The 2017 season kicked off with much fanfare regarding weedy rice. Thanks to the vigilance of the entire rice industry, the UCCE Rice Advisors received many calls regarding weedy rice, starting in late June, as growers finished their herbicide applications. Calls continued to come in through July and August. The California Crop Improvement Association (CCIA) began inspecting fields as the rice headed, and suspected plants were pulled and sent to the UCCE Weedy Rice Team for genetic testing.

By the end of the season, we had a total of:

• 53 samples submitted for testing
• Out of the 53, 15 have been confirmed to be weedy rice
• 7 are still pending genetic testing

Eight seed fields were found to be infested with weedy rice and rejected as seed fields:

• 3 were new medium grain seed fields
• 1 was an established medium grain seed field
• 4 were specialty variety seed fields

Commonly Confused with Weedy Rice

There were many calls throughout the season, which indicated that everyone was out inspecting their fields. Thankfully, many of the calls were not weedy rice!

Some commonly things that can be mistaken for weedy rice:

1)     Sprangletop: like rice, it has a ligule, so early in the season, before heading, it may be easy to confuse it with weedy rice. However, sprangletop has a white stripe down the middle of the leaf (mid-vein).

2)     Elongated Upper Internode (EUI): this is a genetic abnormality of common medium grain rice varieties that causes the part of the stem attached to the rice panicle to elongate. The panicles stick up above the canopy, just like weedy rice. However, the rest of the plant will look just like the variety planted in the field.

3)     Bakanae: this disease of rice causes the plants to elongate and appear taller than the surrounding plants, and they also appear lighter in color. However, any panicles produced by the infected plants will be blanks.

4)     Fertility Differences: if the field has more or less nitrogen in certain areas, some of the rice plants may appear lighter in color than others.

Elongated upper internode (left) and sprangletop (right) can be confused with weedy rice plants.

New Information for Management

We have been working on characterizing some of the biological characteristics of the weedy rice populations. We made considerable progress over the summer, and are happy to report that the dormancy and shattering status of each of the five populations is now known.  

For images of all the weedy rice types found in California, go to www.caweedyrice.com

Implications for Management

All weedy rice types found in California so far are high shattering, which means that many of the seeds will fall off the panicle before harvest. Therefore, it is critical to remove plants from the field before they can shatter completely. Any seeds shattering on the soil surface will have the potential to be deposited into the soil seedbank, lengthening the amount of time weedy rice will be infesting the field.

Three of the types found in California have high dormancy: Type 1, Type 3, and Type 4. High dormancy means that once the seeds are in the soil, they will remain there for a long period of time without germinating, even if the grower is doing everything possible to get the plants to germinate so that they can be killed. Two of the types have low dormancy: Type 2 and Type 5. These types will readily germinate the following spring if they are close to the soil surface, so they can be more easily eradicated from a field, if a grower follows all best management practices.

Summary: We Are Working Together!

Overall, grower and PCA participation in scouting for weedy rice was really high in 2017. Since we can only get rid of it if we know that it is there, this is very encouraging, and we hope that the participation continues into the future. Likewise, those growers that already know they have weedy rice infestations are working hard to eradicate it, following the Best Management Practices. Again, this is very encouraging, as the only way that we can eradicate this pest is as a group, working together.

In the past two years, I have received several reports of fields suffering yield loses due to stem rot. Last year I saw several affected fields. This year, I am starting to get reports of fields being affected. I visited one such field last week.

After the water was drained, plants in the affected area seemed to burn down and dry much quicker than the rest of the field. Much of the rice was down. 

Inspection of the tillers showed outer lesions that were already dry. However, when cutting open the tillers, the inner sheaths and main culms were rotted. In some cases, mycelium and sclerotia, the resting state of the fungus, could be seen growing inside of the culm. Most panicles were partially filled. This field most likely will suffer a yield loss.

Stem rot infections start at the water line. Surviving sclerotia from previous seasons float to the surface of the water and infect plants during tillering when conditions are favorable. Infected young tillers may die; later infections on older tillers can reduce panicle size and grain quality, and increase lodging. 

Management of this disease should incorporate several tactics, probably the most important is residue management. The severity of this disease has been shown to be related to the amount of inoculum present in the field. As the number of viable sclerotia in the seedbed at the beginning of the season increases, the severity of the disease late in the season increases. Straw burning is the most effective way to reduce the amount of inoculum in the field, but is not always feasible. Experiments have shown that sclerotia survive better if they are inside plant tissue, therefore chopping and incorporating the residue to promote decomposition can help reduce the amount of inoculum. If baling, cutting the straw as close to the ground as possible will remove inoculum from the field.

Nitrogen has been shown to affect stem rot. Excess levels of N can increase stem rot severity. In fields with stem rot problems, adjusting the N rate can help reduce the impact of the disease.

Fungicide trials conducted in 2012 and 2013 seem to indicate that currently registered fungicides have an effect on stem rot incidence and severity. Further trials are being conducted this year to confirm those results.

I was evaluating armyworm injury in plots and noticed there was quite a bit of panicle blanking not caused by armyworms. When armyworms injury panicles, they feed on the rachis of panicle branches, causing those branches to dry out. Sometimes the kernels in those branches may be partially filled, but since they stop receiving nutrients and water due to the biting injury of the armyworm, they dry and turn straw colored. Most of the time, the branches brake off at the point of injury and can be seen hanging on the panicle. If you look closely at where the rachis is broken, you can see the biting mark of the armyworm.

The other blanking I was noticing was a little different. It was mostly on the panicles under the canopy, although some could be seen in the panicles on top. Blanked branches turned white, almost as if bleached, and kind of translucent. This  might be because these panicles were young. Most likely they will dry out and also turn straw color. None of these injured branches were hanging on the panicle and no biting marks could be noticed.  I suspect this blanking was caused by high temperatures. Temperatures above 104 F during flowering dry the germinating pollen tube before fertilization and cause blanking. It seemed that some areas in the field were more affected than others. 

Going back to armyworms, I found very little injury in my Butte County trial. Two trials in Colusa had no injury. The number of armyworm moths caught in the traps as of 8/24 have come down again. I suspect we won't see the numbers climb up again, therefore the risk of armyworm injury now is very low.

Click on the images below to see a close up of armyworm injury (left) and high temperature blanking (right).

The 2nd Annual Rice Weed Course will take place: 

Friday, September 15, 2017

from 8:30AM to 4:25PM (Registration begins at 7:30AM)

at the 

Hamilton Road Field (on West Hamilton Rd. between Hwy. 99 & Riceton Hwy.)

and Rice Experiment Station, Biggs, CA

The day will begin with an interactive tour of the Weed Science research plots at Hamilton Road. Participants will also spend time learning about weed identification for important rice weeds both at emergence and at heading (including identification of weedy rice).Presentations will cover herbicide resistance management and the latest information on weedy red rice. Attendees will also hear an update about rice pesticides in California. This course is a great opportunity to interact directly with the UCCE and UC Davis Rice Weed Research Team! 

For a full agenda and registration go to: 

http://wric.ucdavis.edu/events/rice_weed_course_2017.html

For questions, please contact Whitney Brim-DeForest at 530-822-7515, or by email at wbrimdeforest@ucanr.edu 

Credits for PCA, QAC, QAL, Private Applicator, and CA Certified Crop Adviser are pending.