It can take a while for some pathogens to develop symptoms and impact their hosts, and by the end of last week (June 29/30th), reports were coming in on dying plants from the areas of the state that received 2 to 5 inches of rain during the week of June 20th. Technically this is called the latent period, from the time of infection until symptom development or sporulation (the next generation of spores that are ready to infect the crop). Phytophthora stem rot was evident in many of our historical fields that did not have the full resistance package. Symptoms include dying plants, wilting and a characteristic root rot and brown coloration at the base of the stem. The key here is the root rot and the decay moving up the stem.
Symptoms on young seedlings (left) and older plants with characteristic brown canker
In Ohio, the Rps genes will only give some protection. Ohio’s Phytophthora sojae populations have adapted to all of the Rps genes that are commercially available. Rps1a, first deployed in the 1960’s is no longer effective, and like an elephant once it recognized this gene it does not seem to want to forget it. This is challenging because we would like to be able recycle Rps genes periodically, but this is not going to be possible for Rps1a. Rps1c, Rps1k, and Rps3a are now recognized by the P. sojae populations across the state – but not every individual in every field. So we recommend to continue to utilize them. Rps8 is recognized by P. sojae populations in the southwest corner of the state – although it is still holding in the rest of the state. What is needed is to give these genes a bit of backup – they need their guards, defensive team and that is with partial resistance – companies have been calling this field tolerance. The Rps genes are like the star player on the basketball team that can hit the 3 point shots or the pitcher – but they can’t win the whole game on their own! Every company uses a different scoring system – most are on a 1 to 9 scale where 1 is perfect growth, no root rot (this is for the Rps genes) and 9 is dead. OR they use a 9 to 1 scale where 9 is no disease and 1 is totally dead. You must read the fine print. On the OSU scale – (1 is no disease and 9 is dead) a scores of 3 to 5 are optimum for providing a protection to the soybean plant during the mid-season. When soybean varieties have this score – we have never observed Phytophthora stem rot in the field.
Flooding injury – Laura Lindsey gave a nice explanation in last week’s C.O.R.N. on how this can occur. Symptoms of flooding injury will also occur on the roots, the outside cortical tissue can easily be pulled off leaving white “rat tails” or the correct term the root steles. The Rhizobium nodules will be dead, not pink and overall poor health. I have seen plants recover from flooding injury, the weather over the weekend, cloudy and cool was perfect. Some of these areas may recover, it just depends on how long the field was saturated.
Flooding injury with white roots – the outer root tissue can easily be pulled off (left) and a root with healthy Rhizobium nodules
As you can probably tell, the conditions that can cause flooding injury in soybean are the same as Phytophthora. Phytophthora is a watermold and requires saturated soils to produce the swimming zoospore that can find and infect plants. When a susceptible variety is grown in the fields that have these flooding events, it is a double hit and even more important that the right protection was used. For this week, on those fields that received the flooding rains, walk them. Flooding injury will be in pockets where the water sat for a long period of time – then move to the other areas, to see if your Phytophthora package worked. If you find the wilting yellow and dying plants then you need to rethink your Phytophthora management. Work with your seed dealer to get a better package for the fields on your farm – and the focus should be not on the Rps gene but on the partial resistance side of the package.