Jaime A. Cummings and Gary C. Bergstrom, School of Integrative Plant Science, Plant Pathology and Plant-Microbe Biology Section, Cornell University
A potentially yield-reducing disease called ‘brown stem rot’ (BSR) was confirmed for the first time in New York soybean fields in 2013, and was found again in 2014. It showed up in some plants from soybean fields in Cayuga, Herkimer, Niagara, and Yates Counties collected by Cornell Cooperative Extension Educators Kevin Ganoe, Keith Severson, Michael Stanyard, and Bill Verbeten, with support from the New York Soybean Check-off Program. The disease was diagnosed in the Field Crops Pathology Laboratory at Cornell based on characteristic symptoms and the laboratory isolation of the causal fungus and confirmation of a portion of its signature DNA sequence. So far, BSR has not been detected outside of the four counties mentioned above. It is noteworthy that BSR was not detected in soybean fields in northern New York scouted in 2013 and 2014 by CCE Educators Michael Hunter and Kitty O’Neil, with support from the Northern New York Agricultural Development Program.
Brown stem rot is caused by the fungus Cadophora gregata (syn. Phialophora gregata) and occurs in most soybean production regions of the US, but this is, to our knowledge, the first confirmation in New York or the northeastern U.S. Reported yield losses in the Midwest have ranged from minor to in excess of 25%, so the presence of the pathogen is considered a significant factor for soybean production. Yield loss is often a function of the relative susceptibility of varieties that are planted; varieties vary from susceptible to resistant. BSR is a disease of priority to soybean seed companies. Resistant varieties are widely available, and most seed catalogs provide resistance ratings for BSR. If BSR becomes more prevalent in New York, selection of resistant varieties may become more important for New York producers.
The foliar symptoms of BSR are similar to those of other soilborne diseases that restrict the movement of water and nutrients to the leaves. So BSR can be confused with northern stem canker and sudden death syndrome, all of which result first in yellowing and then browning of leaf tissues between the veins during pod-filling stages. However, not all soybean varieties exhibit foliar symptoms when infected with the BSR fungus. What is distinctive about BSR is the browning of the internal tissues of infected plants (Figure 1). This discoloration is often most obvious near the nodes when stems are split lengthwise. Dead leaves may remain attached to the plant. Stem discoloration symptoms may be confused with those caused by white mold, northern stem canker, or Phytophthora stem rot.
Infection by the fungus occurs early in the season, through the roots, from where the fungus continues to grow throughout the plant’s water-conducting tissues. Temperature has the greatest impact on disease development, and is favored by temperatures between 60 – 80F. But, temperatures above 80F may halt BSR development and spread. Because infection occurs at early stages (around the three leaf stage) of the crop, foliar fungicides applied during flowering and pod-filling stages will not be effective in suppressing BSR.
The fungus survives on soy residues and in the soil in the field for many years. Luckily, the pathogen survives on few other plant species, and in severely infested fields, a rotation of at least 3 years out of soybean and deep plowing of infected soybean residues would reduce the incidence of BSR in a subsequent soybean crop.
The most important thing that a New York soybean producer can do at this time is to learn to recognize the symptoms of BSR and other soilborne diseases and to get a diagnosis of problems that they observe in their fields. If BSR or other soilborne diseases are confirmed, producers should talk to their seed supplier and order soybean varieties with appropriate levels of resistance for the soilborne diseases observed on their farm.
Acknowledgements: This research received financial support from the New York Soybean Check-off Research Program, the Northern New York Agricultural Development Program, and Cornell University Hatch Project NYC153473.