Symptom Identification and Control Methods of Spruce Bud Rust

Spruce bud rust is a newly discovered disease in the forest regions of northern China. It has been reported in the natural forest areas of the Greater Khingan and Lesser Khingan Mountains. Abroad, it is mainly distributed in countries such as the United States, Canada and Sweden, where it damages white spruce, black spruce and Norway spruce. It harms the terminal buds and lateral buds of seedlings, saplings and mature trees, seriously affecting seedling quality and the normal growth of plantations.

Symptoms appear in late May when new leaves begin to grow. During the same period, diseased buds grow faster and longer than healthy buds, but soon stop growing. The diseased leaves growing from diseased buds are initially grayish white, gradually turning yellow or purplish red. The diseased leaves swell into a round shape, only about 9 mm long, less than half the length of healthy leaves, and the longest new diseased shoot is 6.5 cm. The newly sprouted diseased branches and leaves resemble a flower and are often mistaken for spruce cones. At the tips of the diseased leaves, small slightly raised black dots begin to grow; these are the pycnidia of the pathogen. They gradually extend downward and produce more pycnidia. Subsequently, orange-yellow elongated small bumps appear; these are the aecia of the pathogen, mostly arranged in two rows on the inner side of the leaf and in a single row on the outer side. After the aeciospores mature, they break through the sac and release many yellow powdery aeciospores, leaving a very short white membranous covering on the diseased leaves. The diseased buds quickly die, shrivel and turn black. After the terminal bud is damaged, the seedling takes on a forked shape. After being damaged for successive years, it becomes clustered and branch-like.

Pathogen: Chrysomyxa woroninii Tranz., known as Woronin's golden rust fungus, a fungus of the subphylum Basidiomycotina. The pycnidia of the pathogen mostly occur in the middle and upper parts of the leaf, rarely at the leaf base, measuring 10.0-17.5 μm × 10.0-14.0 μm. The pycnidiospores are colorless, round or oval, 1.0-1.2 μm × 3.0-3.5 μm. The aeciospores are single-celled, yellow, oval or round, catenulate, with a warty surface, measuring 19.5-46.9 μm × 13.7-38.5 μm.

Disease characteristics: The pathogen overwinters as mycelium in diseased buds, and the disease begins in late May. As the diseased leaves grow, pycnidia are produced, followed by aecia and aeciospores. The aeciospores begin to disperse in early June, and the diseased branch buds begin to die in mid-to-late June. The disease period lasts only about 30 d. Seedlings and saplings under the forest canopy have a higher incidence rate than those without shade, and mature trees on shady slopes have a higher incidence rate than those on sunny slopes. As far as a single tree is concerned, in most cases the incidence rate of terminal buds on branches on the shady side is higher. In mid-to-late June, the diseased branch buds die, and the disease period lasts only about 30 days.

Control methods

(1) Agricultural control.

When afforesting, choose plots with few suspected hosts for planting; in the nursery, check in late May whether the seedlings are infected. Once the disease is found, cut off the diseased branch buds, put them in plastic bags, take them outside the nursery and bury them deeply or burn them to prevent the spread of aeciospores.

(2) Chemical control.

In the early stage of the disease, promptly spray the seedlings with 75% chlorothalonil 600-1000 times dilution or dithane sodium 500 times dilution for control.

Commonly used fungicides: chlorothalonil and dithane sodium.