NPIPM:Pod and stem blight on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Causal Organism
Pod and stem blight are caused by fungal agents: Diaphorte phaseolorum var. sojae, Phomopsis longicolla and other Phomopsis species. These pathogens infect a wide range of host plants including common bean, pea, and cotton. Velvet leaf and pigweed, two common weeds on soybean fields are also reported as hosts of pod and stem blight pathogens.
Symptoms and Signs
The most common sign of pod and stem blight is the rows of dark specks on senescing stems, petioles and pods. These specks are pycnidia, the fruiting bodies of the causal pathogen. Pod and stem blight may result in premature death of plant parts. Infected seeds may develop Phomopsis seed decay. Symptomatic seeds have cracked and shriveled seed coats, covered with chalky white mold and smaller in size. Not all infected seeds show symptoms. Seedlings growing from infected seeds may develop brown lesions on the cotyledons and usually the seed coats remain attached to cotyledons by strings of fungal mycelium.
Disease Impacts
Seedlings grown from infected seeds have reduced germination rate and vigor, resulting in stunted plants and reduced productivity. Later infection of pod and stem blight may reduce yield by shortening seed filling period as the period is truncated by premature plant death. The greatest impact of Phomopsis infection may lay in seed quality reduction, especially affecting food-grade quality seeds.
Life Cycle and Epidemiology
The causal pathogens of pod and stem blight survive in plant debris and within seeds. The characteristic pycnidia on infected senescing plant parts produce conidia that survive on soybean residues in the field. These conidia are transported to stem of soybean seedlings by rain splash or wind. Infected seeds are another pathway of pathogen introduction into a field. Thus, soybean infection by Diaphorte or Phomopsis pathogens usually occurs early in the season, but the infection remain latent until later reproductive stages.
Pods are infected between R5 to R6 stages, but infection remain latent until the pods starting to mature. Colonization of soybean pods decreases when the pods reach R7 stage. High rainfall is correlated with increased pod infection. The pathogen invade the seeds through contact with infected pod walls. Seed moisture content of 19-35% and temperatures around 77˚F (25˚C) favor seed infection. Seed moisture content below 19% hinders seed infection by Phomopsis spp. Thus, warm and wet weather during pod development and maturation favor seed infection.
Management Approaches
Cultural Methods
As the conidia of Diaphorthe/Phomopsis pathogens survive on plant debris, incorporation of soybean residues into the soil with tillage should reduce the disease inoculum. Planting infected seeds introduces new source of inoculum into the field. Usage of certified seeds will decrease the chance of new inoculum introduction. The incidence of Phomopsis seed decay was reported to be higher in fields with continuous soybean system than in those with corn-soybean rotation. Both wheat and corn are non-hosts of the pathogens causing pod and stem blight, thus rotation with these crops are recommended.
Host Plant Resistance
Resistance to pod and stem blight has been identified, but soybean varieties are not typically rated for this in seed catalogs. Soybean varieties also vary in their response towards Phomopsis seed decay.
Chemical Control
A single application of foliar fungicides between R3 and R6 was shown to reduce seed infection. Due to the added cost, fungicide application is deemed more practical on fields grown for seed production rather than cash grain.
