By Yeshi Wamishe, Extension rice pathologist
Not all rice fields in Arkansas show the problem of hydrogen sulfide toxicity. However, it is always advised to scout starting two weeks after permanent flood has been established. Hydrogen sulfide toxicity can cause up to 50% grain yield loss if left without intervention. Although weak pathogenic fungi are involved at later stages of the problem, hydrogen sulfide toxicity is mainly a problem associated with the field soils or well water. The solution is simple: “Drain and dry.” But there are precautions to follow.
Questions and answers related to hydrogen sulfide toxicity:
Question: What are the common symptoms of hydrogen sulfide toxicity in rice?
Answer: Below are some symptoms of hydrogen sulfide toxicity. However, one should not wait to start scouting until symptoms start showing up. Damage may have already been done by the time symptoms are evident. When rice roots from bay are compared with roots from levee, they usually show differences in color if the problem exists as in Figure 1. In addition you probably see all or some of the following:
- The growth of rice crop in bays is slower than those on levees as shown in Figure 2.
- Lower leaves may start yellowing as in Figure 3.
- Due to yellowing in lower leaves, the field may show a yellow cast (Figure 4) more at irrigation inlets and in a deeper flood spots.
- Although not necessarily true to some fields, the roots may have a rotten egg type of smell.
- Black roots when exposed to air for an hour or so turn normal (Figure 2.)
Question: At what part of the field do I need to scout?
Answer: Usually symptoms are more evident at irrigation inlets. Start from there. Make sure you wash the roots thoroughly to clearly see the color differences.
Question: If I see symptoms 2 weeks after flood established, should I drain immediately?
Answer: No, you have to give at least 3 weeks for the crop to use the pre-flood nitrogen. Follow the DD50 recommended timing for straighthead of rice. That is the recommended timing.
Question: How long should I wait to put flood back?
Answer: It takes 3 to 5 days for new roots to start to grow around root crown. Once you start seeing new roots, re-flood. If your field size is too big to drain and re-flood in a few days and your water source and pump capacity are low, drought stress may do more damage. Therefore, it is your judgement call.
Question: What if I discover the problem after the mid-season at any of the reproductive stages?
Answer: Lowering the water depth or leaving it muddy for a few days may suffice. Reproductive stages are more sensitive for drought stress than earlier developmental stages. Combine art and science here.
Question: Is autumn decline (Akiochi) different from hydrogen sulfide toxicity?
Answer: Hydrogen sulfide toxicity precedes autumn decline. Some weak pathogenic fungi take advantage and house the rice root crowns of the weak and blackened roots affected by iron sulfide coating and hydrogen sulfide injury. Then the fungi block the passage way of nutrients and water from soil to upper part of the rice plant as shown in Figure 5 resulting in the crop to decline/die.
Question: So, does it mean all the trick is allowing oxygen to enter the soil that solves this problem?
Answer: Yes, this method is the best recommended so far regardless of the difficulties associated with “drain and dry” strategy.