Factors Affecting Soybean Root Colonization by Calonectria ilicicola and Development of Red Crown Rot Following Delayed Planting

Field studies were conducted in 1994, 1995, and 1996 to determine the effects of planting date, cultivar susceptibility, and soil pathogen population on soybean root colonization by Calonectria ilicicola and subsequent development of red crown rot. Early season colonization of roots was important for red crown rot symptom development. Symptom development in the more susceptible cultivar, Sharkey, was reduced following delayed planting and remained low in the less susceptible cultivar, Cajun, regardless of planting date. Taproot colonization was positively correlated with inoculum density during all three growing seasons but was strongest in 1994. Also, lateral root colonization correlated positively with inoculum density in 1994, the only year in which foliar symptoms were detected. A substantial decrease in inoculum density in 1995, along with reduced soybean root colonization, were attributed to high soil temperatures and probably low rainfall recorded during that summer. The effect of soybean plant age on root colonization was examined by exposing plants to the pathogen at different ages. Soybean plants were most susceptible to C. ilicicola during the first week after seedling emergence. By the second week, susceptibility was reduced by nearly half, and it remained near that level for the next several weeks.