NPIPM:Brown spot on soybean
Compiled by: Buyung Hadi, from the materials by: Dean Malvick and Lawrence Osborne
Brown spot is a common disease of soybean worldwide. The disease was first reported in the US in the 1910s. Brown spot incidence is quite common on soybean fields of the Northern Plains, but the disease rarely causes economic loss.
Causal Organism
Brown spot is caused by the fungus Septoria glycines. The fungal pathogen has been reported infect to most plants of the genus Glycine (the most known of which is soybean) and common weeds such as velvetleaf.
Symptoms and Signs
Symptoms of brown spot are usually found first on older leaves in the lower canopy. Plants can be infected early without showing symptoms until after flowering. Symptoms start as small irregular dark brown spots of the leaf surface. The spots grow in size and several spots may coalesce, forming large browning lesions on leaf edges or along the veins. Infected leaves may drop prematurely. The browning areas of infected leaves may contain numerous small, dark, round, raised fungal structures called pycnidia (visible with the help of magnifying glass), tiny dark specks that form the fungus fruiting bodies. Under favorable environmental conditions, infection may spread to leaves in the upper canopy.
Disease Impacts
Brown spot affects yield negatively by inducing premature defoliation of the plant. The greater the degree of defoliation, the larger the yield loss. The disease rarely causes economic yield loss, but severe infection (usually occurs during wet growing periods) may indeed inflict economic damage.
Life Cycle and Epidemiology
Septoria glycines survives as mycelium or pycnidia in the soybean residues. The pathogen produces spores in the spring and the spores are transported to plant surface by rain splash. Infection occurs under the temperature range of 59-86˚F (15-30˚C) with an optimum temperature of 77˚F (25˚C). Free water must be available on the plant surface for infection to occur and long periods of wetness increase the severity of brown spot. Dry and hot weather hampers the development and spread of the disease. Plants weakened by other diseases (e.g. soybean cyst nematodes or Fusarium root rot) or agronomic practices are more susceptible to Septoria infection. Both pods and seeds of soybean are susceptible to brown spot.
Management Approaches
Cultural Methods
Rotation with corn and tillage help reduce inoculum pressure in the field and are recommended in fields with a history of brown spot.
Host Plant Resistance
Complete resistance towards S. glycines is not available in commercial soybean cultivars. Yet, some soybean cultivars show partial resistance towards brown spot and limit disease progress.
Chemical Control
Foliar fungicide application from bloom to pod fill can reduce the severity of brown spot, but since the disease rarely causes economic damage, fungicide application is generally not warranted.
