The Geocarto International Centre in Hong Kong wrote an article in the Aug 2018 Arborist News ("An Integrated Approach to Tree Stress Monitoring"), as a follow-up to the article they published in Apr 2017 ("A New Technology Warns of Stressed Trees").

I wrote a summary on the Apr 2017 article here: http://openplants.blogspot.com/2017/05/monitoring-at-risk-trees-using-spectral.html

In the Aug 2018 article, as mentioned before, the spectral reflectance data were collected from satellites to observe the following in trees:

• Drop in near-infrared band
• Drop/shift in red-edge band
• Rise in red band, which means a drop in chlorophyll absorption

So, how much drop or rise would warrent concern about the tree's health? The article went on to show the spectral reflectance curves in different trees collected within a 4 to 6 years period. Keywords here being "retrospective study" [emphasis mine], as the author would mention later.

 Arborist News, Aug 2018, page 29

Arborist News, Aug 2018, page 30

The author highlighted a case where a 400 years old Ficus benjamina in Kowloon Park, Hong Kong deteriorated due to park redevelopment, branch failure due to typhoon and brown root rot. A treatment of Trichoderma was applied presumably after Sep 2013 when its removal was withdrawn. The spectral reflectance chart below reflected the claim that the tree improved in its health.

Arborist News, Aug 2018, page 31

 According to this chart:

• Rise in near-infrared band <- Indicates better leaf cellular structure
• Rise and fall in red-edge band <- Indicates rise in chlorophyll absorption in 2011, dropped in the subsequent years (coincide with application of Trichoderma) and rised again in 2017
• Not much difference in red band <- Indicates no difference in chlorophyll absorption

In my opinion, the chart does not indicate that the Trichoderma treatment has definitely suppressed the brown rot fungus. And while the tree's physiological health seemed to be improving, it structural condition is uncertain due to the redevelopment, wind-throw (typhoon), and anything prior to its decline.