Underwatering? Sunburn? Pests?

[maymcguffins]

If I may pick your brains…
I live in Houston, TX. The Chinese elm began developing a mustard yellow on a few leaves, eventually browning and falling off. It looks like the yellow starts bottom to mid-leaf then spreads. Temp has been high 80’s to low 90’s. It did fine when the temp climbed to 101. The leaves are sparse because in early June, it was hit with a bad scale infestation that necessitated a complete defoliation, but it recovered for the most part, save a couple bare branches. Thinking it might have been a pest, I used BioAdvanced 3-in-1 about 4 days ago. Any ideas?

Thanks!

 

[Deep Sea Diver]

Sure looks like a form of rust. Usually comes from leaves being too wet, especially poor ventilation during high humidity and watering the leaves before these can get dry at night.

There are a couple things one can do at this stage, but most of all do not water the foliage and keep the tree out of the rain. Give best ventilation possible.

Spray with copper fungicide every two weeks, top and bottom of the leaves. Can also use Macozeb, which is zinc based. Or alternate the two. Please wear appropriate safety equipment. See:

Copper SDS

Mancozeb SDS

Remove the worst infected leaves

There are other chemicals that can also be used, like Infuse. These are the safest.

Cheers
DSD sends