Straighthead is the oldest rice disease in Arkansas, observed since the early 1900s when rice was first grown here. It occurs only in certain fields and will reoccur each time those fields are in rice, although the amount of straighthead can vary according to variety planted, water management, and year.
Early on, farmers figured out that fields drained and dried for a time after the permanent flood had been established, then re-flooded prior to green ring, would not have much straighthead. Draining and drying became a standard practice on many fields.
Fields noted for having severe straighthead included new-ground fields just out of timber, old cotton fields and fields with lighter soils. Clay soils typically do not have problems with straighthead.
Straighthead is a serious disease with yield losses approaching 100 percent if a highly susceptible variety is planted on a severe straighthead soil and not drained and dried. In fact, in the past two years we have observed severe losses in Cocodrie fields that were not drained.
Straighthead symptoms include a panicle that remains straight because there is little or no filled grain to tip it over. Often, many rice kernels (flowers) will also be distorted with one glume reduced in size compared to the other and one or both glumes strongly curved at the tips, giving the flower a parrot-beak appearance.
Rice varieties vary as to the symptoms shown, since varieties also vary in susceptibility to straighthead. Medium grain rice varieties like Bengal often show the most parrot-beaking, while more-tolerant long grains may show no distortion but still have yield losses due to unfilled grains.
In general, the rice panicle is the only part of the plant noticeably affected with straighthead; however, other parts of the plant are affected to a much lesser degree.
While the leaves of straighthead plants tend to be the same size as normal plants, they are often a darker green through much of the season. This may be especially noticed at midseason, when straighthead fields do not yellow-up as they run out of early nitrogen like normal rice can. Of course, this is becoming less noticed as more nitrogen is now being applied preflood and less at midseason.
The cause of straighthead in commercial rice fields is not known. Because straighthead symptoms can be induced by adding high levels of arsenic to the soil in experimental plots, it is generally believed that a toxic element that the rice takes up under flooded (anaerobic) soil conditions is involved.
For this reason, straighthead in fields is found only in flooded paddies and often only in the more deeply flooded areas. Arsenic is used in research plots to evaluate susceptibility of new rice lines to straighthead.
Over the years, rice breeders have produced several more-tolerant varieties to straighthead, but draining and drying remains a staple management tool for many fields in Arkansas.
WITH THE advent of Roundup Ready soybeans in the mid 1990s and the resulting widespread use of glyphosate herbicide on them, it was noticed that rice fields near treated soybeans sometimes developed unexpectedly high levels of straighthead-like symptoms. Based on proximity to treated soybean fields, obvious drift patterns in the rice fields and similar symptoms on glyphosate-injured rice in weed control research plots, it became apparent that glyphosate from the soybean applications was the cause.
During this time, weed scientists worked out a general understanding that rice is very sensitive to very low rates of glyphosate when it is in the seedling stage, moderately tolerant in the vegetative stages, and sensitive again when the panicle was forming from midseason (0.5-inch internode elongation) until early heading.
Low drift rates of glyphosate can produce a lot of injury to the rice panicles while they are being formed. When glyphosate drift occurs, rice closer to Roundup Ready soybean fields is often much more affected than rice further out in the field due to it receiving a higher dose of glyphosate drift.
Symptoms of glyphosate-injured rice at heading differ from straightheaded rice in several ways. Panicles affected by glyphosate often are more compact and more severely distorted than in straighthead.
Distortion of the individual flowers will also often be more dramatic when injured by glyphosate than straighthead with extreme curling of the glumes and spikelet branches, rather than just parrot-beaking.
In severely affected glyphosate injury, the flag leaf will be reduced greatly in size, sometimes only 2 to 3 inches long, while the flag leaf in straightheaded rice will be normal in size and color.
The flag leaf in glyphosate-affected rice may also be somewhat yellowed or lighter green and can even be curled together or rolled somewhat. Plants can be stunted with glyphosate, especially near the drift source of the herbicide where a higher rate lands on them.
Twinning, where adventitious tillers emerge from the sides of the main tiller, can occur in both true straightheaded plants and in glyphosate-injured plants. Also, distorted panicles can be seen inside the developing boots in both cases.
With lower rates or later drift of glyphosate, panicle symptoms can mimic true straighthead more closely, and it may become difficult to distinguish the two conditions in some fields.
The following are some tips that may prove helpful in diagnosing most cases.
- True straighthead should occur only in the paddies. So look on top of the levees or outside the border levee. Symptoms in those dry areas indicate glyphosate injury.
- True straighthead does not follow a drift pattern. Drift patterns from a Roundup Ready soybean field nearby would indicate glyphosate injury. Look for areas protected from the direction of suspected drift to see if the rice is normal there.
- True straighthead does not affect the flag leaf size, color or shape. Glyphosate can, so look for a smaller-than-normal flag leaf on affected tillers.
- True straighthead does not affect all rice varieties the same. Widespread or severe symptoms on straighthead-tolerant rice varieties would indicate possible glyphosate involvement.
- True straighthead does not occur on all soil types. If there is no straighthead history in the field or the field has a clay soil, then glyphosate should be considered.
- True straighthead should not occur on a field that has been properly drained and dried. If symptoms show up anyway, check out the possibility of glyphosate injury.
While glyphosate injury to heading rice has added another wrinkle to our jobs, it remains a fairly minor problem to Arkansas rice because most growers and applicators learn quickly to be very careful around rice fields. Glyphosate is just like any other herbicide — it is a very effective and useful product when kept on target.
Bob Scott is the University of Arkansas Extension weed specialist. [email protected].
Rick Cartwright is an Extension plant pathologist with the University of Arkansas Division of Agriculture.