Arkansas Rice: Root Blackening – 12 Observations About Known and Unknown Factors
The actual cause for root blackening and rotting in rice is still a mystery. We still do not have strong evidence why in some soils roots turn black and rot in anaerobic (flooded) conditions (Fig. 1).
However, we know that opportunistic fungi grow in the crown (Fig. 2) and the whole root system shuts off and the plants eventually die.
Below are 12 observations made by Drs. Chuck Wilson and Rick Cartwright of the University of Arkansas, Cooperative Extension Service in 2004.
- The problem was noticed first and appeared worse where water entered the field (cold water paddies).
- The crown rot symptom were found on scattered plants in cold water checks in many fields across the state; however the widespread problem affecting large areas of a field was confined to only a handful of fields in the counties mentioned at that time.
- All affected fields had high soil pH.
- Soil types varied from silt loam to clay loam.
- All wells observed at the time were pumping iron-laden (orange) water.
- All severely affected fields had water pumped on them a lot (it seemed) so they suspected the water temperature might have stayed consistently lower in these fields than others – and other well water related factors might be spreading further than normal.
- Wells observed at the time were reported as 100-150 ft deep.
- Cultivars affected at that time were Cocodrie, Wells and CL161, although most were probably susceptible.
- The fields had several herbicides, and not all were the same ones. Grandstand appeared to be the only consistent one used in most of the severely affected fields, but it was not thought to be the original cause.
- The paddy rice was affected; the levee rice was not.
- The soil and roots smelled mucky but not like rotten eggs. Sometimes they smelled a little nasty (sewage) but not always. In fields in previous years where hydrogen sulfide toxicity was suspected, the water and roots did smell like rotten eggs.
- They found islands of healthy rice in some affected paddies (Arkansas County) and healthier streaks and patches in other fields (Lonoke Co). These were surrounded by sick rice.
Last week, we visited a field planted in Cl 151 and CL152 in Northeast AR with similar symptoms described above. The rice was at 1 inch internode elongation. Most of the field was affected although symptoms were worse in the deeper water areas and near water inlets.
Many plants had rotted or rotting roots, and the rot was spreading upward into the crowns. Once removed from the water, the blackening on the roots and lower part of the plants disappeared. Although it is likely unrelated, we noted many tiny snails hanging on the leaves in this field (Fig . 3).
In the past, diagnostic soil, water and plant samples have not indicated conclusive or consistent factors associated with the condition. Previously, Wilson and Cartwright indicated that “growers managed the problem by draining to aerate the crowns and roots and after a few days recovery with new white lateral root growth evident, re-flooded the fields and the rice made it.
Afterwards, we advised these growers to simply drain and dry the soil in these fields as for straighthead (straighthead timing), and this has helped prevent the problem from re-occurring. While draining at straighthead timing does not completely cure this problem (it can reappear later in the season), it interrupts the process long enough to allow roots to grow out and sustain the plants to normal harvest date.”
Whatever the cause, growers have learned that aeration of the system is their only management option when the problem is noticed and most have figured out how to carefully drain down until they can see new white roots forming, then reflood. Growers are very careful not to let the soil get dry if the rice is midseason or later, as rice in these later stages is so sensitive to drought stress.
It is a risky balancing act and they watch the field daily during the process, looking for new white roots each morning, and it shows how good our farmers are at managing this crop under difficult circumstances. Most have told us that doing nothing resulted in huge yield losses in the past as root systems completely died as did plants during the late boot to heading and grain fill stages, when water demand gets so high. Without a root system, the plants simply wilted in the flood and eventually passed on.
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