Pome fruit rust or pome fruit skin disease is a viral disease that affects pome fruits, causing symptoms such as rust or speckled fruit. It was first reported in China in 1933 and in Japan in 1935. The disease is distributed and causes damage in Japan, the United States, the United Kingdom, Denmark, Canada, and Italy, among other countries, although it is not普遍 widespread. All apple cultivation provinces in China are affected. In regions with small apple cultivation, such as northern Shaanxi, the central area of Shanxi, and parts of Hebei, the disease incidence can reach 50% to 70%. In Shandong, one of the main apple production areas, the average disease incidence was about 0.9% in 1988, with some heavily affected orchards reaching 16%. In 1985, a heavily affected nursery in Yancheng, Jiangsu, had a 100% disease incidence in the smaller Golden Delicious variety, with fruits losing almost all economic value.The disease symptoms are primarily observed on the fruit, although they can also be severe in some varieties of seedlings. There are three main types of fruit symptoms:1. Rust-type: Affected fruits develop light green, water-soaked spots on the top of the fruit about one month after flowering. These spots gradually turn into five distinct vertical lines, changing from yellow-green to dark brown and becoming scaly. Between the five vertical rust spots, many irregular horizontal or patchy rust spots often occur, creating a rough, irregular fruit surface. Sometimes, fruits may show no obvious rust spots but develop many deep vertical and horizontal cracks, causing the fruit to shrink and fall off easily.2. Speckled-type: Affected fruits develop many round, yellow-green spots on the surface after coloration, resulting in a speckled appearance when the fruit matures. In yellow variety fruits, the coloration is uneven, with distinct light and dark patches, and the fruit texture becomes harder and the flavor deteriorates.3. Rust-speckled complex-type: Before fruit coloration, affected fruits develop obvious rust spots on the top or scattered spots on the surface. After coloration, areas without rust spots or around the rust spots develop colorless spots, resulting in a combination of rust spots and speckled symptoms on the fruit surface.These three symptom types are not strictly distinct, with early-maturing varieties typically showing speckled symptoms, late-maturing varieties displaying rust symptoms, and mid-maturing varieties presenting a combination of both. Varieties such as海棠, 沙果, and 檳子 typically show speckled symptoms, while 小國光 and 雞冠 show rust symptoms. Affected trees have fewer branches, smaller canopies, and thinner, weaker, and more upright branches.Infected seedlings exhibit two main symptoms:1. Curling of leaves and rust spots on stems. For instance, diseases in varieties like 國光, 雞冠, 大猩猩, andEc夷衣 result in leaves curling upwards and stems developing irregular, scaly rust spots.2. Necrosis of stem and bud areas, forming slightly raised, slightly sunken ulcerous spots that are initially dark brown before turning grayish brown.The etiology of the disease is controversial:1. It has long been believed to be caused by a virus due to its graft-transmissible nature.2. Since 1983, both China and Japan have detected the pathogen as a viroid using polyacrylamide gel electrophoresis (PAGE). This viroid is a circular, low-molecular-weight RNA composed of 330 nucleotides and is called ASS-RNA-1. Another linear double-stranded RNA viroid, ASS-RNA-2, is also present in infected branches. The disease's various symptom types are caused by the same viroid.3. In 1990, China isolated a non-culturable bacterium from diseased apple plants and discovered that it could cause typical cukeleaf symptoms in "國光" seedlings when inoculated. Treatment with penicillin could alleviate the disease, suggesting that this bacterium is the pathogen responsible for apple rust disease.Most apple cultivars are highly susceptible to rust disease, although some yellow varieties like 黃龍, 黃魁, and 金冠 show some resistance. The disease is transmitted primarily through grafting and contact between healthy and diseased tree roots. It can also be mechanically transmitted through tools used in orchards. Seeds and pollen from diseased trees do not transmit the disease.Control measures include planting virus-free seedlings, prohibition of using scion or rootstock from diseased trees, and avoiding cultivating apple seedlings in pear orchards. When establishing new apple orchards, it is recommended to be at least 150 meters away from pear orchards to prevent cross-contamination. Regularly inspect nurseries and remove diseased seedlings, such as those of the 國光 variety, and promptly remove diseased trees to prevent spread of the disease.