CCN-98-8A

Possible Problems with Roundup Ready Cotton

Keith Edmisten - Extension Cotton Specialist

Most folks have heard of the fruit abortion problems with Roundup Ready Cotton found in Mississippi last year. Unfortunately we have found some fields this year that seem to have similar retention problems. I will try to describe the symptoms we have found in several fields this year. I do not want to alarm everyone as all fields do not show problems. However I do think it would be prudent to closely check Roundup Ready fields for fruit retention. The farmers with this problem are not poor farmers but rather very good cotton farmers which is probably why they have noticed the problem. One area with problems has had drought stress while the other area has not. Therefore the problem does not seem to be associated with stress.

The majority of the fruit on the lower 4 to 6 nodes of the plant has aborted, especially on first positions. Figure one shows a plant from a problem field with the leaves removed to show poor fruit retention. The scars associated with this abortion are large indicating that plant bugs did not cause the problem. These larger scars also suggest that the fruit aborted as a boll as opposed to a small square as you would see with plant bug damage. In the fields I looked at there was some damage due to plant bugs but it was minimal and would not have reduced yields. In some fields small bolls that have just bloomed are falling of the plant and appear to be abnormal. This does not appear to be due to stress as we have seen it in well watered field with essentially no boll load. If there was a boll load and stress this might be explainable.

Figure 1. Cotton plant from a problem field with the leaves removed to show poor fruit retention.

Figure 2 shows some of the small bolls that were still on the plant or had aborted recently and some normal small bolls. The small bolls in the fields exhibiting damage had black looking seed and were not filling. This appears to be due to lack of pollination.

Figure 2. The two bolls on the outside came from a field with poor fruit retention and appear to have pollination problems. The two bolls in the middle are normal bolls from a Roundup Ready field that had no fruit retention problems.

In most of the damaged fields you can find a higher than normal level of hawk-billed bolls. In fact in some fields the only bolls retained on the plant with much age at all are mal shaped. Figure 3 shows some hawk-billed bolls. This problem is most likely caused by incomplete pollination.

Figure 3. Hawk-billed shaped bolls caused by incomplete pollination.

Figure 4 shows a cross section of boll with incomplete pollination. The seed counts in the part locks of the boll with any developing lint are lower than usual. You can see that some locks have no viable seed and therefore no developing lint.

Figure 4. Cross-cut view of incomplete pollination.

There have been reports that problems with cotton in Mississippi this year were due to cavitation. There some cavitation in the problem fields I looked at that had been under stress but cavitation did not account for the major portion of the fruit loss. This problem is not simply a cavitation problem as problem fields with no stress have little or no cavitation. Figure 5 shows a boll that has cavitation symptoms. Notice the scar along the fruiting branch running from the peduncle of the affected boll down towards the main stem. Cavitation is caused by a "break" in the xylem water column whereby the transpiration can no longer pull water into the boll. You usually see cavitation on small bolls shortly after bloom. Cavitated bolls turn brown and dry. They stick on the plant as there is not enough time to develop an abscission layer.

Figure 5. Cavitation symptoms.

Now I realize that it is natural that some folks will want to blame everything under the sun on Roundup Ready. There has been a lot of fruit shed due to dry weather this year. The difference is that natural shed due to stress does not occur on the bottom fruiting branches. Even in extremely dry conditions we will normally set 3 or 4 bolls on the first positions of the lower fruiting branches and then abort the fruit above those branches. The problem fields I have looked at have almost no fruit on the 1^st and 2^nd positions of the lower fruiting branches. In fact the only fruit some of these fields have are squares, a few misshaped bolls, and small bolls that may still abort.

Some folks will go look at their cotton and say I have that problem too - no fruit on the bottom of the plant. I have looked at several fields where this claim had been make that had no problem whatsoever. The cotton was leggy due to lack of any pre herbicides and warm weather. The cotton started fruiting on the 6^th or 7^th nodes and due to dry weather started stacking internodes closely together. This does appear like a problem with early fruit retention but is not. You should be able to see big scars if small bolls were aborted after a failure in pollination.

How long will fruit abortion last? Last year in Mississippi the problem usually lasted for 4 to 6 nodes and then the cotton started retaining fruit. With the late fall many growers made a respectable crop. In some of the fields I looked at it appeared that the white flowers had a pretty good amount of Pollen in them. Based on this I would expect that they would start keeping fruit. However in some fields the smallest bolls that had not shed appeared to have pollination problems. I could not tell if the white blooms had sufficient pollen as it was wet and hard to look at pollen. If you shake a white bloom on your hand you should be able to see pollen.

If you suspect you have problems look at other fields (preferably non Roundup Ready) for comparison. On one farm I saw tremendous problems in some fields of the Paymaster 1220 Roundup Ready but none on the stacked 1220. Careful mapping of questionable fields and near by normal fields would be helpful in terms of determining the problem and the extent of the problem. Mapping should include notations of large versus small scars, misshaped bolls, bolls that have cavitation, and small bolls with black seed where no lint is developing.

I sincerely hope this problem is not too widespread. This is another example of what can happen when varieties are "hurried" to the market and growers put to many eggs in one basket by adopting varieties too quickly and on too large a scale. There is nothing that can be done to help this cotton that I know of. I have had growers already tell me they are already booking seed for next year. I would hold of on booking Roundup Ready seed at this point until we can determine how widespread this problem is.