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Early Season Diseases in Soybean

Overall the soybean crop is developing nicely but a few reports of potential root rot problems in the southwest are coming in especially in areas which have experienced significant rainfall in the past week or so. As you assess your stands, keep in mind the most common early season soybean root rots in Ontario are:

Phytophthora Seed and Seedling Blight

Areas in the southwest portion of the province which had significant rainfall last week and are expecteding more over the next week may be seeing this disease. Often the disease is most prevalent on clay and clay loam soils but if saturated soils persist for a day or so, all soil types can show Phytophthora injury under these conditions. Phytophthora along with Pythium love saturated soils and both are classified as "water moulds". In most incidences Pythium infection slows when temperatures become warmer whereas Phytophthora is more heat tolerant (but still requires moist soil conditions).

Phytophthora has been a major disease of soybeans since the 1950's and there are many races of Phytophthora present in Ontario. Soybeans are susceptible to Phytophthora at any stage of development and can result in pre- and post-emergence damping-off in the seed or young seedlings. The fungus also causes seedling blight in older seedlings. Infected plants will exhibit a purple/brown stem lesion that moves up the stem. In severe cases the plant will wilt, collapse or die. In older plants, the lesion may extend from the soil line to the 2nd/3rd node and into the lower branches. Purpling discolourization of the stem is common with leaves becoming brown but DO NOT FALL OFF. Dead plants are easily pulled since lateral roots are absent with a small (rotted) tap root. It often occurs in patches throughout the field.

Rhizoctonia Root Rot

Fluctuations in moisture and temperature may benefit Rhizoctonia root rot this year. This disease continues to increase in the province and there is very little verified tolerance in commercially available soybean varieties.. Rhizoctonia causes a variety of symptoms but the most characteristic is reddish-brown spots that appear at the base of the seedling stem and on roots just below the soil line. These spots can enlarge into sunken lesions which may girdle the stem resulting in damping-off. Seedlings that survive the initial infection have a characteristic reddish band on the stem at or just below the soil line. These lesions will be brick-red in freshly pulled soybean plants.

Fusarium Root Rot

Fusarium wilt of soybeans (also referred to as Fusarium blight) can affect soybeans at any stage of development. Fusarium wilt is most problematic when soybeans are under stress. Stress can interfere with the normal activity of the plant's root system and therefore, will affect growth and potentially increase root diseases. Fusarium symptoms include stunting, overall poor growth, wilting and death.

Figure 1. Rhizoctonia root rot

Figure 2. Fusarium Root rot