From what I've gathered, and forgive me it's been a while, the spores float in the air and hit an emerging needle, the fungal spores respond to cuticle wax and stick there. They then wait until they get some water on them, after which they build a piercing needle and inject themselves into a foliar palisade cell. The fungus then invades one cell, while numbing any immune responses by gene silencing; a biochemical process that can dampen a response by making use of how genes are activated and translated. Once it gets a dose of sugars from the plant, it can continue to spread to about a millimeter of its surroundings in all directions. It then grows a fruiting body at the site where it first penetrated the foliage, which in turn matures and spreads new spores.
To my knowledge, there are no intermediate hosts like juniper rusts require to spread - which is also why I believe it's perfectly safe to keep infected junipers: the spread back and forth is as likely as a new infection coming from the intermediate host, and in spores that's just a numbers game: 0.000001% of infection is ten times higher than 0.0000001% but it's still a slim chance. If the intermediate host is downwind, that chance decreases another million fold.
Anyhow, the way to combat needle cast can be done in two general ways and two not-so-proven-to-be-effective ways:
1. Active treatment on the foliage, building a shield to provide protection. Oils like neem do not work for this, as it's the hydrocarbons in the cuticle wax they mix with, and that's what triggers the germination in part. So copper sulphate, peroxide, fungicides (that can be mixed in oils! The fungicides make them toxic to the fungi). This kills the fungus before it can enter the plant. We do these treatments at needle emergence and expansion.
2. Systemic treatment of the plant. I do this with a healthy microbiome and one specific unidentified fungal culture that seems to protect my plants from getting it - it produces something in the soil, some kind of antibiotic. Other people might prefer systemic fungicides, but those in turn tend to ward off any protective fungi in the soil. This kills the fungus after it has entered the plant, we do this from before needle emergence until mid summer or continuously.
(3. Application of microbial cultures like trichoderma viride, tichoderma harzarium, bacterial cultures: these should work in theory by attacking the spores when they germinate, by using a similar missile-puncture-approach that the fungus itself uses to infect the plant. These however are still much being researched and developed, they're not commonly regarded as useful in this case. See the part about biochemical arms race below.).
(4. Bio-fortifyers like silicium, silicon, DE, sulfur. In theory, they should strengthen the cell walls and prevent penetration. In practice fungi have evolved alongside plants all over the world, growing in conditions where these minerals and nutrients are plentiful, and they are generally unaffected by the use of these fortifyers because otherwise those fungi wouldn't exist anymore. That is the rule of the biochemical arms race: If plants would have won, the fungi wouldn't exist anymore, or if the fungi would have won, the plant wouldn't exist anymore.)
Since pines can infect themselves, it can be useful to remove parts of the needle below the affected band and removing it from the area. The needle part that's left on the tree will show some browning but will otherwise function as it did before.
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