Arkansas Rice: Mostly Dry, Many Reasons for Discolorations

Hydrogen sulfide toxicity in rice. Photo: University of Arkansas

Parts of the state received scattered rainfall amounts yesterday mainly in the northeast, which was very welcome by those that caught it.  However, there were more ‘have-nots’ than ‘haves’ with this event.  Little measurable rainfall has been received in recent weeks, making it more difficult to keep up with irrigation and for some to finish planting soybeans.

An old rice farmer from my area years ago was fond of saying, “be patient and wait on the rain we usually get around the 4th of July, but if you don’t get it you’re a plucked chicken.”  Only he didn’t say plucked.

Certainly, none of us are looking for a really big rain, and too frequent rains will crank up disease pressure that we don’t want or need.  It would be nice to get a couple inches across the entire Delta and get us into a better place overall with crop management though and start steering this ship through to heading.

The current 7-day outlook won’t you feel great if you need a rain, but at least the hurricane is headed east.  The only thing for certain about forecasts anymore is that they’ll change.

NOAA 7 day precipitation forecast

Fig. 1. NOAA 7-day Precipitation Forecast. Click Image to Enlarge

Deficiencies, Disorders, or High Yield Disease

As we hit the midseason stride with rice, there is a great deal of discoloration showing up that is giving growers and consultants fits in their attempts to push the crop and maximize yield potential.  There are a wide range of issues responsible for the variety of discoloration observed, some of which we should be concerned about and others we should not.

The first issue to note that isn’t really an issue, is one that has been referred to as “high yield disease” (Fig. 2).  This leaf tip effect is often noted on the 2nd or 3rd leaf from the top of the plant during reproductive growth (boot stages).  The newest young growth will not have the leaf tip ‘burn’ and neither will lower leaves.  This placement on the plant is one of the keys to distinguishing this particular leaf tip discoloration.

The simple way to define it is plants growing rapidly under high yield potential conditions are likely exhibiting a nutrient imbalance at the tips as they draw mobile nutrients from leaves to amplify their growth.  It could be argued this is exaggerated by the wet conditions this year creating a more shallow-rooted crop, further necessitating the rice to draw more from leaves as it pulls from the roots at the same time.

It is always wise, anytime we see any disorder, to look over the entire plant to best determine what’s going on.  In some cases where similar leaf effects have been observed but occur more frequently among all leaves on the plant, is a nutrient draw related to hydrogen sulfide toxicity (Fig. 3).  A quick look at the roots can clearly show if there is a significant amount of blackening related to hydrogen sulfide toxicity.

Hydrogen sulfide is toxic to cells and when present in the soil can lead to root cell death and the blackening that occurs.  The reaction that generates hydrogen sulfide gas occurs in anaerobic soils and when it kills the roots prevents them from delivering needed nutrients to the plant.  When this issue occurs during reproductive growth stages, it is much more difficult to safely and effectively overcome.

To truly eliminate the problem, draining of the soil is required to return oxygen or aerate the soil and reverse the reaction.  However, in reproductive stages there is a risk of a severe drain reducing yields, so instead the attempt is to reduce the flood to a muddy / firm state to get some oxygen to the roots and then reflood.  This often allows us to outrun the issue and maintain yields, but it is still very difficult during the hot and dry period of summer.

Finally, we do have instances of true potassium (K) deficiency out there (Fig. 4 & 5).  Typically, K deficiency appearing on leaves and leaf tips will take on a yellow/red burnt appearance and as the deficiency increases, the ‘burn’ will progress down the leaf margins.  Typically, the new leaves will not show this, but generally all lower leaves will display this.

For cultivars that are susceptible to brown spot, the brown spot disease can be a pronounced indicator of K deficiency and further modify the appearance of the deficiency.  Check deep water areas such as levee ditches to help confirm, as these areas will have more pronounced K deficiency.  Potassium deficiency can be corrected up the late boot stage, so we prefer send in Y-leaf (uppermost fully expanded leaf with a collar) samples to confirm the K deficiency prior to making a corrective application of K.

If making an application, 100 lb potash/acre (60 lb K2O/acre) should be the rate used, preferably with water stable in the field as potash is very water soluble and will move with moving water.

High yield disease of rice

Fig. 2. “High yield disease” in rice. Click Image to Enlarge

Hydrogen sulfide toxicity

Fig. 3. Hydrogen sulfide toxicity. Click Image to Enlarge

Potassium deficiency of rice

Fig. 4. Potassium deficiency of rice. Click Image to Enlarge

Collection of potassium deficient rice leaves

Fig. 5. Collection of potassium deficient rice leaves. Click Image to Enlarge




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