloom with good yield potential is the only cotton to consider foliar feeding.
Roundup Ultra Being Phased Out by Roundup UltraMAX. (Culpepper). Monsanto is in the process of phasing out Roundup Ultra during the fall of 2000. Although there most likely will be some Roundup Ultra available during the 2001 season, Monsanto is replacing Roundup Ultra with Roundup UltraMAX. The main difference between these two products is the amount of active ingredient per unit volume of material (Table 1).
Table 1. Comparison of Roundup Ultra and Roundup UltraMAX.*
|Surfactant in Formulation||RR
|RR Corn||RR Soybean||Surfactant Recommendations|
|4 lb ai / 3 lb ae
5 lb ai / 3.7 lb ae
0.25% at low rate/high GPA
* Table provided by Dr. Larry Hawf, Agronomic Research Manager for Monsanto.
** One quart/A of Roundup Ultra is equivalent to 26 oz/A of Roundup UltraMAX.
The UGA cotton team conducted several studies comparing Roundup Ultra and Roundup UltraMAX during the 2000 cotton season. No differences in weed control or cotton tolerance were noted when these products were applied at the same amount of active ingredient per acre.
With the phase out of Roundup Ultra and conversion to Roundup UltraMAX, growers should be careful to adjust application rates of Roundup UltraMAX, (to avoid excessive cost) and to add adjuvants when Roundup UltraMAX is applied alone or in tank mixtures.
Silverleaf Whitefly. (Roberts) Silverleaf whitefly (SLWF) has been a troublesome pest of cotton during recent years, primarily in the Tift County vicinity. Infestations were much more intense and more widespread during 1999 and caused significant yield loss in some fields. Silverleaf whitefly has been observed in cotton for several weeks this year. However when compared with last season, populations are significantly less as a whole, but some fields have high populations. Correct identification is important. Many of us are more familiar with the banded winged whitefly which is much easier to control. The banded winged whitefly has faint but visible grayish bands on the wings whereas the silverleaf whitefly is solid white. Silverleaf whitefly is generally a localized pest.
The silverleaf whitefly adult is small, about 1/20th inch in length, and holds its solid white wings roof-like over a pale-yellow body while at rest. Eggs are oblong, pointed, and yellowish brown. The first stage upon hatching is known as the crawler that moves about searching for a suitable site to attach itself on the underside of the leaf. Once becoming sessile, three more molts occur as a flattened, oval nymph. These scale like nymphs remain stationary, sucking plant sap. As the nymphs develop and molt, red eye spots will become visible on the yellowish scale like immature. The pupal stage has very prominent red eye spots. It requires as little as 18 days to develop from egg to adult under warm temperatures. The host range of silverleaf whitefly includes over 500 species of plants. Among these are weeds and cultivated vegetables, agronomic and ornamental crops.
Both adult and nymphal silverleaf whiteflies feed on the lower surfaces of leaves by sucking sap with their piercing-sucking mouthparts. Whiteflies produce honeydew upon which sooty mold can grow; thus reducing light penetration; hence, reducing yield and quality. Honeydew accumulation on open lint may also cause problems. Excessive feeding may also cause premature leaf defoliation.
Silverleaf whitefly is not a predictable pest in Georgia. Potentially significant damage may occur and infestations tend to be most severe during hot and dry conditions. Infestations can build to extremely high numbers in a short period of time. Cultural practices such as destroying crop residues which harbor whiteflies will help reduce overall population buildup. Conserving beneficials, planting smooth leaf varieties, and avoiding late planting are also a means of reducing the likelihood of whitefly problems. However, when and if the problem develops, control of silverleaf whitefly with insecticides is expensive and difficult. We basically have three options when faced with economic infestations of silverleaf whitefly; 1) do nothing, 2) contact insecticides, or 3) the insect growth regulator Knack. 1999 demonstrated to us that silverleaf whiteflies can significantly reduce yields. This pest is unpredictable, but failure to control damaging infestations can be very costly.
Contact Insecticides: Endosulfan (Thiodan/Phaser) or a tank-mix of the pyrethroids (Danitol or Capture) with Orthene, Ovasyn, or Provado can provide temporary relief. However, multiple applications will be needed since we will probably have continued migration and emergence of whiteflies in severely infested areas. Control is difficult. A starting point for a threshold would be 5-10 adults per leaf. To scout silverleaf whiteflies, gently turn leaves and count the number of whiteflies on the underside.
Insect Growth Regulator (Knack): Knack is the most consistent treatment for management of silverleaf whiteflies. It is also the most expensive. Knack has a long residual (several weeks) and is slow acting in general. When females feed on foliage treated with Knack, her eggs will be sterile. Knack will control immatures when they pupate (red-eye stage), thus established nymphs will continue feeding for several days. A starting point for a threshold when using Knack is 5 immatures found in an area the size of a nickel. For best results Knack must be used before whitefly infestations become severe.
Management decisions concerning silverleaf whitefly are difficult and must be made on a field to field basis. Consideration must be given to the length of time the crop has to mature, yield prospects, and the potential for numbers to increase. As we gain experience with this pests hopefully our decisions can be more direct.
When Is a Boll Relatively Safe from Insect Damage? (Roberts) A percentage of cotton in the state is approaching or at the stage at which a decision concerning terminating insecticide applications arises. These decisions were made on some dryland fields several weeks ago. Fortunately, the insects themselves often answer this question for us. Once plants cuttout and few squares remain on the plant, tobacco budworm and corn earworm moths will often look for more suitable host sites such as later planted or lush cotton. However, number and size of larvae must be considered when terminating corn earworm and tobacco budworm sprays. Will larvae be able to size on developing squares and move down the plant to feed on developing bolls? Historically, bollworm larvae have difficulty becoming established in fields when squaring rates drop below 100,000 per acre (approximately two to three squares per plant). In fields where bollworms do become established, we must be concerned about losing bolls already set on the plant. Until bolls are about 20 to 25 days old, they remain susceptible to bollworm damage. Damage is less likely to occur on these older bolls but they are not immune from bollworm damage. Control of fall armyworm may be needed for a more extended period of time. This is evidenced by damage observed to bolls low in the plant canopy during past years. Falls generally penetrate the softer basal area of bolls and thus are capable of damaging more mature fruit. Excessive defoliation by foliage feeders such as loopers and beet armyworm can affect a developing boll's ability to fill properly. Thus, foliage which is feeding these developing bolls must be protected until bolls are physiologically mature. Honeydew producing insects such as whiteflies and aphids must also be monitored and controlled if necessary until it is time to defoliate the crop. Treatments for these sucking pests may be needed if excessive honeydew is accumulating on open bolls. As stink bugs have become more of a concern, questions concerning termination of stink bug controls have arisen. At this point in time we have limited data. However, one year of research suggested that bolls which were 21 days from white bloom were not significantly impacted by stink bug feeding. We still need more data and at this point in time would be more conservative and suggest protecting bolls for 30 days past white bloom. To use these rules, we must identify the last harvestable boll which will significantly contribute to yield. This position will vary from field to field and thus these decisions will need to be made on a field by field basis.
Harvest Aid Formulation Changes and New Generic Products. (Brown) Over the past couple of seasons, Finish has gained acceptance in the harvest aid market. The initial formulation was a 4.0 lb/gal product, but last year Finish 6, a 6.0 lb/gal product, was introduced. The primary formulation in the marketplace this year is Finish 6. For the more concentrated product, 1.33 pints of Finish 6 provides the same active ingredient as 1.0 quart of the old formulation.
GRAMOXONE MAX, a 3.0 lb/gal concentrate, is now the primary paraquat formulation from Zeneca. It replaces Starfire, which was used at a variety rates to supplement weed and crop desiccation. To boost weed control with standard mixtures, GRAMOXONE MAX rates should be cut in half as compared to Starfire. Rates of 1 to 3 oz/A may provide some help for weed control in defoliant mixtures.
There are a couple of new generic entries in the arsenal of harvest aid products for the 2000 season. Both are from Griffin LLC and both are limited in quantities. Free Fall is a 50WP which contains the same active ingredient as Dropp, while Boa is a 2.5 lb/gal paraquat product.
December 13-14, 2000, Savannah, GA
January 9-13, 2001 Anaheim, CA
Steven M. Brown, Extension Agronomist-Cotton
A. Stanley Culpepper, Extension Agronomist-Weed Science
Glendon H. Harris, Jr., Extension Agronomist-Soils & Fertilizer
Bob Kemerait, Extension Plant Pathologist-Cotton
Phillip Roberts, Extension Entomologist-Cotton