Algal spot, also called green scurf, is caused by algae (Cephaleuros virescens) that are common on magnolias and other ornamentals in the southern parts of the country. During moist weather, cells of the algae enter the leaves or the other tissue of twigs and spread rapidly. The invaded twig tissue may swell and crack. If the crack encircles the twig, the twig dies. The infected magnolia leaves turn greenish brown at first. If the algae develop spore-producing bodies (tiny round heads on fine, dense reddish hairs), the patches appear reddish brown and velvety or cushiony. The algae spread rapidly when rains are frequent and heavy. Trees weakened by drought or poor growing conditions are most susceptible to algal spot.

This is a common problem with acid-loving plants such as magnolia that grow best in soil with a pH between 5.5 and 6.5. The soil is seldom deficient in iron, but iron is often found in an insoluble form that is not available to the plant, especially in soil with a pH above 7.0. A high soil pH can result from overliming or from lime leached from cement or brick. Regions where soil is derived from limestone or where rainfall is low also have high-pH soils. Plants use iron in the formation of chlorophyll in the leaves. When iron is lacking, new leaves are yellow.

Neolecanium cornuparvum This is the largest scale insect found in the United States. The immature purple insect spends the winter on the twigs and branches of the tree. Starting in spring, it feeds by inserting its mouthparts and sucking the sap and nutrients from the bark tissue. The scale is unable to digest fully all the sugar in the plant sap, and it excretes the excess in a fluid called honeydew. A sooty mold fungus may develop on the honeydew. The scales mature in August, and the female lays her eggs beneath her powdery white or shiny brown shell. In the fall the young scales, called crawlers, settle on the twigs to spend the winter. Repeated heavy infestations kill branches and possibly the whole tree. Several other types of scales also infest magnolia.

Leaf scorch is caused by excessive evaporation from the leaves. In hot weather, water evaporates rapidly from the leaves. If the roots can''t absorb and convey water fast enough to replenish this loss, the leaves turn brown and wither. For optimum growth, magnolias require moist soil. Leaf scorch is most severe when water is unavailable because the soil is dry. If the weather is extremely hot, scorch may also develop when the soil is moist, however. Drying winds, severed roots, limited soil area, or low temperatures can also cause scorch.

Toumeyella liriodendri The tulip tree scale is a serious pest of tulip trees, lindens, and magnolias. The immature insect spends winter on twigs and branches. Starting in spring, it feeds by sucking the sap and nutrients from bark tissue. The scale cannot digest all the sugar in the sap, and it excretes the excess in a fluid called honeydew, which drops onto objects below. Sooty mold grows on the honeydew, making the tulip tree leaves appear black and dirty. Honeydew also attracts bees, wasps, flies, and ants, which feed on it. Scales mature in August, and females lay up to 3,000 eggs underneath a brown shell. Young scales, called crawlers, settle on twigs in the fall. Crawlers are dark tan, looking like tiny aphids. Repeated heavy infestations kill branches or the whole tree. Several other types of scales also infest magnolias.