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Symptoms and Control Methods of Korean Pine Blister Rust

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Symptoms: The disease occurs on the bark of branches and trunks. In the early stage of infection, the affected bark swells slightly and softens, cracks begin to form in early May, and yellowish-white blister sacs, which are the rust pustules (aecia) of the pathogen, emerge from within. In early to mid-June, the rust pustules mature and turn orange-yellow. From mid-May onward, the sacs rupture and release powdery rust spores, finally leaving a membranous white covering that gradually falls away and disappears. By the end of June, most blister sacs have ruptured and dispersed; the old diseased bark becomes rough and often develops a layer of sooty mold fungi, appearing black. Because the disease occurs year after year, the bark thickens and swells, and resin flows out. In late August and September

Symptoms: The disease occurs on the bark of branches and trunks. In the early stage of infection, the affected bark swells slightly and softens, cracks begin to form in early May, and yellowish-white blister sacs, which are the rust pustules (aecia) of the pathogen, emerge from within. In early to mid-June, the rust pustules mature and turn orange-yellow. From mid-May onward, the sacs rupture and release powdery rust spores, finally leaving a membranous white covering that gradually falls away and disappears. By the end of June, most blister sacs have ruptured and dispersed; the old diseased bark becomes rough and often develops a layer of sooty mold fungi, appearing black. Because the disease occurs year after year, the bark thickens and swells, and resin flows out. In late August and early September, honeydew oozes from the diseased parts, initially milky white and later turning orange-yellow, with a sweet taste. When the diseased bark is peeled off, pycnia can be seen in the cortex, appearing bloodstain-like and dark red after drying.

Pathogen: The pathogen is Cronartium ribicola J.C. Fischer ex Rab., a fungus belonging to the Basidiomycota, Teliomycetes, Uredinales, genus Cronartium.

Disease cycle: The pathogen produces teliospores at the end of July, continuously from mid-to-late August to early September, and these germinate to form basidia and basidiospores. The basidiospores are spread by air currents onto pine needles, germinate, and invade through stomata, with a few entering through young twigs. The hyphae spread continuously into the cortex of branches and trunks, and after 2-3 years, lesions appear on the bark of branches and trunks. From August to September, honeydew is produced, which is a mixture of pycnidiospores. In the spring of the 3rd to 4th year, the diseased parts produce aecia and aeciospores. If the diseased tree does not die, it produces aeciospores year after year. The aeciospores are spread by air currents onto the leaves of the alternate hosts, Ribes (currant/gooseberry) or Pedicularis (lousewort), germinate, and invade through stomata. After 10-11 days, urediniospores are produced for re-infection. From August to September, teliospores are produced. When weeds grow densely around trees, especially when alternate host plants such as Ribes or Pedicularis are abundant, trees are prone to the disease.

Control: (1) Strictly inspect seedlings for disease and prohibit the use of diseased seedlings. (2) Use herbicides such as sodium pentachlorophenate or dipotassium tetrachloride, atrazine, fenuron, and thiobencarb to eradicate Ribes and Pedicularis within 500 m around Korean pines, at a dosage of 1-5 g/sq m. When young trees are affected, before the aeciospores disperse, brush the diseased parts with undiluted pine tar to kill the aeciospores; after continuous application for 2-3 years, the diseased trees can recover to health. (3) Timely pruning is an effective measure for controlling this disease.

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