Corn is one of the most heavily cultivated crops in the world, and is especially important to the economy of the United States, which ranks first in its production. Accordingly, any pests or diseases that may affect the growth or quality or corn plants are a serious concern. Losses can often be minimized by planting certain genetically engineered hybrids, and this practice has helped greatly reduce the severity of many threats to the crops. Disease-resistant hybrids are not always available for all types of corn, however. For instance, though there is rarely any need for additional control measures for rust among field corn crops when disease resistant varieties are grown, sweet corn and popcorn are much more susceptible to the fungal infection.