We are receiving questions and also samples this year that are similar to fields we visited during the 2007 and 2008 growing seasons. As a reminder, the focus for many of those field visits were specifically if there was a breakdown in the Rps 1K gene for Phytophthora. A summary of that information is available here.
Similar to those two years, samples we have looked at this year have not necessarily been typical and it has not been easy to identify a primary disease of interest (i.e., the primary cause). Samples submitted to the Plant Disease Diagnostic Clinic have often yielded evidence of multiple pathogen species in a given sample. Recent results from the Wisconsin DATCP Phytophthora root rot survey of 45 fields between 16 June and 9 July indicated presence of Phytophthora in 15 fields (33%), which was an increase from the previous two years (20% in 2008 and 18% in 2009, respectively). In spite of the increase, the results still indicate that it is important to make sure you have a proper diagnosis of suspect soybean plants. Two diseases that can often be confused during the later stages of soybean development are Northern stem canker and Phytophtora. Below are a description of the two diseases and associated symptoms/signs:
Northern stem canker (Diaporthe phaseolorum var. caulivora): reddish brown to black discoloration on stems and petioles that can first appear around flowering. Lesions originate at the nodes and appear sunken and may girdle the stem. There can be a yellow and brown discoloration of leaves around the veins and plant death is associated with petiole and leaf retention.
Phytophthora stem and root rot (focus is on symptoms after V4): brown to black lesion that extends above and below the soil surface. A root rot can be found. Leaves turn yellow and petioles will droop. Wilting where tip of the plant forms a shephard’s hook. Plant death associated with petiole and leaf retention.
For further information about soybean diseases, please consult Soyhealth.
For a field diagnostic guide of common soybean diseases in Wisconsin, please click here.