Heterodera glycines Infection Increases Incidence and Severity of Brown Stem Rot in Both Resistant and Susceptible Soybean

Phialophora gregata (Allington & D. W. Chamberlain) W. Gams, the causal agent of brown stem rot (BSR) of soybean (Glycine max L.) (1), and Heterodera glycines Ichinohe, the soybean cyst nematode, are widely distributed throughout the midwestern United States (32). The current recommended management strategy for both pathogens is the use of resistant soybean cultivars combined with rotation to nonhost crops (26,33). BSR resistance is believed to be controlled by three nonallelic Rbs genes (2,4,21,31) and one or more unknown genes derived from PI 88788 (6), a soybean line that also is a source of H. glycines resistance (20,27). Both H. glycines and P. gregata are common and both infect soybean; therefore, they may interact. In 1984, Negishi and Kobayashi (12) reported that H. glycines infection increased symptoms of BSR of adzuki bean, Vigna angularis (Willd.). In the early 1990s, researchers in Iowa noticed increased symptoms of BSR in both BSR-susceptible and BSR-resistant soybean cultivars grown in H. glycinesinfested fields (H. Tachibana, personal communication). Preliminary experiments based on these observations also suggested increased BSR symptoms when soybean plants were grown in H. glycines-infested soil in a greenhouse (25). In 1997, Sugawara et al. (22) reported that, in a greenhouse experiment, H. glycines increased both percent stem length and percent node discolored in a soybean genotype susceptible to both P. gregata and H. glycines. They, however, did not find an increase in symptoms in two other genotypes, one resistant to BSR and another resistant to H. glycines. Given the limited numbers of genotypes that have been tested in controlled experiments and the conflicting data available, it is necessary to further study the effects of H. glycines on BSR symptoms. Data also are needed on the effect of H. glycines on the colonization of stems by P. gregata. To date, the only information available on whether H. glycines affects BSR is based on observation of stem symptoms. To conclusively test whether H. glycines increases susceptibility of soybean to P. gregata, the extent of stem colonization by the fungus needs to be assessed. It is possible, for example, that simultaneous infection of soybean by P. gregata and H. glycines could result in changes in the host that could lead to increased severity of BSR symptoms without a corresponding increase in colonization by the fungus. In addition, the relationship between colonization and the presence of BSR symptoms is not clear, even in the absence of H. glycines. Development of internal stem discoloration is greatly affected by the environment (19,28), but whether this is due to an effect on the fungus or on stem symptoms is not clear. In addition, Tabor et al. (23) reported that soybean plants could be infected by P. gregata without expressing stem symptoms. For example, 12 weeks after inoculation and incubation in a growth chamber, the majority of stems of both the BSRsusceptible genotype Sturdy and the BSRresistant BSR101 were extensively colonized but did not exhibit visible internal stem discoloration (23). Thus, any effect of H. glycines on the susceptibility of soybean to BSR needs to be confirmed by assessing colonization of the host by the fungus. Another aspect of the H. glycines–P. gregata interaction that needs to be examined is the effect of H. glycines on the progression of infection of the host by P. gregata. Weber et al. (29) reported that yield loss from BSR largely was due to a reduction in seed number rather than seed size. Thus, the timing of infection by P. gregata relative to seed set may be important in determining yield loss. Early season colonization by the fungus might result in more yield loss than late-season colonization. The difference in the timing of colonization by the fungus also appears to be an important difference between resistant and susceptible soybean genotypes. For instance, in growth chamber experiments, Tabor et al. (23) showed that stems of the BSR-susceptible genotype Sturdy were colonized earlier than stems of the BSR-resistant genotype BSR101. Thus, the potential of H. glycines to affect the progression of colonization by P. gregata should be determined in both resistant and susceptible soybean genotypes. The objectives of this research were to determine if H. glycines (i) increases the incidence and severity of internal stem discoloration in soybean genotypes with various combinations of resistance and susceptibility to both diseases, (ii) increases incidence and severity of P. gregata colonization of the soybean stem; and (iii) affects the progression of colonization of stems by P. gregata over time.