By Yeshi Wasmishe, Extension Rice Pathologist; and Jarrod Hardke, Extension Rice Agronomist
Healthy roots promote healthy plant growth. Scouting for hydrogen sulfide toxicity in submerged rice is an essential practice to avoiding the risk of crop damage. Regular scouting for hydrogen sulfide toxicity starting two to three weeks after permanent flood is established is highly recommended. We have received more reports each year from Arkansas rice production fields since 2012. Although this may not imply expansion of the problem, it indicates increasing awareness among rice producers.
Early detection is the key to making timely management actions. Once hydrogen sulfide predisposes rice plants for infection by opportunistic fungi, the damage usually is irreversible. Therefore, as a general rule, scouting for the toxicity problem and further autumn decline in all flooded rice is highly advisable regardless of history of the field.
Hydrogen sulfide toxicity has been reported in Arkansas from various soil types on different rice varieties in the past five years. Some soil types are more prone than others. Observations have shown fields having a clay loam soil texture to be more prone to hydrogen sulfide toxicity problem than others commonly cropped to rice. Likewise, we have observed differences in varietal tolerance levels. But no rice has been seen fully clean from the problem in our field experiments when grown under continuous flood in a field with history.
Hydrogen sulfide toxicity predisposes rice plants to root crown invasion by opportunistic fungi causing autumn decline or akiochi. In such a situation, grain loss can reach up to 50 percent. Early detection and timely actions are the key to minimizing the risk of damage by the hydrogen sulfide toxicity and autumn decline complex. The problem can start at any stage of the rice crop grown in flooded conditions. Therefore, regular scouting is highly recommended.
Scouting tips for hydrogen sulfide toxicity at early stage of rice crop
- Where to scout:
- Scout in areas closer to water inlets. Symptoms start near water inlets and fade further away (Figure 1).
- How to scout:
- Pull out rice plants to examine roots from bar ditches, paddies and levees.
- Wash the mud off of the rice roots (Figure 2).
- For early symptoms split tillers (Figure 2) and examine the root crown base right below the soil surface (Figure 3).
- Compare rice roots from bar ditches, paddies and levees (Figure 3).
- If hydrogen sulfide toxicity exists, roots from bar ditches and/or paddy would show black unlike roots from the levees.
- Blackened roots gradually change color to normal when exposed to an open air for an hour or so.
- In some fields, the flood water smells like rotten egg.
- As symptoms progress, yellow cast similar to symptoms of sulfur or nitrogen deficiencies may be seen (Figure 4).
- Affected rice may show yellowish lower leaves and rice plants appear stunted.
- Gradually, nearly all roots turns black (Figure 5).
- Usually there is difference in root mass between rice roots from levees and bays (Figure 6).
- Root crown infection can occur at any developmental stage of the crop depending on the severity of the problems (Figure 7).
Management tips for hydrogen sulfide toxicity
For rice fields with history, a preventative approach is recommended:
- Limit or avoid the use of sulfur-containing nitrogen fertilizers such as ammonium sulfate.
- Limit or avoid use of irrigation water with excess sulfur.
- Start scouting 2-3 weeks after flooding and follow the timely “drain and dry” strategy.
- Use a straighthead drain timing based on the DD50 program.
- Be careful not to lose your preflood nitrogen.
- Before you drain, wait at least 3-4 weeks for the crop to utilize the preflood nitrogen applied.
- After draining, monitor for new root growth before re-flooding. Greenhouse research and field observations showed growth of new roots in 3 to 5 days. Note that the concept behind draining and drying is to allow oxygen to enter the soil environment.
- Continue scouting.
For late discovered hydrogen sulfide problem rice fields, rescue approach is recommended:
- If the problem is identified before midseason, drain and dry similar to a field with straighthead.
- Since water is a critical need to the rice plants at reproductive stages, if you need to drain, drain the field only to lower flood depth but drying is not recommended. Again, the concept here is to allow oxygen into the soil without stressing the rice crop with drought. Therefore, knowledge about the field size, water resource capacities and management ability need to be considered before draining. If the field size is huge and takes days to re-flood, the damage from drought stress may exceed than that of hydrogen sulfide toxicity and/or autumn decline.
- Once the flood level is lowered, wait until new roots start to grow before you re-flood.
- Re-flood once new roots start to show up.
Consult with your county extension faculty for symptom confirmation and to discuss management approach.