• By Yeshi Wamishe and Jarrod Hardke •
The production of hydrogen sulfide in some soil types due to an interplay between soil chemistry and microbes under anaerobic/ flooded conditions may affect rice starting early in its development. Does your field have a history of hydrogen sulfide toxicity? How long has it been since you applied the pre-flood nitrogen fertilization and established a permanent flood? It may be time for you to start scouting for root-related problems to relieve or rescue your rice before it is late.
Start scouting two or three weeks after a permanent flood is established. As shown in Figure 1, color differences in rice roots from a bar ditch or bay (left) with roots from a levee (right), respectively, are indicators of the disorder. Make sure the roots are well rinsed before you compare them.
The disorder is the result of high levels of sulfur and iron in soil and irrigation water which leads to a reaction in the root zone. This reaction occurs in anaerobic (no oxygen) conditions and results in the formation of iron sulfide that coats and blackens roots.
Hydrogen sulfide formed due to anaerobic condition is often toxic to roots to extent of killing them. The iron sulfide coating on roots can limits oxygen exchange worsening the problem. Although complex, at this level the problem is named hydrogen sulfide toxicity.
The rice plants may appear stunt and lower leaves may start to turn yellowish. Symptoms are usually severe around groundwater inlets.
What happens next?
Weak pathogenic (opportunistic) fungi invade the damaged roots and make root crowns their home. As the fungi grow and multiply, the passage for minerals and water from roots to the above-ground parts of rice plants get clogged partially or fully marking the beginning of autumn decline (Figure 2). Such a situation makes the rice plant decline or dies resulting in an estimated yield loss of up to 40 %.
Re-introducing oxygen to the root zone (draining) reverses the reaction. If this correction is made early enough, complete or near-complete recovery is possible and the approach is called protective strategy. However, opportunistic fungi once entered the crowns of the root system, their growth clogs the crown – once this happens complete recovery is very unlikely.
How early should you drain?
Although you start scouting a few weeks after a permanent flood is established, give it time for the rice to use the pre-flood nitrogen. The earliest you may drain the field is at the straight head drainage timing based on DD50, which would be a little before mid-season. If drained and dried for up to four or five days, you should start seeing new roots growing from the side of the crown. The re-introduction of oxygen initiates new root growth. Then after that, it can be re-flooded.
What if the farm size is huge and draining takes days?
Oh! That is tough! This is one of the reasons that we have to keep our field sizes manageable particularly if the field has a history. However, you need to rescue your rice.
You may start draining from the top part of the field and by the time the lower part is drained, you may start re-flooding starting from the top. Here comes a judgment call. It all depends on your resources such as water resources, pump capacity, etc.
What if the problem is discovered at some point during reproductive stages?
This one is tough too! However, again something has to be done to rescue the crop. In such cases, lower the flood depth, but do not completely dry the ground. The most it can go is until it is muddy.
Rice plants are very sensitive to dry conditions at reproductive stages. Moreover, remember diseases such as blasts may take advantage of the drought situation.
• Not all fields for rice production in Arkansas have shown the problem of hydrogen sulfide toxicity or autumn decline.
• We encourage every field to be scouted for root health once in a while for this or any other root disorders.
• We understand draining and drying is not the best option for producers with limited resources and large fields. So far, we hav no other way to alleviate this problem. With the problem starting early and left unmanaged, yield would be highly affected. You’ve got to act!
Dr. Yeshi Wamishe is a University of Arkansas Extension plant pathologist. She may be reached at email@example.com. Dr. Jarrod Hardke is a University of Arkansas rice Extension entomologist. He may be reached at firstname.lastname@example.org.