Hi @Abigail,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Magnolias, especially when grown in pots, will occasionally shed older leaves as part of their natural cycle, particularly during seasonal changes or after feeding. The yellowing you’re seeing on your magnolia leaves looks quite typical of mild nutrient stress or natural leaf ageing. The yellowing between the veins, while newer growth remains green, suggests it’s not a major deficiency. Since your plant has plenty of healthy new growth, I wouldn't be too concerned.
The fertiliser for acid-loving plants you used is suitable, and applying Seasol two days later shouldn't have caused any harm. However, it’s best to space fertiliser and tonic applications by about two weeks in future to give the roots time to absorb nutrients without overlap.
The insect in your photo looks like a small fly, likely a fungus gnat or a harmless visitor rather than a pest that damages leaves. These insects are often attracted to damp potting mix rather than the plant itself. If you notice a few hovering around the soil, let the top couple of centimetres dry out between waterings, as that discourages them from breeding.
The dark, round patch on the leaf appears to be a minor fungal or bacterial spot rather than pest activity. It’s quite common on magnolias, especially when moisture sits on the leaves or after physical damage. You can safely remove any affected leaves to prevent spread and improve the plant’s appearance. Ensuring good airflow around the foliage will also help.
Overall, your magnolia looks healthy and well cared for. Remove the yellow or spotted leaves, continue watering consistently but avoid overwatering, and hold off on fertilising again for a couple of weeks. Keep an eye on new growth, as long as it remains green and strong; the plant is thriving. This minor yellowing is normal and should resolve as the plant continues its seasonal growth.
Keep us updated. If anything changes and you are concerned, don't hesitate to reach out.
Jacob
