Texas Blacklands Cotton: Foliar Diseases and Premature Defoliation an Arising Issue

Cotton target spot.

GENERAL

A much-needed rain for cotton came this past weekend and the first part of this last week and should provide enough moisture to finish out the cotton crop.

The downside of the rain is prior to its arrival plants were shedding fruit due to depleting soil moisture and heat stress, but this recent round of rain will lead to the plant holding on to some of these late pollinated bolls which will bring some late season management options to ponder about as we approach harvest.

Foliar cotton diseases and premature defoliation are also an arising issue in some area cotton fields. The premature defoliation is a function of either foliar disease, Potassium deficiency, or the combination of the two. There are no cotton insect pest issues observed this week in fields in the scouting program.

We also need to start thinking about wheat planting, especially if you caught seed during harvest to plant this fall thanks to all the rain received once the crop was ready for harvest. The biggest issue we will likely see with caught wheat seed from this spring is reduced germination due to pre harvest sprouting.

COTTON

The area cotton crop received a much-needed rain to help finish out what appears to be a great crop for Central Texas. Insect issues are non-existent at this point as the aphids I was seeing the last couple of weeks are no longer being found.

The recent rains will lead to some late blooms to stay on the plant, and this may lead to some producers wanting to hold on and wait for these bolls to mature, but this may not be the best agronomic decision for multiple reasons. Some area cotton fields are starting to experience premature defoliation caused by foliar diseases like Target Spot and Stemphylium leaf spot, as well as Potassium deficiency.

There comes a point in the cotton growing season where you must no longer worry about setting and maturing more bolls on the top of the plant, but the delayed growth due to the excess moisture during the month of May is making the decision a little harder. In Central Texas we are not stressed to set bolls by a certain date to beat the annual first frost date like areas of west Texas, but we are stressed to get the crop out before the rainy season arrives.

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The recent rains will help the plant to hold some of these recently bloomed bolls but waiting for them to mature could hurt your crop’s lint quality and loan rate. It takes roughly 850 heat units for a cotton boll to go from white flower to open boll. If we accumulate 20 heat units per a day which is normal for this time of the year it would take roughly 34 days for the boll to mature.

If a boll was pollinated on Monday (8/16) would not be ready for harvest until around September 19th, add another 10 to 14 days for the defoliation process and the field will not be harvest ready until between Sept. 29th and Oct. 3rd.

Additionally, the bolls that are set this high on the plant are typically smaller and do not contribute much to yield, and the micronaire and other fiber qualities can also be low due to the interaction between the plant and environment and the environment alone with risk of late season rains staining cotton lint.

I am seeing areas of some fields across the area to be prematurely defoliated due to three different factors, 1) cotton root rot, 2) foliar disease, and 3) Potassium deficiency setting in. At this point in the growing season the premature defoliation will have little impact on the yield but may have some implications on the fiber quality as some of the younger bolls may not fully mature due to lack of photosynthetic area on the plant.

Foliar fungicides are rarely justified in Texas cotton, even in the Rio Grande Valley and Gulf Coast. Symptoms of target spot are irregularly shaped lesions with concentric rings. Target spot lesions can be found on multiple parts of the cotton plant including the leaf, bracts, and stems.

A second disease I am observing in some are cotton fields is Stemphylium leaf spot. This disease produces smaller lesions that are dark brown to black in color and do not have concentric rings. The center of Stemphylium leaf spot lesion also can fall out giving the leaf a shot hole appearance where target spot lesions stay intact.

Stemphylium leaf spot is typically associated with Potassium deficiency, where as target spot can infect healthy plants. Another distinguishing factor between target spot and Stemphylium leaf spot is where the infection starts, target spot will first be observed in the lower canopy and move up the plant and conditions remain favorable, and Stemphylium leaf spot infections can start in any portions of the crop canopy.

Another explanation for the premature defoliation of cotton plans is the onset of Potassium deficiency due to reduced uptake of Potassium from the soil causing the plant to move the Potassium stored in the leaves to the developing bolls. The deficiency and movement of Potassium from the leaf to the boll causes the leaves to develop a interveinal chlorosis that eventually progress to a gold and red color before the leaf falls off the plant.

Since Potassium is mobile in the plant the deficiency symptoms typically start in the lower canopy and moves up the plant as the deficiency increases. This late in the season the premature defoliation from Potassium deficiency will not have a major impact on yield but may cause some issues with lint quality.

The premature defoliation could be looked at somewhat of a good thing as it can be an indication of a good boll load, and the premature defoliation could make harvest preparation easier with less leaf tissue needing to be defoliated.

WHEAT

I know we all want to forget the nightmare of 2021 wheat harvest, but the excess rain and pre-harvest sprouting could have negative impacts on wheat stand establishment this fall if seed is being saved for planting. The pre-harvest sprouting, we saw earlier this spring will likely reduce the germination percentage and vigor of seeds planted.

The preharvest sprouting was caused by the continuous rains on harvest ready wheat, causing the seeds to break dormancy and start converting the starch to sugar leading the radicle and shoot starting to form. When the starch is started to be converted to glucose and then redried the seed loses potential energy it will need to germinate and be a vigorous growing plant.

It is highly recommended to conduct a germination test on saved seed that is going to be planted later this fall. I would also recommend calculating the number of seeds per pound, so you can properly adjust your seeding rates to obtain your desired plant stand.

Another test that is commonly conducted to test grain quality is a Falling Number Test, which is a measure of the number of seconds it takes a plunger to fall through a slurry of grain and water. Industry usually prefers this number to be around 300, and number below 275-250 is a cause for concern if wanting to plant this seed.

The Falling Number Test can only be conducted in a laboratory setting. This test does not need to be conducted to get an idea of if your caught seed is good to plant since germination percentage and vigor test can tell you how much of the seed will germinate and how vigorous the seeds will grow.

There are seed testing laboratories in every state in the United States, but the two most common seed labs I hear about are the Texas Department of Agriculture’s Giddings Seed Lab, and the Kansas Crop Improvement Association Seed Lab in Manhattan, Kansas.




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