Arkansas Rice: Start Scouting for Hydrogen Sulfide Toxicity

With temperatures increasing, early planted rice will soon reach the reproductive stage.

In fields with a history of hydrogen sulfide toxicity (also referred as autumn decline or akiochi), draining the flood and drying the field for a few days before midseason (DD50 for straighthead drain and dry timing) is the recommended protective strategy to reduce potential grain yield loss.

Hydrogen sulfide toxicity starts when it blackens rice roots (Figure 1) starting as early as two weeks after permanent flood has been established. Symptoms include black root rotting with stunted and yellowish rice foliage (Figure 2).

Fig. 1. Early symptoms of hydrogen sulfide toxicity to rice roots.

Fig. 2. As hydrogen sulfide toxicity progresses rice plants remain stunt and turn yellowish.

The problem is often most severe where cold well water first enters a rice field (Figure 3) and may spread throughout the field, except on levees. Rice roots from the levees remain clean due to the aerobic conditions around the root zones. The disorder in anaerobic conditions becomes progressively worse throughout the season if unmitigated.

Fig. 3. Hydrogen sulfide toxicity is often severe where cold well water first enters a rice field.

Fungi grow into the crown (Figure 4) which ultimately limit the normal function of the whole root system and prevent translocation of water and nutrients from the soil to the rice plant.

The real question is, “if I drain, can I ever get the fields flooded again in the summer heat?”  Some rice producers are reluctant to drain due to water constraints, the field size or because of the growth stage of the rice.  As long as the flood remains, the problem will most likely persist and probably become more severe.

It is true that you do not want to drain during the reproductive growth stage. Therefore, if you know the field had hydrogen sulfide disorder in the past, it is better to drain earlier than later.

Fig. 4. As the disorder progresses, fungi grow into the crown (right). Roots from levee remain clean (left)

Make sure not to drain before the rice plants utilize their preflood nitrogen.  If detected late, it sometimes becomes necessary to drain at reproductive stage.

The question then becomes, “is there enough sick rice in the field to jeopardize the good rice in the field by draining?”

In the cases of autumn decline, it is likely that nutrients such as potassium or phosphorus may be deficient.  The rice plants may even appear they are deficient of nitrogen or sulfur (Figure 5).

Fig. 5. To start with the rice plants may appear deficient of nitrogen or sulfur

Lower leaves start yellowing and later die and leaves of severely affected rice show brown spots. However we would only add these nutrients if we are reasonably sure they were deficient.  Under situations where roots have been already compromised, we do not want to fix an oil leak in our car by adding more gasoline.

The only real solution we have so far is the “drain and dry” strategy.  The degree of drying could vary with the crop stage. However, early detection of the disorder is the key to act timely with the available management approach without compromising yield.

Tips to Scouting for Hydrogen Sulfide Toxicity

• Pull rice plants to examine roots both from the paddy and levees
• Wash the mud off the rice roots
• Compare rice roots from the paddy with the rice roots from the levees. Roots from the levee should not be blackened.
• For early symptoms, split tillers not the crown and examine the root area right below the soil surface (Fig )
• For additional confirmation, expose the blackened roots for an hour or so to air. The disappearance of the blackening
  confirms the disorder.
• Rotten egg smell of blackened roots sometimes confirms the presence hydrogen sulfide.
• Consult with your county extension faculty for symptom confirmation and to discuss management approach.