Exercise 3 solution

Example answer (yours will differ):
Lots of leaves, few fruit: the plants may be too heavily fed with nitrogen-rich feed, or too shaded, or poorly pollinated. Move or prune back shading leaves, switch to a feed higher in potash (such as a tomato feed) and encourage pollinators with flowers nearby (check).
Huge marrows: the fruit were left too long. Pick courgettes small, every few days, so the plant keeps producing. Cut the marrows off now and eat or compost them.
White powder: powdery mildew, common in late summer, helped by dry roots and crowded leaves. Remove the worst leaves, water the soil well at the base (not the leaves), mulch to hold moisture, and space plants properly next time.