New publication available at:

http://coststudies.ucdavis.edu/files/2012/RiceSV2012.pdf

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 80^o F, by 2 pm we are close to 90^o F, and late in the afternoon temperatures can easily reach 100^o 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 95^o 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 104^o 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

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.