General Thoughts On Defoliation As Conditions Change
DR. TYSON RAPER
JACKSON, TENNESSEE
I’ve had a flurry of calls wondering what should change as we move into slightly cooler and *possibly* wetter conditions. I’ve also had several questions about why we’ve seen different responses to our concoctions this year than in years past. In this short blog, I list a few points to keep in mind.
One shot or two?
Many have called to ask about my concoction for a single shot. After, they ask about my concoction for my first shot in a planned double shot approach. My single shot and my first shot in a planned two shot approach are identical. Why, you might ask?
It helps to understand that two things generally cause us to have to apply twice. The first is poor coverage; we often cannot coat the lower distal fruiting positions, subtending leaves and mainstem leaves (skirt) on a lush plant. The second is slow development of abscission zones and/or sutures. Sometimes this is due to us applying prematurely to immature positions. Other times (like this year) it is because the plant is extremely stressed and the physiological processes causing boll opening and leaf shed are simply taking longer. Applying a second application- particularly on stubborn bolls- can make a big difference in rate of opening.
The goal of my first shot in a planned single and planned double shot approach is the same; I want to open all bolls and remove all leaves as cheaply as possible. Hopefully (and this has been happening quite a bit this year) the second shot of the planned double shot doesn’t get applied.
Boll opening
This also falls into the conversation about one or two shots, but in my experience, there is little use in dumping a tremendous amount (32+oz) of ethephon in a single shot. On a lush plant, coverage is the issue, not rate, so upping the rate generally doesn’t generate acceptable results. Instead, it is better to run a 24 oz shot initially, then follow it (if necessary) with a second shot of 18 oz. Again, in my experience, coating a boll once with an excessive amount of ethephon is not as effective as coating a boll twice with a reasonable amount of ethephon.
Regrowth
For most fields, significant rainfall will generate a large flush of regrowth at all axillary buds, likely beginning at the base of the plant. The materials we have that prevent regrowth (thidiazuron, cyclanilide) will struggle to prevent regrowth this year, given the amount of residual fertilizer present.
What can we do about it?
- Attempt to time the picker approximately 14 days after the initial application. If it looks like you may be 21 days out, delay the initial application.
- Thidiazuron is very good at removing juvenile growth- this includes regrowth- but coverage and temperatures are critical. Use a nozzle that generates small droplet sizes and 15 GPA+.
- If temperatures slip below 60F at night, understand the activity of thidiazuron drops considerably. I’ll be keeping thidiazuron in the mix if we only have one or two nights below 60F, but if we have four or five days after the application below 60F, I’m pulling thidiazuron from the mix and saving that money.
- After pulling thidiazuron from the mix, we will have to significantly increase the rate of Folex for it to cause shedding of juvenile growth.
- A one shot has been working great, but moving forward, if you see basal (at the base of the plant) regrowth, a second shot will likely be required.
Folex
Many have called with concerns using tribufos (active in Folex) in the first shot when conditions are warm for fear of sticking leaves. Instead, they were interested in a PPO option. PPOs generally work much faster than tribufos. Subsequently, PPOs are never in my first shot concoctions. This is particularly true in a drought year, as the abscission zone develops slowly in an unhealthy plant; a PPO in the first shot has a good chance of killing the leaf before abscission zone develops.
I don’t stick leaves with tribufos. Many may recall our old standard defoliation mix was a pint of Folex and a pint of Ethephon. I’ve previously walked fields where an applicator applied 24 ounces of Folex instead of 24 ounces of Ethephon, and even then, it didn’t stick leaves!
What rate of Folex should be applied?
- Since these plants are tough, I’ve included 4 oz of Folex in temperatures we usually wouldn’t think we would need Folex.
- As temperatures slide into the mid 80s with night temperatures near 60F, I’m increasing the Folex rate in my mix to 6 oz.
- If temperatures continue to slide closer to 55F with daytime temperatures below 80F, I’m pulling thidiazuron out of the mix completely and my Folex rate will jump to 8 oz.
Rainfall
Rainfastness varies by product. Thidiazuron (24hr) generally needs considerably more time than ethephon (6hr), which needs more time than tribufos (1hr). I don’t like applying in front of a decent chance for rain, as rainfall in a cracking boll can promote boll rot and or hard lock.
My concoction based on our forecast as of 9/22
As soon as we dry back out (Friday?), I’m running 2 oz Thidiazuron, 6 oz Folex, 12 oz Finish and 12 oz Ethephon. I’ll update this through this coming Thursday (9/25) based on changes in the forecast.
**Thursday update- fields are wet and night temps are low over the next couple of days. The earliest I would start back spraying would be Saturday. By Monday, night temperatures will again be high enough to generate acceptable activity with Thidiazuron. ∆
DR. TYSON RAPER
UNIVERSITY OF TENNESSEE