By ANNE DORRANCE
Water molds are the plague of Ohio soils and we have plenty of them. Over the past eight years, we have recovered 30 different species of Pythium that can infect and cause seedling blight and or root rot on soybeans and corn.
In addition to Pythium, we have also recovered a Phytophthora that can infect both corn and soybeans.
Don’t forget that in soybeans we also have a great diversity of Phytophthora sojae where now we have some isolates in the state that can infect plants with all of the Rps-genes, including Rps8.
As a group, many of these Pythium and Phytophthora spp. survive in the soil as oospores.
They are very capable of surviving these long winters, and in many cases they do not germinate immediately in the spring. They require a period of time where the soils remain wet for at least two weeks.
Perfect conditions
Based on earlier work at the OARDC, two weeks is required to get maximum germination of many of these oospores. In 2011, in Ohio, we have now had four weeks of perfect conditions, to prime, many of these soil borne pathogens.
Management of these soil borne pathogens is best done at the very beginning — First: select soybean varieties with the best disease resistance package — Rps1c, Rps1k, Rps3a, Rps6, PLUS high levels of partial resistance for Phytophthora sojae.
The second: put a seed treatment on — at the right rate. The low rate of Allegiance or Apron XL won’t touch Phytophthora.
Our field data suggests, that the best return on the seed treatment investment for Phytophthora is the high rate at 1.5 fl. oz/cwt of metalaxyl or 0.6four fl oz/cwt of mefenoxam. At this rate, we have gotten a yield benefit over several years and locations of testing.
We will evaluate this again in 2011, in a couple of locations.
Note: this is for the fields that historically have to replant due to seedling damping-off not submergent issues.
Not effective
We also know from our data, that metalaxyl and mefenoxam are not effective against the full spectrum of watermolds. In some cases, a strobilurin can help knock some of them back. Different companies have different strobilurins — each of these also have a bit different spectrum of control. We are currently confirming this in field evaluations.
Conditions are also right for Rhizoctonia, Macrophomina (charcoal rot), Fusarium virguliforme (SDS) and Fusarium graminearum (head scab of wheat/seedling blight of soybean). The strobilurins will provide some reductions on all of these but do not provide total control; but Fludioxinil (Maxim) has the best data at the moment. I don’t have Ohio data, but others are reporting that ipconazole is effective toward Rhizoctonia as well as Fusarium graminearum.
The best seed treatment is a mixture of several active ingredients that target each of the different types of seedling pathogens.
(The author is a plant pathologist at the Ohio State University and the Ohio Agricultural Research and Development Center.)
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