- Author: Luis Espino
Research conducted by Albert Fischer and James Eckert, Department of Plant Sciences, UC Davis, has confirmed that some populations of smallflower umbrellasedge from rice fields in the Sacramento Valley have become resistant to propanil. Following is a summary of their findings.
Smallflower umbrellasedge seed from eight fields where resistance was suspected was collected and germinated in the greenhouse. Plants were grown in pots and, at the 2-3 or 4-5 leaf stage they were sprayed with SuperWham, UltraStam or RiceShot at half field rate, recommended field rate, and twice field rate. Both spray timings produced similar results, but the earlier application produced the most striking results.
Table 1 shows the percent control of smallflower umbrellasedge when sprayed with 3 formulations of propanil. For simplicity, I'm showing only the results of the application with 6 lbs a.i./a. Percent control of smallflower umbrellasedge from fields 4, 6 and 8 was very poor, between 6 and 45%. Control of smallflower umbrellasedge from field 3 was mediocre, between 62 and 74%. And control of smallflower umbrellasedge from fields 1, 2, 5 and 7 was very good, between 68 and 100%.
Table 1. Percent control of smallflower umbrellasedge with 3 formulations of propanil applied at 6 lbs a.i./a.
Smallflower umbrellasedge from fields 1, 2, 5 and 7 can be considered susceptible to the formulations of propanil applied. They were tested because control failure had been observed in the field. Results from the greenhouse tests suggest that something went wrong with the field application. For example, the application could have been made too late, coverage may have not been appropriate, there could have been incompatibility in the tank mix, wrong application rates might have been used, etc. Populations from fields 3, 4, 6 and 8 can be considered resistant. This prompts the question, what are the options to control these populations?
To answer this question, another set of greenhouse tests were conducted to evaluate herbicide options for propanil-resistant smallflower umbrellasedge. Results (Table 2) show that propanil-resistant smallflower umbrellasedge was also resistant to Londax, resistant or partially resistant to Granite SC and Sandea, and susceptible to Shark H2O when applied as a foliar.
Table 2. Percent control of smallflower umbrellasedge with alternative herbicides.
Results suggest that in fields with propanil-resistant smallflower umbrellasedge, Shark H2O could be used to achieve control. In fields where resistance is not a problem yet, the best approach is to alternate modes of action whenever possible. If you suspect you have propanil-resistant smallflower umbrellasedge, collect seeds at the end of the season and take them to the Weed Science project at the Rice Experiment Station in Biggs for screening.
- Author: Luis Espino
While scouting for blast, a PCA noticed some yellow leaf tips. Close inspection revealed aphids. He got a little worried since he's never seen aphids on rice during his long career. They turned out to be yellow sugarcane aphid. These aphids are common in small grains like wheat and barley, and in corn, so it is not surprising that they can be found feeding on rice. Actually, they are considered a pest in southern rice, where they feed on rice during the seedling stage.
Photo by Bart Drees, Texas A&M University
We went to the field to see how prevalent they were. There were some yellow leaves above the canopy level. These were mostly from bakanae infected plants. There were some yellow leaves here and there at the canopy level, on these we found aphids.
Yellow sugarcane aphid injects a toxin into rice plants during feeding that causes the foliage to become reddish.
In California, aphids are not considered a pest of rice. Most likely, they are present in some rice fields some years at very low levels, and mostly go unnoticed. The infestation in this field was very low, and probably the aphids will disappear in a few days. Nothing to worry about - just something to be aware of.
While looking for the aphids, we noticed some feeding scars that, at first, looked like rice water weevil scars. After looking closely, we realized these were caused by something else. After some search, we found quite a few beetles feeding on the rice next to the road.
I haven't identified the beetles yet, but they look and jump like flea beetles. The field is next to some unmanaged vegetation, and most likely their food source is drying off, so they are looking for some green material. I think this is just an "incidental" infestation. Again, nothing to worry about - just something to be aware of.
- Author: Luis Espino
In the past few days we've seen some unusual high temperatures in the Sacramento Valley, and looks like the hot weather is going to continue for a few more days. By mid morning temperatures are reaching 80o F, by 2 pm we are close to 90o F, and late in the afternoon temperatures can easily reach 100o F or more. Rice at the moment is heading (40% headed by August 12, according the USDA's Statistics Service report), putting some rice at risk of high-temperature sterility.
Anthesis (the opening of the flowers or spikelets) is the most susceptible stage of rice to high temperature damage. Very high temperatures cause indehiscence of anthers (anthers not opening), reduce pollen production and viability, and can dry the germinating pollen tube before fertilization occurs. In rice, spikelet sterility by heat occurs during and up to 3 hours after anthesis. Once fertilization is complete, spikelet sterility does not occur. Generally, anthesis is believed to occur between 9:00 am and 2:00 pm. In one study*, anthesis in M-202 was found to occur between 9:30 and 10:30 am. Given these times, rice would be susceptible to high temperature spikelet sterility from morning to mid afternoon.
In the above referenced study, when the average daytime temperature during anthesis was 95o F, spikelet sterility increased by more than 70%. Unfortunately, the study did not indicate the temperatures right at anthesis, just the daytime average. It is believed that temperatures should be above 104o F during anthesis to cause sterility. We had a couple of days last week when temperatures reached this level, putting rice at risk of spikelet sterility. We'll have to wait to evaluate if this actually happened.
*Prasad, P. V. V., K. J. Boote, L. H. Allen Jr., J. E. Sheehy, and J. M. G. Thomas. 2006. Species, ecotype and cultivar differences in spikelet fertility and harvest index of rice in response to high temperature stress. Field Crops Research 95: 398-411
