Botrytis (grey mould)
Grey mould (Botrytis cinerea) may occur on the foliage of many native plants in still, humid or drizzly conditions. In Western Australia it is commonly observed on native plants of arid regions in the cooler months of autumn, winter and spring. Solanums, Ptilotus, Sturt peas and boab trees may be affected as they are dormant in the cooler months. Attacks are worse on species with dense foliage or those with soft hairy leaves. Most grey mould infestations cause little damage unless there are long periods of suitable weather. Infection of developing flowers can also occur, which results in flower abortion. If plants are grown in well ventilated, sunny positions, both leaf and flower infection is usually not as severe as on plants in crowded, shady conditions. Grey mould can also be very damaging to seedlings and cuttings in glasshouses. Iprodione is registered for chemical control of botrytis and other fungal diseases on ornamentals in home gardens.
Galls
Galls are growth malformations caused by the stimulus of an insect, small animal, fungal or bacterial attack. Insect feeding, insect egg deposits or fungal/bacterial infections may result in an increased level of hormones flowing to the site of irritation. This promotes abnormal growth around that point which leads to formation of galls. Galls vary widely in size and shape, and the shape of a gall may be specific to its causal agent.
Galls should be removed manually and destroyed when first noticed, in case they are caused by insects. This will curtail a build-up of the insect pest. Chemical control with systemic insecticides is usually ineffective. Good general health and vigour of the trees is the best prevention of gall damage. Keep up water and nutrient supplies to the plant.
Ink spot
Ink disease is an attack by a fungus called Alternaria alternata which affects kangaroo paws. Ink spot shows up as blackening of the leaves and flower stems. The disease usually starts at the leaf tips, then spreads down the leaves into the rhizomes. If uncontrolled, ink spot may kill weak plants of dwarf hybrids and kangaroo paw species including Anigozanthos manglesii and A. viridis. More vigorous species, like A. flavidus, are resistant to infection.
A range of cultural control measures can be tried. Manipulating the microclimate, as well as nutrient modifications can be of benefit. The best way to control fungal diseases is to keep the plants vigorous and healthy. The addition of essential trace elements, especially potassium and calcium, may improve the vigour of the plants and reduce the plants’ susceptibility to ink spot. If the plants are situated in a shady, cool, moist environment either move them or increase ventilation and sunlight by thinning neighbouring and overhanging plants.
Cut foliage affected by ink spot off at the base to encourage healthy new foliage.
Phytophthora
Phytophthora cinnamomi, also known as Jarrah dieback and Phythophthora dieback is a pathogen that attacks the root system of plants.
Phytophthora dieback is common in Western Australian home gardens, parks and nurseries and is readily transported in infected soil which may stick on shoes or vehicle tyres.
Once plant roots are infected, the pathogen produces mobile zoospores at an optimum temperature of around 15°C. These swim in water to the root tips of other plants which they may infect. In warm, wet conditions such as heavy rainfalls in summer, the fungus can complete its life cycle within 24 hours. Little infection takes place below 15°C.
There are more than 50 species of Phytophthora and they cause a variety of symptoms, depending on the species attacked and the soil conditions where it grows. Plants infected with the disease may suddenly collapse and die within a few days after having been apparently healthy. About 40% of native WA plants are suseptible. Plant collapse is common in susceptible dry climate plants like many Banksia spp., Dryandra spp., Adenanthos spp., Isopogon spp., Hakea spp., Grevillea spp. and Eucalyptus spp.
The collapse is thought to be the result of the breakdown of the plant’s root system. Plants may have survived for a long time as the remaining functional root system supplied sufficient nutrients and water under mild conditions. However, in period of high evaporative demand, the damaged roots supply insufficient water to maintain effective transpiration and cooling.
Phytophthora becomes more of a problem in waterlogged soil, and only resistant plants survive.
In the well drained soils of most Perth metropolitan gardens, Phytophthora may not kill the plants but may cause stunting, slow growth and affect the growing tips. The small feeder roots are attacked and the leaves are often yellow with brown margins and apices. After warm, wet periods of 24 hours or longer, the fungus may cause sufficient damage to the roots of the plants to kill them.
Cultural control can be achieved by improving the soil drainage through installation of underground drains, raised beds and sunken garden paths. In areas of known infection, only tolerant species should be grown. Sowing infected areas to grass for at least three years should reduce the chance of fungal re-infection.
The home gardener can achieve chemical control with phosphorous acid (also known as phosphite) which can be sprayed onto the foliage of plants or injected into trees.
Pythium, Fusarium and Rhizoctonia
These are damping off diseases which affect seedlings all year round, but mainly in wet conditions. Seedlings die before they emerge or after emergence. In the latter case, rot develops on the stems and constricts water and nutrient movement near the soil surface. The stems are physically weakened and the plants collapse.
To avoid damping off disease, do not over-water. Sow seeds into sterilised soil thinly, to avoid crowding of the seedlings. Pythium may attack plants which have soft tissue due to an over-use of nitrogenous fertiliser. Treat with sulphur and mancozeb.
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.
Unfamiliar pests and diseases
The Department of Agriculture and Food, Western Australia (DAFWA) is on the lookout for animal and plant pests, diseases and weeds that could pose a threat to agriculture and the environment.
If you discover something unfamiliar please send a photo to the Pest and Disease Information Service (PaDIS) by email: info@agric.wa.gov.au or phone them on Freecall: 1800 084 881.
Please read the sending specimens for identification web article before sending samples to the Pest and Disease Information Service, 3 Baron-Hay Court, South Perth, 6151, WA.