Nitrogen management can help control rice diseases and boost quality.
• By Bob Johnson •
Knowing how much nitrogen to apply to rice, and when, is the key to good yields and to managing diseases such as blast and stem rot. Too much fertilizer can encourage either of these important diseases, farm advisors say. Scaling back to just enough can make a large difference in bringing the crop back to health.
“I know of a couple fields where they were able to reduce stem rot quite a bit by adjusting their nitrogen,” said Luis Espino, University of California Cooperative Extension rice systems advisor.
Stem rot begins as black lesions at the water line on the leaves, which eventually die. The disease can also survive in the soil and crop residue.
Espino made his remarks on the role nutrient management can play in disease control as researchers discussed the benefits of targeting rice fertilizer applications during the UC’s 2021 virtual rice grower meeting.
“Excess nitrogen can also aggravate blast,” Espino said, referring to the fungal disease that has been widespread in Sacramento Valley rice fields the past two years. “Keep an eye on your fertility. Use as much nitrogen as you need to maximize your yield, but don’t apply any extra.”
Economical nitrogen evaluation
Researchers are developing new, relatively low-cost ways for farmers to evaluate whether they need to apply additional nitrogen at midseason.
“It can be difficult to eyeball whether you need a top-dress application,” said Bruce Linquist, UC Cooperative Extension rice specialist. “Do a nitrogen-rich test strip and use the GreenSeeker NDVI response index to help you make a decision.”
Many grain growers already use nitrogen-rich test strips.
They are relatively small strips within the field that receive unusually high fertilizer levels to let the farmer see whether the rest of the field would turn greener if it had enough nitrogen.
The GreenSeeker is a hand-held device that measures the normalized difference vegetative index in the nitrogen test strip and the rest of the field.
The response index is the ratio between those two numbers.
“We’re suggesting a 1.09 response index to make a top-dress application,” Linquist said. “That should give you about a 200-pound yield increase, which will pay for the top dress.”
Researchers are also looking closely at granular nitrogen sources that can be used when it is impractical to apply all the fertilizer before the field is flooded.
“In general, the later you applied the granular nitrogen or split it, the higher the yield you got,” Linquist said.
Although researchers caution that applying too much fertilizer in the middle of the season can cause problems, they also advise against applying all the nitrogen at the beginning of the season.
“When you apply all the nitrogen up front, there are a lot of ways you can lose it to the environment,” Linquist cautioned.
For every ton of rice yield, UC research shows there is an average of 22 pounds of nitrogen in the grain and an additional 13 pounds in the straw.
This detail in deciding how much nitrogen to apply, and when, is worth the trouble, the UC specialists said. It not only results in the best crop at the lowest fertilizer cost while maintaining environmental values, but it also helps manage important diseases.
Blast is a fungus that can cause kernel blanking. The pathogens then remain in the crop residue and ground, ready to infest the crop the next year.
“We saw a lot of blast in the northern end of the Sacramento Valley in 2020, in Glenn and Butte counties and the northern end of Colusa County,” Espino said. “We saw a lot of blast in 2019, too.”
Though precise nitrogen management is an important economical tool for managing blast, variety selection can also help, he said.
“M-210 is a blast-resistant variety,” Espino said. “M-210 is almost identical to M-206 genetically, so if you grow M-206, you know how M-210 is going to respond.”
In four years of UC trials, the blast-resistant M-210 out-yielded its familiar cousin M-206 by 28 to 180 pounds. But M-210 was more prone to lodging, which is not a prime virtue of the widely grown M-206 variety.
“M-206 lodged significantly more than M-105 across all nitrogen rates, and applying all the nitrogen up front did not increase lodging relative to split applications,” Linquist said.
Another strategy worth considering under high blast pressure, Espino said, is a fungicide application to knock down pathogen levels.
“If you see leaf blast, I would consider an application,” he advised.
Stem rot is another disease favored by too much nitrogen and less than the optimal 80 pounds of potassium per acre, according to Espino.
Fungicides can also knock down stem rot pressure, but nutrient management, residue management and the use of less susceptible varieties are the preferred methods of control, he said.
Bob Johnson is a reporter in Sacramento. He may be contacted at firstname.lastname@example.org. This article originally appeared in the California Farm Bureau Federation’s weekly publication, Ag Alert.