Keep the greenhouse pest free by staying alert for signs of pests and diseases. This is the time of year when many come out of hiding and start to feast on your tomatoes, cucumbers and melons, turning them from healthy plants to sickly, mottled and struggling ones – so it's worth taking action early.
Use a yellow sticky trap as an early warning of trouble ahead: you'll often see greenhouse pests like whitefly on sticky traps long before you'd notice them on your plants.
Keep humidity high to discourage red spider mite. This nasty pest turns leaves mottled yellow – you'll also see their gossamer 'webs' on plants. Spray the floor and plant leaves to keep populations at bay.
Consider biological controls for infestations which are really taking a hold, using natural predators to keep pests at bay. Ask for advice from the staff at your favourite garden centre – they'll point you towards the right one for the pest you're trying to tackle.
Keep plants growing strongly and they'll shrug off most pests and diseases more easily. Water thoroughly (keeping the roots moist will ward off mildew) and feed weekly with a liquid seaweed or, when fruiting, tomato feed to keep them in tip-top condition.