My apple tree in the garden has had these weird brown spots on the leaves for about three weeks now. The spots are getting bigger and some leaves are already falling off. Could this be a fungus or more likely a pest? How can I treat it without immediately resorting to chemicals?
Sounds pretty much like scab or powdery mildew, both fungal diseases that are super common on apple trees. Scab typically creates those brown, kind of irregular spots and spreads quickly, especially when it's damp and warm. Powdery mildew looks more like a gray coating, but scab also fits your description with the falling leaves. Pests would rather eat holes or leave webs behind, so my guess leans more toward a fungus.
What you can do without chemicals: First, remove all the affected leaves and any leaves lying on the ground and throw them away - not in the compost, that just spreads the spores. After that, spray regularly with water, but down at the plant, not on top of the leaves, because wet leaves just feed the fungus even more. Nettle or horsetail broth supposedly helps too, you just gotta try it out. Then in the fall when you prune, thin out the crown a bit so air can circulate better.
Plan ahead for next year: In spring already spray preventively with neem oil or sulfur lime before it starts again. Some people swear by replacing the top layer of soil in winter too, because the fungal spores overwinter there. If it still gets bad next year anyway, maybe think about more resistant varieties, but I'd try these methods first.
When brown spots get larger and lead to leaf drop, timing matters a lot - three weeks fits perfectly with apple scab, which really takes off after humid spells in summer. The thing is, you should already be collecting the affected leaves now before they hit the ground, because the spores settle in there for next year. Air circulation helps a ton - if your tree's growing too dense, you can carefully thin it out a bit. For this year you probably don't need to do much, but next spring I'd use nettle slurry or copper fungicide (yeah, that's still pretty natural) preventively as soon as the first leaves come out.
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