Safeguarding the Future of US Agriculture - Global Crop Diversity Trust

Apple scab (caused by Venturia inaequalis)This fungal disease occurs throughout the world, especially in cooler applegrowingregions. In the United States, apple scab is a major economicproblem for the Mid-Atlantic region, in particular. The fungus blemishesyoung fruit and leaves. Once established, the fungus can overwinter inorchards, and re-infect apples the following growing season.Most major apple cultivars are susceptible to apple scab. But USDA-fundedexpeditions have collected 1,200 apple accessions in Kazakhstan. Thesevarieties are now conserved in genebanks and being grown in the field. Over40 percent of the seedlings have been found to resist apple scab. Furthertesting continues on these trees as they mature. Scientists continue huntingfor apple scab resistance genes in Kazak collections by screening them bothfor scab resistance traits and for the presence of DNA markers associated withspecific scab resistance genes. Unfortunately, rare apple collections inKazakhstan are in imminent danger of loss due to insufficient resources tomaintain the orchards. (See “A System Under Stress,” below.)Cedar apple rust (caused by Gymnosporangium juniperi-virginianae)Cedar apple rust fungus requires two hosts: apple or crabapple and analternate host, such as cedar or juniper trees. Cedar apple rust is of majorimportance in southern New England, in particular. The fungus blotchesleaves and fruit with yellow and orange spots.Citrus are native tosoutheast Asia(northeastern India,southern China, and theIndochinese peninsula).Christopher Columbusfirst brought the orangeto the United States. USorange production isnow the second largestin the world, behindBrazil.© USDA-ARS, Scott BauerIn most cases, apple and crabapple trees are not seriously damaged by cedarapplerust, but severe loss of leaves can shrink fruit size and weaken trees.Cultivar susceptibility varies. Some economically important varieties, such asGolden Delicious, have only intermediate resistance to the fungus. To findresistance to cedar apple rust, scientists are currently evaluating 200 appleaccessions from the NPGS collection, as well as 11 species from Central Asia,China, Turkey, Russia, Armenia, and Germany.CitrusCENTER OF ORIGIN AND DIVERSITYSoutheast Asia. Citron, lemon, lime, sweet orange, sour orange, pumello,mandarin, kumquat, and trifoliate orange all originated in this area. ChristopherColumbus first brought the orange to America.ECONOMIC VALUEAnnual value of US production: $2 billionIn an average year, the U.S. may export 10 times more citrus tonnage than it imports.SELECT DISEASES OF CONCERN Citrus Canker (caused by Xanthomonas axonopodis pv. citri)In the early 20 th century, the agricultural industry worked hard to removecitrus canker, a feared bacterial disease, from the US landscape. Althoughthe disease was declared eradicated from the U.S. in 1947, it has sinceresurfaced multiple times, and is currently spreading through Florida’sorange groves. Infected trees ultimately produce no fruit at all. Citrus cankeris highly contagious and can be spread rapidly by rain.16 SELECT CROPS AND DISEASES OF CONCERN