New Breakthroughs for Phytophthora Control
Midsouth soybean producers who farm on clay soils have long known about the damage potential that is associated with Phytophthora root and stem rot [causal organism Phytophthora sojae, an oomycete]. This soilborne pathogen causes damping off of seedlings and root and stem rot of older established plants. The damage potential from the pathogen is most severe for soybeans that are grown on soils that are poorly drained and thus stay wet for extended periods during the spring (Delta clay soils) when soybeans are planted and/or are in the seedling stage. Rotation and tillage are not control options since oospores of P. sojae can remain viable in the soil for many years.
The best defense against Phytophthora injury has been and still is genetic resistance and seed treatments that contain metalaxyl or mefenoxam. However, P. sojae has many races, which can make variety selection difficult since a resistant variety will not be resistant to all races that may be present in a given field. Furthermore, the seed treatments are only effective at preventing seed decay and damping off; i.e., their protection is short-lived.
Two recent news items offer increased potential for combating this damaging pathogen.
On the genetic front, a Sept. 13, 2017 article titled “Breakthrough soybean research by Purdue and Dow AgroSciences could save farmers millions“ that appeared in the Purdue Univ. Agriculture News tells of the discovery of a novel soybean gene [Rps11] that provides strong resistance to multiple isolates of P. sojae. This discovery will allow the development of molecular markers that can be used to rapidly incorporate this resistance gene into soybean varieties through conventional breeding techniques. Purdue’s Dr. Jianxin Ma, professor of Agronomy, states that as more resistance genes are identified, they might be stacked to strengthen soybean resistance to the pathogen. The research describing this finding is reported in Theor. Appl. Genet. [2016] 129:445-451.
On the seed treatment front, DuPont Lumisena is a new fungicide seed treatment that uses a new class of chemistry and new mode of action (Oxathiapiprolin–FRAC code U15) to control P. sojae at all life cycle stages of the disease; it has preventative, curative, and eradicative activity in soybeans, and is effective against all P. sojae races. It is the first seed treatment ingredient since the introduction of metalaxyl/mefenoxam to offer control of the pathogen. Click here for the Lumisena Technical Bulletin and here for the Lumisena label. Resistance management is required since the resistance risk is assumed to be medium to high because of its single site inhibition. Thus, a seed treatment program that includes fungicide rotation and/or mixing Lumisena with a seed treatment fungicide having a different mode of action is essential.
Pioneer will add Lumisena as a component of its Pioneer Premium Seed Treatment for 2018, and this will negate having to add a higher rate of mefenoxam (as Apron XL) that is recommended for Phytophthora control on the ApronMaxx label. However, it is assumed that metalaxyl will remain a component of Pioneer’s seed treatment package since it will be needed to provide control of Pythium in the clayey soils.
Producers are encouraged to identify those soybean production sites (clay soils, low-lying fields subject to casual flooding) that are likely to harbor populations of P. sojae so that either or both methods (genetic resistance, seed treatment) for control of the pathogen can be used to avoid stand loss and yield-reducing plant damage associated with this pathogen.
Composed by Larry G. Heatherly, Sept. 2017, larryheatherly@bellsouth.net