Verticillium is a fungi that gets into the xylem (wood) and its effect is to clog it up, stopping the flow of water and minerals upward. Its manifestation is that the tree will leaf out normally and then suddenly the leaves will wither and dry. The dry leaves remain attached to the tree. The only other exterior signs are an area of bark damage near the ground where the spores splashed up from the soil and through which it gained entry into the tree. Verticillium is soil borne and doesn't grow at temperatures much above, so it manifests itself primarily in spring.
The damage in your pix looks like winter sun/wind burn. In dry sunny climates the bark on the sunny/windward side gets desiccated and the cambium underneath the bark dies leaving what you now see as the brown coloration (it can be darker for a while sometimes). The yellowish halo is just an area of thin or weak cambium. So, think back to its location over winter and see it this makes sense. If this is indeed the problem, there is nothing you can do but let the tree grow. It will get uglier as the cambium around the edges grows. It will form a thickening lip over the course of the next several years before it finally closes (aka 'heals over'). It is still okay for prospective bonsai use, this may prove to be an interesting feature or your bonsai composition uses a front where this damage is to the back of the view and not seen.
Thank you! I was afraid of verticillium.
I'm in Buenos Aires Argentina.. now is autum, here is very humid but we usually have our maples with sun burnt.
I'm waiting for winter to repot it in a better soil (that one is awfull).
I also have this trident, what do you think about it's leaves?
Please put your location in your BNut profile. Also add that it is a USDA zone 9b.
Regarding the leaves, all maple leaves get a bit tattered near the end of the season. All I see in your pix is the tips of some leaves have browned. A bit of a breeze and maybe some bright sun draws a lot of water out of the leaves. The tips are at the end of a long pipeline from the roots, up the trunk, up the petiole and leaf vein and will run low on water first --> most vulnerable to being 'burnt'. I don't see any indication of a pathogen.
My quick take is that you may need a sun shade over your trees, just to reduce the intensity of the sunlight. It may only be needed around the time of the summer solstice when sunlight is most intense.
The USDA zones may be getting to be out of date. The zone really only reflects how cold it might get - it is an index of winter hardiness. Some tropical species cannot survive freezing temperatures (below 34F/0C). When water in the living cells freezes, the ice crystals puncture the cell membranes causing large numbers of cells and even the whole plant to die. Temperate species are able to sugar-up the water in cells allowing them to survive to lower temperatures. Since all it takes is one time, the USDA hardiness zone just reflects whether a species can survive through the winter.
Many temperate species have bud chilling requirements - a need for an accumulated number of hours below 40F/5C. Obviously we would expect serious problems keeping these plants in USDA zone 11 or higher because it never gets below 40F/5C. Further, we might infer that zone 10 would be problematic. Even though it gets to 5C, we might suspect that it is not for a long time - in other words, growing p. parviflora might not work out in a USDA zone 10.
This is pretty much how we use the USDA zone here at BNut. But, you are right, it doesn't give the whole story. I live in USDA zone 8b. Brian Van Fleet, in Birmingham Alabama is in an almost identical USDA zone 8a, but summers are very very different. It rarely gets much above 72F/22C here (in Anacortes) whereas it is much hotter in the summer in Birmingham. So he can grow pretty much the same trees as I can, just his grow a whole lot faster than mine. The American Horticultural Society has mapped the US into heat zones to reflect this. I'm in AHS heat zone 1 with no days in the average year hotter than 86F/30C. Birmingham, on the other hand is in AHS heat zone 8 having 91 to 120 days with temperatures above 86F/30C. San Francisco is a USDA hardiness zone 10 and an AHS heat zone 1, as another example.
I looked up your location and can find that it is USDA zone 9b, but I cannot find anything about its heat zone directly. I can only tell that your climate is somewhere in between mine and Brian Van Fleets without some serious research because the AHS heat zones are not mapped outside of the US like the USDA zones are. I can readily find that you are located about 34 degrees south which means you experience the same summer solar intensities as Birmingham, AL which is at 33.5 north. Me, I'm 48.5 N.
Long story short, USDA zone gives some of us a quick clue about your climate and whether it might pertain to problems you are experiencing. That is all, I think.
Personally, I have no trident maples, but what I see in your pix doesn't worry me. It looks like normal wear and tear from a the course of a growing season.
Fungicides like Daconil and lime sulfur are sprays and only work as preventives. The standard routine with fungal infections is to remove the clearly infected leaves and stems and then spray. The infected tissues are just a source of sprores = further infection. Spraying then kills any spores that are laying around. Daconil and the like leave a residue that remains active for several weeks or until it is washed off by rain. Different fungicides are effective against different fungi, so some people often alternate between spraying one, say Daconil, and then another, say Phyton, to widen the spectrum of protection. Possibly you should do likewise if you have frequent fungal problems.
Systemics, such as Aliette may cure an existing fungal infection as well as being prophylactic. I've recently seen Aliette recommended for maples, aside from the phytophthora that it was 'designed' for. I am lucky to have only rarely had fungal problems with my trees. I do the standard routine of removing the effected tissue and spraying, but I spray a 300 ppm to 900 ppm solution of hydrogen peroxide instead. Peroxide is an antiseptic, killing bacteria as well as fungal spores. After releasing the reactive oxygen, peroxide turns into just water. So I have no worries about accumulations of copper or what not in my small garden area. I just prefer it, despite the fact that most fungicides, like Daconil have low toxicity.