Grevilleas are small to medium sized evergreen shrubs that produce a wide variety of flower shapes and colors, depending on the species. These Australian plants are popular with gardeners because they bloom quickly and need little maintenance, aside from pruning. They also attract birds and don’t usually have problems with disease. However, there are a few fungal diseases that can infect them.
Grevilleas can become infected with cercospora and phyllostica, two fungal diseases that favor humid conditions. Symptoms of phyllosticta include premature yellowing of leaves and the appearance of round or irregular gray, brown or black spots. Some spots may leave holes in the leaves when they drop out. Small, black fruiting bodies can be seen in the older spots. Leaves on cercospora-infected plants turn bronze or light brown, then gray with fuzzy spore-bearing fruiting bodies. Cercospora usually attacks the inner leaves first and, eventually, as the dead leaves drop, the only leaves left are those at the tips of the branches. Fungicides aren’t usually needed for phyllosticta infections, but are available for treating cercospora.
Armillaria root rot can be a problem for grevilleas growing on poorly-drained soils. Look for creamy white mats of fungal growth between the bark and wood, and black threadlike structures on the roots. Fungal mats can usually be found just above the soil line. In addition, honey-colored mushrooms may grow around the base of the shrub in the fall. Above ground symptoms include discolored leaves and limb dieback. Armillaria can kill grevillea shrubs in less than two years. Fungicides are ineffective. If the disease is caught early enough, exposing the root collar to air for several months over the growing season can slow it down.
Phytopthera cinnamomi, or cinnamon fungus, is another disease that can kill your grevillea. Plants growing in soggy soil for extended periods of time are particularly susceptible. Symptoms include wilting and leaves that turn yellow, red or purplish in color. Look for reddish brown streaks in the wood under the bark. Young trees are more apt to die because of the small size of their root systems. Prevent phytophthora attacks by providing shrubs with good drainage and not allowing them to stand in water. Avoid wetting the trunk and lower branches. The fungicide fosetyl-Al can be used to help prevent infections, but doesn’t replace good cultural practices.
Sooty mold forms a gray to black velvety or crusty fungal coating on the leaves, but rubbing the leaf between your fingers will remove the mold. Sooty mold is an aesthetic problem and doesn’t kill shrubs, although serious infections may interfere with a plant’s ability to do photosynthesis. Sooty mold often grows on plants that are infested by insects like aphids or scales. These insects secrete a sweet, sticky substance called honeydew that attracts sooty mold. Control this fungus by controlling the insect infesting your plant. Sooty mold can be removed from leaves by washing it off with a strong spray of water.