Powdery mildew
Powdery mildew is caused by a group of related fungi. These are usually host-specific. Amongst Western Australian native plants Verticordia spp. and Chamelaucium spp. frequently become infected in autumn and winter. Initially, faint white spots on the leaves indicate a powdery mildew infection. These spots gradually increase in size, until the whole leaf surface is covered in white powdery growth. When young leaves are attacked, they may become distorted and/or fold up. Powdery mildew spores require high humidity to germinate. However, once established, the fungus will continue to grow, even in dry conditions.
For cultural control of powdery mildew, improve ventilation by reducing plant density and treat with fungicides containing bicarbonate of soda, wettable sulphur or triforine as soon as the disease is noticed.
Rusts
The disease which is commonly named as rust, can be caused by many different fungal groups. Generally, the symptoms consist of the appearance of small yellow patches or spots on the upper leaf surface. Powdery pustules form when the fungus within the leaf spots produces spores which burst through the epidermis of the leaf. Most rust pathogens have a limited host range, so infestation from one plant species to the next is unlikely.
In Western Australian home gardens, rust may affect kangaroo paws and Boronias.
Rust disease of kangaroo paws is evident as small, 2mm, reddish-brown pustules on the leaves, gradually becoming more dense until the plant is killed. Rust can be a serious disease of cultivated kangaroo paws, but it is rare in wild populations.
In Boronias, rust (Puccinia boroniae) causes brown pustular growths on leaves and stems which may result in defoliation.
For natural control, try to select rust-resistant varieties. For chemical control, use mancozeb or sulphur or preparations containing bicarbonate of soda.