Magnolia trees, adored for their deliciously fragrant blooms, succumb to common fungal diseases that rob the ornamental beauties of their splendour. Magnolia trees are heavy feeders that may become chloritic under the wrong conditions. Magnolias require iron and nitrogen. Fungal problems or black mildew can wreak absolute havoc on these trees.
Most of the diseases afflicting shade and ornamental trees, such as magnolias, favour cool weather, light, frequent rain, heavy dews, high humidity and crowded plantings. Minimal spotting causes little harm, making them more unsightly than injurious. Leaf spot infections that appear early in the growing season often lead to premature defoliation. Two or more successive years of these infections can cause serious weakness to a tree, making it more susceptible to additional illness. Leaf spot disease presenting late in the growing season does not affect a tree’s health as seriously.
Virtually all leaf spot diseases evolve as small, dispersed, circular to oval dead areas in the leaves; usually in shades of tan, dark brown, yellow, grey, purple or black. Some develop as raised, shiny and coal black while others reveal light and dark concentric zones. Occasionally, leaf spot fungi deform or kill buds, flowers, fruits, twigs or small branches.
Fungi that overwinter on fallen leaves, infected buds, twigs, fruits and branch cankers cause numerous leaf spot diseases. Most fungi attacks only one tree species each, yet a few attack several species. Tremendous numbers of microscopic spores grow on leaf surfaces from early spring to summer. Air currents, splashing rains and insects spread spores to new leaves and infection begins. Water levels and temperatures determine the severity of each fungal infection, allowing infected leaves and plant tissues to survive through multiple growing seasons. Leaf spots thrive in extended cool, moist periods.
Protective control measures do not contain, control or remove most leaf spots. In early spring, fertilise previously defoliated trees properly. Fertilisation stimulates vigorous growth, as does thorough watering. Regularly prune trees, thinning out dense crowns and removing weak, diseased wood.
Several fungal attacks result in leaf spots. Examples of common fungi found on Magnolia trees are Alternaria, Cercospora, Coniothyrium, Cristulariella, Sphaceloma, Mycosphaerella and Phyllosticta.