Volume XXVII No. 17 July 22, 2002 GENERAL SITUATION: Most of the cotton producing areas of the LRGV received little rain this week. Areas west of McAllen had as much as 3.5 inches of rain. But, not much cotton has been grown there in many years. Otherwise, the Valley did get some much needed water behind Falcon Dam and that is certainly positive news. Cotton harvesting was increasing slightly and grain sorghum and corn harvesting were near completion. The leaf killing problem mentioned in the last issue of Pest Cast had some light shed on the issue this week. Several people have put forth explanations for the problem, at least in part. Charles Stichler, Extension Agronomist-from Uvalde and Dr. Tom Isakeit, Extension Plant Pathologist-College Station, have both visited several field sites in the LRGV within the last two weeks and have provided some insight into the possible issues at hand: Pre- mature Senescence-Charles Stichler When cotton matures, there is a hormonal shift in the plant and leaves. As leaves age, they finally reach the stage of senescence, or maturity, when they naturally fall off the plant. Some cotton throughout the region has shown evidence of either a foliar "disease" or malady, that is prematurely defoliating cotton, or nutrient deficiencies. Samples of leaves from the Coastal Bend region was sent to plant pathologists to determine if there was a "new" disease of cotton that we needed to be concerned about. There were several fungi isolated on leaves, but none are listed as primary pathogens on cotton. The condition seems to be a combination of mature cotton, generally with heavy boll loads and stress from either too little moisture and or nutrients, followed by the recent rains and high humidity which promote fungal growth. There have been almost two weeks of cloudy high humidity and rainy weather in the regions most affected. In the Rio Grande Valley we have seen less of the "disease" than in the Corpus Christi and Upper Gulf Coast Region. The irrigated cotton was generally irrigated late enough to keep the plant growing and the dryland cotton in the eastern part of the Valley, was stressed early and has also received late rains which rejuvenated the plant. The problem seems to show up on cotton that is close to opening, or has already begun to open. It is associated with cotton that was in a stressed stage just prior to the recent rains. The leaves were beginning to mature, (senesce) as the plant went into cutout. In addition, on the Upper Coast, the malady seems to be associated with fields that are low in potassium. Potassium deficiency shows up in the cotton plant with red leaves in the upper portion of the canopy. Earlier it was thought that some varieties were more susceptible than other varieties, but this has not been shown to be the case in most situations. At this point, we are still trying to determining the condition and cause. We do not expect this to be a "new disease" that growers need to become overly concerned with at this time. Investigations will continue to determine its cause and prevention. With better growing conditions in another year, we may not see the "disease" or malady again.

Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomic level, race, color, sex, religion, disability or national origin.
The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.

PEST CAST continued

Premature Defoliation of Cotton-Dr. Tom Isakeit

I recently examined several cotton fields in Hidalgo county, including John Norman's variety trial at the TAMU - Hiler Farm, to determine why leaves had abnormal coloration and were defoliating. These symptoms did not fit the pattern of any known foliar pathogen of cotton. Some of the leaves had circular lesions that resembled those caused by the fungus, Alternaria. I am doing laboratory tests to determine whether fungi are causing these lesions. However, Alternaria is a weak pathogen of cotton and, in decades of observations, it has never been reported as a problem on cotton in the Lower Rio Grande Valley. From what I've seen so far, I feel that the problem in the LRGV is not caused by infectious organisms. I do not know the cause, but I feel it is an environmental stress that exerts a stronger influence on plants with a heavier boll load.

The foliar symptoms of this phenomenon resemble potassium deficiency in many fields. This does not necessarily mean that there is a deficiency in the soil. If tissue analysis does indicate that there is a deficiency, the problem could be the result of impaired uptake by plant roots. For example, high populations of the reniform nematode feeding on cotton roots can sometimes cause foliar symptoms of potassium deficiency. (I am quite sure that the current situation is not a reniform nematode infestation, since I did not see the stunting pattern that consistently occurs with it).

The cotton problem in the LRGV looks quite different from what I saw on July 5 in Wharton county. In the LRGV, defoliation was noticeable, but it was not as severe as in Wharton county. There are some fields in Wharton county that will have a yield reduction, but this does not appear to be the case in the LRGV. In Wharton county, the defoliation seemed to be associated with certain varieties. However, I would not feel one could draw a certain conclusion from this, unless several varieties were planted in the same field. This is the case in John Norman's variety trial, where all the varieties were affected to some degree and the most severe defoliation was first seen with the earliest maturing varieties, but progressed to later-maturing varieties.

I am continuing to gather information about the occurrence of premature defoliation in different parts of the state. I am not close to understanding the cause, but I can be confident that pathogens are eliminated as a major cause of the problem.

Boll Weevils in Large Numbers

Boll weevils continued to show increases in many fields across the Valley this week. Many fields which had few blooms left had weevil numbers in the teens inside some blooms. Damaged square counts were as high as 100 percent in a few fields. Many fields which had any squares left were being punctured at very high rates. Late maturing fields will need large amounts of insecticide to prevent loss of small bolls. Blooms at point in the season still will need 45 days to be open bolls.

That will be too late to get the field defoliated and harvested before September 1, the stalk destruction deadline. So, only bolls which are less than 18 days old need protection from weevils for another week or so.

Other Pests Light

No bollworms, beet armyworms or other larval pests were reported this week. Silverleaf whiteflies and aphids were in most fields, but not at economic levels in most.

Cotton Harvest Aide Study Results

Attached are two tables which include results of the cotton defoliation (harvest aide) trial and costs of each treatment. Defoliants were applied to a test field at the Texas Agricultural Experiment Station Annex farm north of Weslaco on July 10. The results were measured on July 18. The cotton defoliated was a small field of Stoneville 4892BR planted on February 20, 2002 and which was irrigated one time. The plant height ranged from 3 to 3.5 feet. A large boll load was present on the plants. If anyone desires to see the plots, they will be maintained for about a week or so more before harvesting commences. The plots are located north of Weslaco. Follow FM 1015 north of Expressway 83 to the intersection of Mile 10 North. Turn right (east) on Mile 10 North and go approximately ½ mile. There a fence ends. Turn off of Mile 10 North and drive along the drain ditch until you begin to see the defoliated plots. Orange flags with black numbers will mark each plot. You can use the attached tables which show the treatments numbers to guide you through the plots.

THE INFORMATION GIVEN HEREIN IS FOR EDUCATIONAL PURPOSES ONLY. REFERENCES TO COMMERCIAL PRODUCTS OR TRADE NAMES ARE MADE WITH THE UNDERSTANDING THAT NO DISCRIMINATION IS INTENDED AND NO ENDORSEMENT BY THE COOPERATIVE EXTENSION SERVICE IS IMPLIED.

Posted at this web site by:
texagnet Internet Services.