Magnolias are attractive trees and shrubs that spruce up the landscape with waxy foliage and fragrant, showy blooms. However, the plants can succumb to a variety of fungal diseases that rob them of their splendour, causing leaves to turn brown and eventually drop. Treat the diseases, and adopt good cultural practices so your magnolia grows healthy and develops lush green foliage.
Different types of leaf spot can cause the foliage of infected magnolias to turn brown. The diseases that inflict these ornamental trees thrive in cool and humid weather with frequent light rains, heavy dews and crowded planting sites. Minimal spotting is not harmful but only causes aesthetic damage. However, left untreated, the conditions will worsen and affect the entire leaf surface, weakening the structure of the tree and making it susceptible to other diseases and pests. Common fungi that cause leaf spots on magnolias include Sphaceloma, Alternaria, Coniothyrium, Mycosphaerella and Cristulariella. Cephaleuros virescens is the parasitic alga that damages Southern magnolia.
All leaf spot diseases start as tiny oval or circular spots on leaf surfaces that are usually yellow, grey, reddish-brown, purple or tan. While some appear as shiny raised surfaces, others form concentric circles over the foliage surface. Often the symptoms spread or deepen in colour when the disease is untreated.
The fungi and alga responsible for leaf spots overwinter on fallen leaves or branch cankers on infected magnolia trees. The spores or pathogens responsible for the diseases spread to foliage through irrigation or rainwater, pests or wind. In severe cases, premature yellowing of the foliage occurs, coupled with leaf drop. Temperatures and water levels determine the severity of each disease, because leaf spots thrive in extended periods of cool and moist weather.
To manage leaf spots, rake and destroy fallen leaves and plant debris from around the base of the magnolia tree. Pick off leaves with minor spots to control spread. Adopt proper irrigation and fertilisation practices to maintain a vigorous tree. Irrigate the tree at soil level, using a soaker hose, instead of wetting the foliage. Prune overhanging branches of nearby trees or plants to improve sunlight penetration and air circulation. Also prune magnolia trees and thin out dense crowns to improve air circulation. Although not usually required, use a copper fungicide over infected parts of the tree until the disease disappears.
Other factors responsible for browning leaves on a magnolia tree include sun scald due to excessive exposure to direct sunlight and improper irrigation practices.