I’m no gardening expert so I’d love to get some advice on how to get my lemon tree back in good shape. It is dropping leaves and you can see from this image that they look either diseased or like something is eating them. Any help is much appreciated!
It could be lemon scab. Get a copper spray to control this happening.
Citrus leaf curl, which this appears to be the start of from your pic, can be caused by a range of conditions, among them over- or under-watering, too high or too low temperatures and insect pests such as scale, mealy bugs and aphids. Pretty much a case of take your pick!
Check your watering - citrus need regular deep soakings so the roots are well moistened but not waterlogged. A 100mm deep mulch over the roots will help maintain moisture levels but keep the mulch well clear of the trunk - you don't need collar rot thrown into the mix!
Check also for insect pests, especially on the undersides of the leaves. A thorough application of white, eco or horticultural oil over upper and lower leaf surfaces and the bark may prove beneficial - it certainly won't cause any harm.
Consider your feeding regime too. Always use a citrus fertiliser that includes boosted magnesium and iron to assist leaf greening and strength.
Hope all this helps.
It's great to see you've already received some helpful advice from our members @Jackson. I trust @Noelle's expert response will have your lemon looking wonderful in no time. If you were looking for a fertiliser with boosted magnesium Scotts Osmocote Citrus & Fruit Controlled Release Fertiliser would be suitable, and here are some white, eco and horticultural oils.
Please let us know if you need further assistance or have questions.