Below are two photographs of a couple of very similar weeds. I know it is well past the normal time for discussion of weeds or weed control, but it came up because I got a phone call about lack of control of Redstem that had been treated with Londax. The first question I asked was whether the plant was Redstem or Toothcup (also known as Rotala) because in my experience Redstem is much easier to control than Toothcup.
The two plants are closely related and very similar in appearnce. Both are succulent, have reddish stems, have opposite leaves in two planes, and produce purplish flowers in the leaf axils (where the leaf joins the stem). The easiest difference to see is the difference in the base of the leaves. Redstem has leaves that appear to surround the stem or at least bases that expand and meet at the stem. Toothcup has more traditional appearing leaves that have a distinct petiole (the stalk that supports the leaf).
Above is a photograph sent to me by B. D. Fontenot. I forwarded it to Dr. Groth who identified the disease as Black Kernel. It is a fairly common fungal disease generally regarded as a minor problem. For some reason this year there is quite a bit showing up and the rainy weather will probably make it worse. I noticed some in our field in Cameron parish but it was scattered and much lighter than shown in these panicles.
It should not be confused with Kernel Smut which also causes black kernels. The smut fungus actually invades the grain and replaces it becoming visible when it begins to ooze out between the hulls. Black Kernel attacks from the outside and covers the grain completely.
Dr. Groth said he has seen False Smut too. This one causes the development of very conspicuous greenish masses in the panicle. They change from greenish to bright orange with maturity.
With the amount of rice flowering for the past couple of weeks I was expecting to get more calls about the disease shown at right. There are several things to look at in the photograph that help us identify the disease. The base of the some of the pedicels (where that part of the plant joins the spikelet) are discolored. Note its discoloration compared to others that are green. Also take a look at the band of slightly darker area of some of the spikelets in comparison to those that are completely straw colored. Third diagnostic aids are the green panicle branches. What cannot be seen in the photograph is the absence of grain in the affected spikelets.
The disease is bacterial panicle blight. It is associated with high night time temperatures during flowering. Just as a reminder, no matter how much fungicide is used it will not control this disease. Fungicides control fungi NOT bacteria. We have nothing to control bacterial panicle blight yet. The only thing we can recommend is to plant a variety that is resistant or moderately susceptible or plant early to avoid flowering when night time temperatures have risen.
Dr. Linscombe took the photograph shown below in his plots on the Research Station. It looks like borer damage, but is severe rotten neck blast in the California variety M-206 which is very susceptible to blast. Dr. Groth said he now has rotten neck blast in M-202, CL162, and Rex. In the Field Notes supplement sent out May 21st, he reported he had leaf blast on CL151, CL152, CL162, CL182, CL261, Cypress, Jupiter, Rex, and Wells. So it is not totally surprising to see the rotten neck form on some of the same varieties.
It is clearly a year to use fungicides. As I said to Owen Taylor who writes Rice Fax, it is not a matter of whether to spray or not, but a matter of what to use, how much and when. If you need help with any of that give your local county agent a call and we will be glad to help.
Earlier this week Barrett Courville and I visited a field where quite a few plants had this type of injury. It is late planted, water seeded rice. While we did not identify the specific culprit it is some sort of insect damage. Barrett visited some other fields later that showed much the same injury and found billbugs in the field. (See Fields Notes editions of April 30, 2011 and July 30, 2007). It is possible that billbugs could cause this type of injury although it is usually associated with an insect with piercing sucking mouthparts like stink bugs and leaf hoppers.
Below I used a piece of paper and a pencil to demonstrate how the injury occurs. The rolled up sheet of paper with a pencil stuck through it is to simulate the young rice leaf rolled up in the whorl. The pencil represents the piercing mouthpart of an insect. When the leaf later emerges from the whorl and unfurls the injury points form a dotted line across the blade width. This weakens the blade frequently causing it to break along that line.
Following are a series of 4 photographs depicting various aspects of rice borers. The first photograph shows a rice stalk with the typical orangish lesion on the leaf sheath where a borer entered the plant. The eggs hatch and very small (about 1/16th inch long) borers move behind the leaf sheath where they begin to feed and eventually bore into the stem. This is the only time foliar insecticides are effective on them. Once they have bored into the stem we cannot get to them. Dermacor seed treatment is labeled and effective against borers.
The next photograph is of a mid-instar sugarcane borer, the borer that has become the most common borer in rice over the past several years. It is distinguished from the rice stalk borer and Mexican rice borer by the dark head capsule and dots on its back. This one was about an inch long.
The third photograph is a closeup of the throacic region of a rice stalk borer. Above the second and third legs there are two dots. They are the base of a hair (seta) like the others on the body, but because I shot the photograph head on you cannot see the hairs. This was taken with a camera so I know they are visible with a good hand lens. This is the definitive distinguishing characteristic to separate the rice stalk borer from the Mexican rice borer. The rice stalk borer has two setae above each leg while the Mexican rice borer will have only one above each leg.
The borer in the bottom photograph is a very young rice stalk borer that could easily be mistaken for a Mexican rice borer. Normally the head capsule of the rice stalk borer is dark. This one may have recently molted and the head is still very light colored. The lines on its back are fairly continuous in contrast to the Mexican rice borer which has lines resembling a series of dashes.
The plant shown below is exhibiting symptoms that could be the result of several things. The rust colored leaves can be associated with zinc deficiency (bronzing), localized decline or as in this case iron toxicity.
The next photograph shows heavy iron accumulation on debris under the water, a common site near water wells throughout much of the state.
One possible solution is to aerate the water as it enters the field as is done when pumping well water onto crawfish ponds. Even though the water looks crystal clear it has low oxygen levels and the iron in it is in a reduced form. According to Dr. Harrell reduced iron is more mobile; therefore it is likely to be taken up rapidly by the plants. If oxygen is added to the water much of the iron is converted from reduced to oxidized form which is less toxic and less likely to be absorbed by the plants.
There are indications that this could solve the problem because the problem decreases as it gets farther from the inlet. In many cases I have seen small paddies deliberately set up near the inlet to serve as a sacrificial area. Once water moves through this paddy the others are not affected or at least not as severely.
Keep in mind we have not conducted research to support this recommendation; we are relying on past experience and some knowledge of the behavior of iron.
The symptoms are very similar to Localized Decline because Localized Decline is also associated with high levels of iron and aluminum uptake by rice plants. The difference is that the exact reason this is happening is still undetermined. We do know high levels of zinc, especially where soil pH is high will correct the problem in some cases.
Shown below is a problem we see at least some of every year. It requires looking at both the roots and the panicle to determine the cause. We determined this to be 2,4-D injury caused not by incorrect timing, but because the herbicide was applied to a field with too little water to protect the crown of the plant. Another clue was greater injury to the tillers than to the main stalks which again points to the crown being hit by the herbicide. Uptake resulted in the formation of odd shaped adventitious roots and panicles emerging from the side of the flag leaf sheath. Some of these had significant enough damage to reduce seed formation; however most of the panicles were flowering normally in spite of the irregular exsertion. Even though 2,4-D is considered a broadleaf herbicide it can and does affect grasses like rice. I have seen similar injury on both corn and grain sorghum when 2,4-D was used over the top on them. How much yield might be affected will depend on how extensive the problem is throughout the field.