Botrytis and Pythium can cause root rot and blight on seedlings in the greenhouse. These diseases are promoted by wet conditions and can be prevented through good sanitation and ventilation. Botrytis can damage flowers and foliage outdoors during extended periods of wet weather. Remove blighted plant parts and old flowers from production beds to reduce further spread. Several species of the root-knot nematode, Meloidogyne spp. , can attack yarrows and cause stunting, decline, and reduction of flower yield.
Heavy infestations can kill plants.
Root-knot infection is recognized by the lumpy appearance of the roots. Always check roots carefully when dividing plants. Have soil tested for nematodes if areas in the field decline or if you notice lumps on roots. Chemical nematicides can be applied only to fallow ground as they are toxic to plants. Control of plant-parasitic nematodes must be done prior to planting.