Gary Breitenbeck, Johnny Saichuk and Joseph Kraska
For more than 10 years, rice producers in southwestern Louisiana have occasionally encountered a mysterious disorder in their crops. It usually appears early in the season when young rice plants fail to tiller – or send out shoots. In some fields, the affliction is confined to a small area; in others, the symptoms can rapidly spread unless the field is drained. While drainage can often arrest the spread of this disorder, afflicted crops usually do not recover and produce grain.
This localized decline in early-season rice has baffled producers, consultants, agents and scientists. As a result, this disorder is commonly called the "mystery disease" or "mystery malady," though few believe it is caused by a pathogenic agent. While the rapid spread of the disorder suggests the action of a pathogen or other pest, inspection of afflicted areas usually shows no evidence of any organisms. Many believe the disorder is occurring more frequently and that it afflicts a greater acreage each year. In 2005, a systematic study was initiated to identify the cause of this mystery malady.
The LSU AgCenter researchers embarking on this study did not get a consensus from growers, agents and others regarding the specific symptoms of this disorder other than general 2 agreement that it caused stunting and a failure of young plants to tiller. Therefore, a primary goal of this initial study was to determine whether a specific set of symptoms was associated with this disorder or whether it was the result of a complex of causes that contributed to poor stands.
The general suspicion that this disorder was becoming more widespread caused speculation that recent changes in production practices were responsible. Some associated the disorder with crawfish production, while others suspected that it was the result of subsoil exposure caused by precision land-leveling. Some speculated that it was caused by conservation management practices such as no-till planting or stale seed beds. Still others suspected that this disorder was related to the widespread use of the NewPath (Imazethapyr)/Clearfield production system to control red rice. Identifying the Cause
To investigate the causes and contributors to the mystery malady, parish extension agents conducted inspections of all fields reported to have the disorder. If the cause of the symptoms was not apparent, a research team was sent to the site to collect samples of above- and below-ground plant tissue, soils from zero to 6 inches deep and irrigation water. A 5-square-foot area was harvested to determine the volume of vegetation and the numbers of stems and tillers. For comparison, a similar set of samples was collected from a nearby area under similar management where no symptoms were evident. To identify possible relationships between production practices and the disorder, growers were asked to complete a detailed survey documenting current and previous production practices.
More than 50 sites were reported during the 2005 growing season, and 29 of these sites were extensively sampled. Some sites were not sampled because heavy insect infestations or other factors were clearly contributing to the decline of the rice plants. The 29 sites sampled all contained silt loam soils. Symptoms
Despite initial ambiguity, it soon became apparent that a specific set of symptoms could be associated with the mystery malady. The most definitive of these symptoms was the presence of reddish-brown spots on the lowest erect leaf. Examination of these spots with a hand lens showed that they were unlike the lesions typical of disease but were caused by discoloration within the interior of the leaf – a symptom more commonly associated with metabolic dysfunction.
Roots were partially coated with reddish-brown iron plaques, but the blackened roots associated with high sulfide concentrations were generally not present. Afflicted crops were stunted, and their root systems and stands severely reduced. In afflicted areas, plant height averaged 64 percent, root mass averaged 35 percent, and stand weight averaged 18 percent of corresponding healthy areas. Most of the stand reduction was due to reduced tillering, though death of seedlings was also observed at a number of sites. In fields where the disorder had spread rapidly over a large area, rice plants displayed a reddish-brown hue. The onset of this problem was not reported after flowering began, indicating an early-season disorder. Iron Toxicity
The symptoms are consistent with a nutrient toxicity, especially the reddish-brown spots on the lower leaves and the bronze color displayed when large areas are afflicted. Analysis of above-ground tissue supports this possibility. Afflicted plants accumulated iron at levels far in excess of the tissue concentration considered toxic to rice. A number of healthy plants also contained high levels of iron, but apparently these accumulations did not cross the toxic threshold (Table 1). Rice plants, especially in afflicted areas, also accumulated high amounts of aluminum, but the characteristic symptoms of aluminum toxicity were evident at only a few sites.
Iron toxicity is a common problem in many parts of the world, but it has not been identified as a concern in Louisiana. Many popular rice varieties grown in Louisiana are more tolerant of iron than those grown in other rice-growing areas. Even so, the high levels of iron uptake observed in this study suggest that some conditions lead to iron uptake that exceeds the tolerance of the popular varieties.
Most of the soils used for rice production in southwestern Louisiana contain high levels of iron, but this iron is usually present as insoluble iron oxides that pose no threat to plants. When soil pH falls below 5.6 or when soils are flooded, iron is reduced to a soluble form that can be readily taken up. Rice plants have the ability to regulate iron uptake by transferring oxygen from their leaves to their roots, causing soluble iron to precipitate as a reddish-brown coating on root surfaces. Where the amount of soluble iron exceeds this ability, iron toxicity can occur. When fields are drained, microorganisms rapidly oxidize soluble iron and reduce its availability. This process apparently accounts for the success of timely draining in arresting the spread of the disorder in susceptible fields.
Tissue analyses also suggest that zinc (Zn), potassium (K) and nitrogen (N) deficiencies may have contributed to the onset of this disorder. Excessive iron uptake causes a number of adverse effects, including a reduced ability of the plant to take up and utilize zinc. Table 2 shows the percentages of plants with zinc tissue concentrations below the deficiency level established for rice. Three-fourths of the afflicted plants were below this level, whereas only 20 percent of the healthy plants may have been zinc-deficient. Average potassium levels in afflicted plants were also less than those in healthy plants and were frequently below the deficient level. This may have been because of reduced root development, since many of the soils in afflicted areas contained adequate potassium. Nitrogen uptake was low in all plants and may have contributed to the onset of this disorder by retarding early-season growth and the ability of the crop to adequately oxidize its root zone. Effects of Management
No consistent pattern emerged regarding management practices that rendered fields more susceptible to this disorder. It is noteworthy that in nearly half the fields, the disorder originated near a riser where water flowed into the field. While irrigation water invariably contained significant concentrations of iron, the same water was applied to nearby healthy fields displaying no symptoms. It is possible that cool water temperatures or a slight increase in water depth, rather than the composition of the water, was responsible. In several fields, the disorder originated away from the riser in depressions only a few inches lower than the surrounding area.
Most fields investigated were irrigated with well water, but the disorder was also observed in several fields irrigated with surface water. Symptoms occurred with no-till, conventional till and stale seedbed management. They occurred after drilled, water-seeded and broadcast planting. Symptoms occurred in fields previously used for soybeans, crawfish or rice. Several fields had been in fallow, some for several years.
No clear relationship was found between varieties and the frequency of the disorder. It was most frequently observed in the varieties Clearfield 161, Cocodrie and Chenier, but that was likely because most of the acreage in the study area was planted with these varieties. The disorder also occurred in two fields planted with Wells, an Arkansas variety only distantly related to the popular Louisiana varieties. The disorder occurred in a number of fields treated with the NewPath system but also occurred in fields with no history of this herbicide. There was little likelihood that herbicide drift was responsible for the disorder in any of the fields examined.
The mystery malady appears to be caused by excessive iron uptake, possibly augmented by excessive uptake of aluminum and deficient levels of zinc and potassium. Draining fields as soon as symptoms appear is often effective in preventing spread of the disorder throughout the entire field. In fields with a history of the disorder, growers should consider delaying the flood as long as practical to allow sufficient growth to adequately oxidize the root environment. Ongoing research is focused on developing a soil test to identify susceptible fields and to develop management practices that prevent the onset of this disorder. We are also seeking treatments to arrest or offset the symptoms once they occur, though preliminary work suggests achieving this goal will be particularly challenging.(This article was published in the summer 2006 issue of Louisiana Agriculture.)