Situation. The crop is still all over the board in terms of growth stage and condition. Some fields are well past mid-bloom while others are just about to flower. A very few areas might be water logged, but a wet August would improve the potential of most. It’s prime time to be alert for stink bugs and escaping worms.
What seems to be stress-induced potassium (K) problems are widespread. Deficiencies express as leaf discoloration, marginal leaf decline and burn, and secondary leaf spots. If drought/heat stress occur during vegetative growth, K uptake can be restricted even if soil levels are adequate to high. So this can occur even in fields with a good fertility program. Symptoms include leaf reddening, chlorosis, and curling. Leaf spot infections from Stemphylium, Alernaria, and Cercospora are often associated with K-related leaf decline. Because the K-deficient leaf loses health and vigor, these fungi establish and cause a variety of leaf spots as secondary invaders. These specific fungi are not the primary cause of leaf decline. Foliar K applications may sustain the crop a few days longer and be a valuable contribution to yield, but since such treatments provide only limited K, they are meant to supplement an overall program.
Growers know more about their own fields and management programs (fertility, water, etc.) than anyone and thus have the best gauge of expected canopy growth. Such knowledge helps adjust plant growth regulator rates and timing. Prevailing hot conditions compel a conservative approach; conversely, good moisture, strong fertility, and aggressive varieties require a more aggressive approach. (Brown)
Weed Management. Due to prolong drought conditions this summer, many cotton fields have significant Palmer pigweed problem due to slow cotton growth and high water use efficiency of pigweed. The best treatment for this situation is Roundup Powermax 32 oz + Xtendimax 22 oz or Engenia 12.8 oz + EPA-approved DRA, followed by Liberty 29 to 42 oz + Dual Magnum 16 oz within 7 days from the first application. Addition of liquid AMS may enhance Liberty performance on big pigweeds, sicklepod, and other big annual broadleaf weeds and does not burn cotton. Use 42 oz of Liberty on large pigweed escapes that are over 12 inches tall.
Include Dual Magnum with the first application if rainfall is expected within a day or two. If large pigweeds survive sequential applications, either pull them by hand or spot spray with 25 percent v/v Gramoxone (wear proper protective equipment), unless cotton receives adequate rainfall or irrigation and will quickly canopy to shade out these remaining injured pigweeds. Remember, dicamba cannot be sprayed beyond 60 days after planting or mid bloom stage in cotton, even with a hooded sprayer.
PRE and POST treatments were evaluated for teaweed (arrowleaf or prickly sida) control this summer. No singular PRE worked well -- they provided only 50 to 70 percent suppression. Cotoran 2pt + Brake 24 oz was the best combination and provided close to 90 percent control at 4 weeks after planting. Liberty 32 oz + Staple LX 3 oz was the best POST treatment, consistently delivering over 90 percent control of 6 to 12-inch tall teaweed. Roundup PM 32 oz + Xtendimax 22 oz + Staple 3 oz also provided about 90 percent control. Roundup PM 32 oz + Xtendimax 22 oz provided 85 to 90 percent control, while Liberty + Dual magnum delivered only 80 to 85 percent control. The addition of Staple LX with Roundup or Liberty also increased the control of large sicklepod and coffeeweed (I have seen some residual control of these two weeds from Staple LX). After the second POST of Liberty + Dual Magnum or Roundup PM + Xtendimax, nearly all plots were above 90 percent control. Two applications of Liberty + Dual Magnum was the worst treatment (only 80 to 85 percent control). More research will be conducted on teaweed next year.
In fields with heavy pigweed and morningglory infestations, make a layby application with Liberty 29 oz + Cotoran 2 pt and/or Valor 3 oz + 1 percent v/v crop oil. If nutsedge, tropical spiderwort, and grass are the main problems, use Roundup 32 oz/A + Dual Magnum 1.33 pt + MSMA 2 pt. Addition of 2 to 3 oz of Valor improves tropical spiderwort control significantly. Leaving escaped, tall pigweed to defoliation is fine since their seeds do not shed easily, but they should be removed (pulled) before harvest. Never run a cotton picker or grain combine over mature pigweeds.
Cotton Insect Control for the Remainder of 2019 Season. In most seasons, we would be entering the home stretch for cotton insect control by early August. While this may be true for fields planted on time in 2019, it is not the case for a lot of late planted, late emerging, late-maturing fields this season. I hope we can narrow our focus to just a few species of insects for the remainder of this season. The primary group would be the bug complex (plant bugs and stink bugs) and escaped bollworms on 2-gene Bt cotton. However, there are several other insects that could arise if weather conditions or other circumstances permit. These are spider mites, late season aphids or even silverleaf whiteflies.
The insects of focus for the remainder of the 2019 season may not be as concerning as how long our management and control programs should extend into September and even October on our later maturing cotton. Let’s look at some general guidelines. If past seasons give us any trends, our escape bollworm issues should end by Labor Day. However, we should continue our monitoring as long as our late maturing fields have squares in the top of plants that would serve as a food source for a one-day old bollworm. Once all the squares are gone, escape bollworms will have a difficult time becoming established. The bug complex should eventually be dominated by stink bugs here in Alabama. This may consist of several species: brown, southern green, and brown marmorated (BMSB), the latter which can now be found in many cotton growing counties. How long should we continue stink bug controls on late maturing cotton? Our general rule with our traditional stink bug species is to continue controls until the top bolls we hope to harvest are about 25 days old. When the BMSB is in the mix, we may need to protect even longer since this species will attack bolls from thumb nail size, up until they begin cracking. With our late maturing fields this season, we will need stink bug controls through at least the month of October. An application for stink bugs usually gives 10 to 14 days of boll protection. However, as some fields mature out, just like with corn, stink bugs will move to younger cotton, or swag areas of fields that are still producing bolls or to late maturing soybeans. In other words, as our crops mature in September and October, we will get field-to- field and crop-to-crop movement of stink bugs. Insecticide such as Bidrin, bifenthrin, or any other pyrethroid at a mid to high labelled rate should give adequate control. The best way to scout for stink bugs will be to examine bolls for internal injury. Just observing or using a sweep net or drop cloth for stink bugs is not very effective and often leads to underestimating the number present. (Smith)
Red Banded Stink Bugs. Following a mild winter, red-banded stink bugs (RBSB) are a soybean pest that we should always have on our mind. Last Tuesday, 7/16/19, we found treatment level of adult RBSB at the Prattville experiment station. We found these in relatively early planted soybeans at R4/R5, and although we don’t have very large plots, we were exceeding 5 RBSB on 10 sweeps. That works out to about 3 times threshold and is a cause for concern for soybean growers in Alabama. This is a pest that we haven’t dealt with as much as the mid-south growers in Arkansas, Louisiana, and Mississippi, but they’ve put out some excellent info for control tactics that we can draw from.
RBSBs are a tropical insect and behave and look different in many ways to the native stink bug pests we often deal with in soybeans, and the control tactics differ as well. RBSB are harder to capture with sweep net samples and time of day can have a large impact on the number you catch in the same field. In our plots in Prattville we caught 3 times threshold on 7/16 and 7/22 on warm mornings when the dew was still wetting the plants, but caught 0 on 7/18 during a hot mid-day. These stink bugs will drop off the plant very quickly when disturbed, and it may be hard to sample them in the heat of the day when they can evade capture or even on very cool mornings when they may be seeking shelter from the cooler temperatures.
RBSBs should be controlled when populations average 4 adults or nymphs per 25 sweeps through R6.5, 10 adults or nymphs from R6.5 to R7, and should only be controlled after R7 if weather conditions are unfavorable. Cool and wet conditions can make mature soybeans more susceptible to feeding from RBSB, but if conditions will be favorable, no control is necessary after R7. RBSB are harder to control than our normal soybean stink bug complex and generally require a tank-mix to get adequate control and residual to keep them knocked back. Bifenthrin at 5.8–6.4oz with a half-pound of acephate is one of the most reliable control options. Bifenthrin can also be tank-mixed with imidacloprid at 3oz or Belay at 4oz and provide reliable control. Endigo at 4–4.5oz is also a good control option, but may not be available for many producers.
Adults are identified by their cigar shape when compared to normal stink bugs, the red band behind their head, and the clear/white spine extended forward between the legs as picture on the left.
Don’t Shed on Me. Several in North Alabama have reported a large number of squares and small (two day old) bolls being shed following large amounts of rainfall last week. Although no one enjoys seeing this, it is not surprising considering recent conditions of the cotton crop and environment. This is the lightest plant bug year I can remember and much of the crop had 90% or greater fruit retention up to this point. Cotton will seldom hold 90% of its fruit even under ideal conditions and it is not uncommon for a cotton plant to shed 40-60% of its fruit under normal growing conditions, especially as a good boll set advances towards cutout.
Fruit shed can be caused by a host of factors such as insects, nutrient deficiencies, environmental conditions, and pathogens. As mentioned before, environmental conditions are likely the cause for this in most situations over the last week. Carbohydrates or photosynthates accumulated in the leaves are responsible for “feeding” developing bolls. Environmental stresses reduce the plants ability to meet carbohydrate demands for these young developing bolls. Cloudy conditions and lack of sunlight which have accompanied recent rains can reduce photosynthate production and result in fruit shed. Cotton under a heavy boll load may not be able to produce enough photosynthates to meet the demand of its existing fruit load under these conditions and gives preference to older more mature bolls and sheds the youngest fruit. When this is coupled with high temperatures and humidity, this places more stress on the plant and can escalate the situation. As soils become waterlogged, photosynthates in cotton plants are reduced due to lack of oxygen which will also result in fruit shed. Many fields were extremely dry leading up to last week’s excessive rains. When prolonged drought stress is incurred and followed by a rainfall or irrigation event, fruit shedding often occurs due to the previously mentioned stresses and factors.
Fruit shed may be worse where cotton is rank, seeding rates are too high, and excessive nitrogen was applied. This is due to lack of sunlight penetration into the canopy. Shading of lower subtending leaves (leaves closest to the fruit) will reduce photosynthate production and can cause shedding. Several have noted that they have observed greater fruit shed on 30 inch row cotton versus wider row and skip row cotton. This stands to reason do to the tighter canopy and shading created by narrower rows.
On a positive note, much of the crop still has a more than adequate boll load and yield potential remains high despite what may seem like excessive fruit shed.
Boron Fertility Management. Boron (B) is an essential micronutrient for plant growth and plays a critical role in cell wall development, cell membrane function, and root elongation. Boron deficiency symptoms in cotton include stunted terminal growth, abnormally-shaped leaves, and aborted flowers. Less frequently, B deficiency can cause dark rings to appear around cotton petioles. If B remains deficient throughout boll fill, deformed or dark-colored bolls may be observed.
The Alabama Cooperative Extension System (ACES) recommends 0.3 pounds of B per acre as a soil or foliar-applied fertilizer for cotton production. If applying to the soil, apply just prior to planting or at plant. If applying as a foliar spray, it is best to apply within the first four to six weeks of bloom. Foliar sprays can be applied with growth regulators or insecticide sprays. Be aware that many products on the market today supply B at very low rates (10 to 100 times less than ACES recommendations) at their recommended rates. Research trials in the Southeast have shown that these products to not supply more boron to the plant than untreated controls. Producers should ensure that the recommended 0.3 pounds of B per acre is applies regardless of the product used, and products which do not supply this amount of boron are not recommended.
If B deficiency is suspected, a leaf tissue test can be used for verification. To collect a plant tissue sample, collect leaves (youngest fully mature leaves) from 25 to 30 randomly-selected cotton plants within the affected area. The optimum boron concentration for cotton leaf tissue analysis is 20 to 60 ppm. If boron is lower than 20 ppm, a foliar application of boron may be used to correct the deficiency during until the eight week of bloom. Later applications of boron will likely not be plant-available. (Gamble)
This issue contributors:
Dr. Aaron Cato, Post-Doctoral Fellow, Extension Entomologist
Dr. Audrey Gamble, Extension Soil Scientist
Dr. Steve Li, Extension Weed Scientist
Dr. Ron Smith, Extension Entomologist, Professor Emeritus
Tyler Sandlin, Extension Agronomist
Dr. Steve M. Brown, Extension Agronomist
