Phomopsis seed decay (PSD) of soybean is caused primarily by the fungal pathogen Phomopsis longicolla Hobbs along with other Phomopsis and Diaporthe spp. This disease causes poor seed quality and suppresses yield in most soybean-growing countries. Infected soybean seeds can be symptomless, but are typically shriveled, elongated, cracked, and have a chalky white appearance. Development of PSD is sensitive to environmental conditions. Hot and humid environments favor pathogen growth and disease development. Several control strategies have been used to manage PSD and reduce its impact; however, the use of resistant cultivars is the most effective method for controlling PSD. Efforts have been made to identify sources of PSD resistance in the past decades. At least 28 soybean lines were reported to have certain levels of PSD resistance in certain locations. Inheritance of resistance to PSD has been studied in several soybean lines. In this paper, general information about the disease, the causal agent, an overview of research on evaluation and identification of sources of resistance to PSD, and inheritance of resistance to PSD are presented and discussed. 1. Introduction Phomopsis seed decay (PSD) of soybean, Glycine max (L.) Merrill, is caused primarily by the fungal pathogen Phomopsis longicolla Hobbs et al. [1] along with other Phomopsis and Diaporthe spp. This disease causes poor seed quality and suppresses yield in most soybean-growing countries, especially in the mid-southern region of the United States [2, 3]. PSD severely affects soybean seed quality due to reduction in seed viability and oil content, alteration of seed composition, and increased frequencies of moldy and/or split beans [4–7]. Development of PSD is sensitive to environmental conditions. Hot and humid environmental conditions, especially during the period from the pod fill through harvest stages, favor pathogen growth and disease development [2, 8–11]. PSD has been reported to cause significant economic losses [4, 12]. Annual yield loss caused by PSD in the United States from 1996 to 2007 ranged from 0.38 to 0.43 million metric tons (MMT) [13]. PSD has been exacerbated in recent years because of the use of the early soybean production system (ESPS), which generally involves planting of early-maturing cultivars in April to avoid the late-season droughts typical of the region in the mid-southern USA [14]. Unfortunately, seed infection by P. longicolla was high in some cultivars that matured in July or August, when high temperatures and high humidity were conducive to PSD development
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