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Hot weather accelerates rice heading

The unbearable hot spell that hit California this summer affected all crops and with rice the impact has been several varieties heading earlier than normal, according to Cass Mutters, UCCE farm advisor for Butte, Sutter and Yuba counties.

Mutter said plant development is about 10 to 14 days ahead of schedule. “I have seen fields of early Calrose varieties (M-202, M-204, M-205) begin heading by 70 days after planting and very early varieties (M-104) start heading by 65 days after planting,” said Mutters. “Although the response to the high temperature is widespread, the accelerated growth appears to be more pronounced in the later planted fields.”

A comparison of degree day accumulation from May 15 to July 26 shows that 2006 has been warmer thus far than 2004 and 2005.

Using a threshold of 58 degrees, DD accumulation for 2006 rice planted on May 15 is 169 units ahead of 2005 and 172 units ahead of 2004. Lower yields are often associated with shortened periods of vegetative growth.

Mutters said another potential consequence of the weather is pollen desiccation. Plants flowering during hot spells may be more susceptible to high-temperature blanking due to the pollen drying out before fertilization takes place. In addition, more lodging frequently occurs in years when high temperatures coincide with internode elongation resulting in tall plants with weaker straw strength.

Warm night temperatures and high humidity over the past couple of weeks may be favorable for rice blast. ”Keep an eye out for rice blast lesions if you are growing in an area that has a history of rice blast. Pay particular attention to M-I04 and M-205. Fields that have excessive nitrogen or have been subjected to drought stress by prolonged drain times for herbicide applications will be at greater risk.

The farm advisor added very hot temperatures that have occurred during the day are not favorable for disease development. “However, the overnight conditions we have been experiencing (in mid-August) are almost ideal for rice blast development,”

The essential processes for rice blast disease development (sporulation, germination and infection) all require periods of high relative humidity and leaf wetness. The time taken to complete each of these processes is dependent upon the temperature during the leaf wetness period. The optimum temperature for these processes is around 82 degrees, said Mutters. Temperatures higher or lower than this optimum temperature slow the progress of the infection process.


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