Mango has gooey/sticky sweat like underside of the leaves

[Lovetoplant]

[tastywaves28]

I couldnt find a direct answer anywhere. Everyone says its probably scales, but your leaves look pretty clean. maybe check the surrounding stems for tiny scales? if not I am not sure. https://www.houzz.com/discussions/4549780/mango-with-sticky-leaves

[mbmango]

Are there fine spiderwebs around?  I'm guessing spider mites, which are hard to see.  They really get out of control when I bring plants indoors.

[sapote]

They look like insects to me.

[Daintree]

Inside a greenhouse, with no signs of any insects, maybe it is just guttation, which IS basically plant "sweat".  I don't know about mango leaves, but my papaya, pepper, and most of my orchids guttate a lot when they take in more water than they need. 

It is a natural process. It is just excess water and minerals leaking out of active leaf hydathodes.
Plants can't transpire in high humidity, so they guttate to release the buildup of fluids.

If insects were causing this, you almost always see SOME sign of the pests - little body parts stuck to the fluid, webs, etc. I doubt it is aphids, since honeydew rarely ever forms actual droplets.

I spent a lot of time looking for non-existent pests when my papaya trees were doing this.
Temperate plants usually guttate at the leaf margins, but many tropical plants guttate on the entire underside of the leaves. 

Anyway, in the absence of pests, that is my guess.

Carolyn

[Lovetoplant]

Carolyn, thank you for your indepth analysis and suggestions.

 I tried to keep the RH high by misting 3X daily, but only use 1 circulating fan along with temperature control exhaust fan. 

I set the thermostat to turn on the exhaust fan at 95 which makes inside of the GH very hot and humid figuring that the young mango plants would love it.  They grow fast, but the underside of the about to mature leaves secreting sticky sweat-like substance  which you call it as "guttate"

I noticed that the plants directly got blown by the fan do not have this symptom.  They "transpire"  easier.

So for the past few days I added another circulating fan and turn off the misting system.  This  substantially helps drying up the underside of the leaves.  I believe that by adding a extra fan may has solved the problem. 

With your second opinion/explanation it really makes sense now.  Are the plants inside your GH still have this "guttation"

[TropicalFruitSeeker]

This is great information! my mango gets really sticky under the leaves as well. I spent weeks looking at them trying to spot bugs or anything but never found anything. I guess I'll try a fan too. Also, watering less helped.

[Lovetoplant]

This is great information! my mango gets really sticky under the leaves as well. I spent weeks looking at them trying to spot bugs or anything but never found anything. I guess I'll try a fan too. Also, watering less helped.

Yes, I am glad I figured this out. It bothered me for months. Also big thanks to Carolyn for the help of sharing her experiences with her plants.

Basically air movement thru plants ' canopies helps them transpire more efficiently.   

[Daintree]

Glad to help! And good idea to add more airflow. Might try that with my orchids too!

Carolyn

[Lovetoplant]

Glad to help! And good idea to add more airflow. Might try that with my orchids too!

Carolyn

Great to know that you are growing orchids. What varieties are you have growing?

 I built this GH especially for growing "Vanda" orchids since 1996.  It used to be 12x8 then my collections got bigger and I had to extend it another 12 feet.   The first 12 feet was built with a single wall, and the addition was a double wall.

At one point my Vanda collections was closed to 300 matured plants bloomed with beautiful flowers.  They were mostly  starting from tiny plants from inside the flask bottles.
 
Over 20plus years of growing orchids. Plus Between a restaurant business and a newly born child got me burned out.    Majorities of them died plus a few were given away leaving the GH emptied for a few years.

Now thanks to all members from this forum. They inspire me to grow my favorite fruits, mango. Now the GH is filled with mango seedlings with a few ready to graft this spring.

Let's get to your question.  The least expensive way to circulate air inside the GH is using box fan.  For the exhausted fan I use thermostat controlled fan as you can see on the very back of the second and third photo