By Albert Tenuta
No indication of resistance or tolerance developing!
By Albert Tenuta
Seed treatments are frequently used to manage phytophthora seed rot and damping-off
in soybean. Metalaxyl (Allegiance-FL and Apron-FL) and mefenoxam (Apron XL LS)
are the primary compounds used to treat soybean seed for control of Phytophthora
sojae. Metalaxyl has been used for many years in several formulations, and
metalaxyl-M (mefenoxam) is a similar chemical with the same site of action that
was introduced in the past five years for soybean seed treatment.
A number of other phytophthora species have developed tolerance or insensitivity
to these two compounds, most notable is the late blight pathogen of potato and
tomato, P. infestans. Questions have arisen recently concerning the possibility
of resistance to metalaxyl or metalaxyl-M (mefenoxam) in Phytophthora sojae.
To address this question, plant pathologists in the US (Anne Dorrance at Ohio
State University, and Dean Malvick at the University of Minnesota) and Ontario
(Terry Anderson with Agriculture and Agri-Food Canada at Harrow) and myself
(Albert Tenuta with OMAFRA at Ridgetown) have started a series of studies to
determine if P. sojae is losing sensitivity or becoming more tolerant to these
Isolates of P. sojae were collected from multiple locations in Illinois, Ohio
and Ontario, and then tested in the laboratory for sensitivity to metalaxyl
and mefenoxam. The research is ongoing, but the preliminary results suggest
P. sojae is highly sensitive to these compounds in all of these areas. In Illinois,
isolates from 20 different counties were tested and all were sensitive to metalaxyl
and mefenoxam. In Ohio, isolates from different locations were tested and all
were sensitive to metalaxyl. In Ontario, isolates of P. sojae from six counties
have been evaluated against mefenoxam and all were found to be sensitive.
Additional isolates from this region are being tested this winter. These preliminary
results are very promising and indicate that metalaxyl and mefenoxam continue
to be effective compounds for control of early-season infection of soybean seed
and seedlings by Phytophthora sojae over a wide geographic area in the
north-central region and Ontario.
Besides seed treatment control, phytophthora root rot requires a combination
of soybean variety selection and good soil management as well. Soybean varieties
with resistance or tolerance to phytophthora root rot are available. Some varieties
have both resistance and tolerance. Select soybean varieties that have both
specific resistance (Rps genes such as 1k and 1c) and good partial resistance
(tolerance) to all races of phytophthora. Consult with your seed company for
variety profiles. Consult the current report, Ontario Soybean Variety Trials,
available from your local OMAFRA office or on the Ontario Soybean Growers web
site (www.soybean.on.ca) for plant loss ratings from phytophthora root rot and
resistance genes included in the varieties.
The North Central Soybean Research Program funded the University of Illinois
and Ohio State University, and the Ontario component of the study was funded
by the Ontario Soybean Growers through the Canada-Ontario Research and Development
Program, administered by the Agricultural Adaptation Council. Funding is being
provided by Agriculture and Agri-Food Canada and the Ontario Ministry of Agriculture,
Food and Rural Affairs under the Canada-Ontario Agricultural Safety Net Management