Hi, our Hydrangea paniculata limelight is suddenly dead and dying, where others in same area are fine. Is it disease or another reason? Ps have been watering them. Warkworth.

Hi Trudy, it looks like your hydrangeas have a root rot disorder, quercifolia and paniculata types are susceptible to this. There are several reasons for this to happen, one is that the plant has dried out at some stage and then received a lot of water which causes the plant to take up too much water too quickly, the roots to burst, fungi in the soil enter through the burst roots and infect the plant. Unfortunately there is little that can be done once root rot diseases take hold. There are products such as trichoderma, which are beneficial soil fungi, that can be sprinkled in the soil and feed on the damaging soil fungi. Soil drenches are also available, talk to your local garden centre for a soil drench for diseases such as phytophthora, armillaria or pithium - common soil root rot fungi. Another cause is poorly drained soils, such as heavy clay soil, which tends to bake in summer and become waterlogged in winter or when watered. Unfortunately the plant looks too far gone to save, so focus on the one that is doing well. If you choose to try and treat the plant, cut the plant back to 50cm from ground level, remove the flowers and dead leaves, burn all plant material or dispose of in the rubbish, not the compost, to prevent the disease spreading, and treat with a soil drench, hopefully the plants can be revived. Clean all cutting tools after cutting back the diseased foliage as tools can also spread the disease. The problem with leaving the plant in the ground and treating it is that there is a risk of the disease spreading to your other healthy plants, so removal is probably the best solution to make sure the other plant does not become infected, dig out the soil that surrounds the infected plants and replace with fresh soil or potting mix. Lianne.