Several questions received last week prompt this week’s comments on a long-term nemesis to Ohio soybean farmers. A difficult word for a number of people to pronounce and for many more to spell, Phytophthora. This disease, sometimes called “Phyto”, is our costliest soybean disease. For the most part, we have learned how to manage this pest well. However, a review of the disease may be in order.

Phytophthora seedling blight and root rot is caused by the soil-borne fungus Phytophthora sojae. This soil-inhabiting fungus can cause seed decay, preemergence or postemergence damping-off, seedling blight and root rot as well as mid- to late-season wilt and death of plants. Phytophthora sojae produces structures called oospores, which enable it to survive from year to year in crop residues or in the soil. In the spring, the oospores germinate to produce sporangia. When soils are flooded or saturated, the sporangia release zoospores, which are attracted to the growing soybean root tip, where infection occurs.

Phytophthora seedling blight and root rot is more severe in areas that are low or poorly drained, in compacted areas or in clay or heavy soils (sound familiar?), but the disease can appear on plants growing in lighter soils or higher ground if the soil remains wet after planting. Significant rain after planting favors the development of Phytophthora. A dry period after planting drastically reduces this disease. Phytophthora may occur at soil temperatures as low as 50 degrees F, but greatest root damage occurs when soil temperatures are 59 degrees F or above.

Numerous races of Phyto have been identified based on their ability to overcome specific Rps genes or combinations of Rps genes in soybean varieties. Phytophthora is found throughout Ohio.

Management options for Phytophthora seedling blight and root rot: 
•Select varieties with either race-specific resistance, tolerance or a combination of race-specific resistance and tolerance, especially for use in fields with a history of Phytophthora. Race-specific varieties contain a single gene or combination of genes (i.e., Rps1c, Rps1k, Rps3a, etc.) that confer resistance to specific rates of Phytophthora sojae. Tolerant varieties have a non-race specific, partial resistance and may also be called field-resistant varieties.

•Plant in good seedbed conditions. Phytophthora is more likely to occur in low, wet areas, poorly drained areas or compacted areas of a field. Tiling to improve drainage and taking steps to reduce or prevent compaction may help minimize disease problems.

•Avoid the application of high levels of manure or potash fertilizer (KCl) just before planting.

•Use an appropriate fungicide seed treatment. Products containing either metalaxyl or mefenoxam as an active ingredient are particularly effective against water mold fungi such as Phytophthora sojae. If high disease pressure is expected, the use of the higher rate of these seed treatment fungicides may be necessary.

Rapid germination, emergence and early growth of the soybean plant lessens the chance for Phytophthora to reduce yields.

Of course, we know many fields are planted in April. Cooler conditions increase the importance of having good seed to soil contact. Good seedbed conditions, proper planting speed and depth control are all important to encourage timely and uniform seedling emergence.

Good luck with the busy planting season. Please prioritize safety. Whether servicing equipment, traveling to a new location, or planting in the field, keep safety in mind at all times. You owe it to your family … and yourself!

The views and opinions expressed in this column are those of the author and not necessarily those of Farm World. Readers with questions or comments for Roger Bender may write to him in care of this publication.