By the OMAFRA Field Crop Team
In Ontario, soybean seedling diseases and root rots are the second most important yield limiting diseases and this year is no different. Cool soil temperatures along with early rains in parts of Essex and Niagara resulted in a large number of soybean fields needing to be replanted and the significant rainfall recently has had a negative impact on soybeans in many areas.
Diagnosing these early season diseases in the field can be difficult since the symptoms of the various seedling blights and root rots are very similar. They can also be easily mistaken for other problems such as pre-emergence herbicide damage (group 14 PPO-inhibitors or group 5- photosynthetic inhibitors). Infected plants are often weak and less vigorous. This week we will discuss four common early soybean seedling diseases and their management.
Phytophthora root rot is caused by the oomycete Phytophthora sojae which has caused major losses in Ontario since the mid-50s. The fungus is prevalent in clay and clay loam soils with numerous pathotypes (formerly races) of the pathogen present in Ontario. Soybeans are susceptible to Phytophthora at any stage of development and infected plants can occur alone or in patches often with a stem rot characterized by chocolate brown/purplish stem lesion which may extend up to the 3rd node on older plants. Phytophthora-infected plant roots and lower stems are soft (mushy) and water-soaked causing the plants to wilt and/or be stunted. Dead plants are easily pulled since lateral roots are absent with a small (rotted) tap root and it is common for the leaves to become brown and shrivelled but do not fall off. Phytophthora root rot occurs across many environments, but is most common in warm soils (>15°C /60°F) and wet conditions.
Rhizoctonia root rot is caused by the fungus Rhizoctonia solani. The characteristic symptom of the disease is a reddish-brown lesion which appears at the base of the seedling stem and on roots just below the soil line. They can enlarge into sunken, dry lesions which may girdle the stem resulting in stem breakage.
Fusarium root rot can be caused by many Fusarium species which reside in the soil and can infect soybean. Infected plants may be stunted and spindly, and roots may have brown or black discoloration. Affected plants may also have poorly developed root systems. Fusarium species can infect plants under a wide variety of environmental conditions and is often associated with stressed plants.
Pythium seedling blight is caused by many Pythium species which cause symptoms similar to Phytophthora sojae, which is also an oomycete. Pythium and Phytophthora sp are referred to as “water moulds” since the infective spores (zoospores) swim in the water film between soil particles until they find and infect the root tip of many crops. These zoospores are not produced until soils become very wet to flooded but standing water is not needed for zoospore production. Three to four hours of very wet “damp” conditions are enough to initiate zoospore production. Pythium seedling blight symptoms include rotten, mushy seeds or seedlings with poorly developed roots.
An on-going Grain Farmers of Ontario supported regional project with Midwest US colleagues has shown an increase in some species of Pythium which are more heat tolerant in Ontario and the US which unfortunately means the disease risk from Pythium is lasting longer into the season.
For more info, see the complete article at fieldcropnews.com
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This Report includes data from WIN and Environment Canada.