Association of Plant Color and Pericarp Color with Colonization of Grain by Members of Fusarium and Alternaria in Near-Isogenic Sorghum Lines

Sorghum (Sorghum bicolor (L.) Moench) pericarp pigments can affect grain quality. Pigments found in vegetative parts of the plant and in the peduncle and glumes of flowers (12,44) also may indirectly impact sorghum grain quality. Pigments associated with the “purple wound response” accumulate following mechanical injury, insect feeding, or pathogen invasion (22,31). Grain with nonpigmented pericarps (white seed), grown on tan plants that lack the purple wound response, is highly desirable as livestock feed and for human consumption (4,39). Infection by fungal pathogens can reduce sorghum grain quality and/or yield (6,36). Additionally, stored grain or plant material used for forage or in the making of silage can be contaminated with mycotoxin-producing fungi (11,42). Although less desirable from an agronomic perspective, plant pigments are associated with protection against invasion by insects and pathogens (2,8,31,44). Sorghum leaf pigments include 3-deoxyanthocyanidin phytoalexins involved in plant defense (30,46). The sorghum pericarp pigments 3-deoxyanthocyanins (3,32) also may have protective qualities (10,48). Sorghum germ plasm has been screened to assess the role pigments may play in defense against pests (5,45). However, grain hardness or plant height (2,16) also may contribute to protection against insects and pathogens. Thus, the protective role of anthocyanin and anthocyanadin pigments is unclear. Recently developed near-isogenic sorghum lines that vary in plant color (purple versus tan) and pericarp color (red versus white) exhibit favorable agronomic qualities (39). Among the four phenotypes (purple/red, purple/white, tan/red, and tan/white), there were no significant differences in germination, seedling vigor, and field performance. However, when plant color and pericarp color phenotypes were considered separately, germination and field emergence were significantly better for plants having either purple plant color or red pericarp phenotypes (39). It was proposed that these differences may be due to greater colonization and/or infection by fungi of plants with the white-grain or tan plant color phenotypes (39). Therefore, it was incumbent to test the hypothesis that sorghum with white grain and tan plant-color are more susceptible to colonization by fungi than plants carrying seed with red pericarps with the purple wound response. In this work, we assessed colonization of field-grown sorghum grain by Alternaria spp. and Fusarium spp. Isolates of these genera were obtained from fieldgrown grain using two different semiselective media (1,28). These fungal species were chosen because of prevalence in both diseased and asymptomatic sorghum grain (23,43,49). Additionally, greenhouse inoculation assays (18) were conducted using an Alternaria sp. isolated from sorghum seed and a Fusarium moniliforme sensu lato isolate pathogenic on sorghum.