October 15, 1996

In This Issue:

BLAST - New Rice Disease Found

BLAST - More About The Disease

Choosing The Best Wheat Variety For Your Field

BLAST - New Rice Disease Found

Blast, the most important disease of rice worldwide, was recently found on California rice for the first time.

On August 30 we found diseased rice plants with typical Blast symptoms in three Glenn County fields on two separate farms. Subsequent isolation from these plants made at U.C. Davis recovered the fungus, Pyricularia grisea, which is known to cause Blast. Isolation of the Rice Blast fungus provided the initial confirmation of the disease.

Since these initial finds, U.C. researchers have surveyed over 500 Sacramento Valley rice fields to assess the distribution and extent of the problem. To-date, the disease has been confirmed in a total of 33 fields. Of the 33 fields, 27 are from Glenn County and 6 are from Colusa County. The disease has not been found in other Counties at this writing.

We have observed the disease on many California varieties including, M-201, M-202, M-204, M-103, M-401, S-102, L-204, Calmochi-101 and other proprietary varieties as well. These observations coupled with other experience tell us that our varieties are susceptible to this disease. Several of the diseased fields had portions that were severely affected, but most where moderately to slightly affected.

University of California researchers are continuing to study the disease to find ways of controlling this new problem. More about the disease follows.

BLAST - More About the Disease

Blast, an important fungal disease of rice, occurs in most rice producing areas of the world including Asia, Europe and in the Southern United States. The disease is typically more of a problem in areas where intensive high-input production practices are used. The disease is caused by the fungus Pyricularia grisea.

Disease Cycle

The fungus can over winter in diseased crop residue or in seed. Seed is considered an important primary source for the fungus especially in temperate areas where only one rice crop is grown per year. Straw management practices are also believed to be an important factor in pathogen carry over from season to season. Greenhouse studies have shown that many grasses serve as hosts for the pathogen, but it is not completely clear what role weeds may play in nature.

Leaf wetness from dew or other sources is needed for the fungus to infect rice. Temperature also impacts infection. At 60 ° F, the fungus needs 16 - 20 hours of leaf wetness to infect, but at 76 ° F it only needs 8 - 11 hours of wetness. Warmer temperatures also speed the development of lesions on rice. Spore production from diseased tissue increases with increasing relative humidity above 93%; no spore production occurs when the humidity is below 89%. Warm nighttime temperatures coupled with high humidity favor spore formation. Spores can be produced on an ongoing basis and are dispersed by the prevailing winds.

Warm night time temperatures, high humidity and long dew periods enhance disease development. This year's warm, humid weather appeared to favor the disease. We believe that more normal weather patterns will be less favorable for disease development.

Symptoms

The pathogen is known to infect all parts of the shoot, but usually not the leaf sheaths. Leaf infections also occur.

To-date the most commonly observed infections have been at the panicle neck node (just below the panicle) and the flag leaf collar (at the junction of the leaf blade and sheath). Infections of the panicle branches and lower stem nodes have also been observed. Older lesions at the leaf collar and nodes are usually brownish, while some node lesions are blackish in color. New developing lesions may be lighter in color.

Neck blast infections can be very damaging, especially if it occurs before the panicle begins to fill. Early neck infections can cause the panicle to die, turn white and leave a completely blank panicle. Later infections may cause incomplete grain fill. Collar blast infections can kill the entire leaf blade.

Control

Control measures commonly used include disease resistance, cultural practices and fungicides. Since resistant varieties will not be immediately available we will need to rely on cultural approaches and work toward registering a fungicide.

On the cultural practice side the disease is more severe when high nitrogen rates are used. In one field, we observed this first hand. The disease was much more severe in spots where excess nitrogen was applied (spill or overlap). It is clear that excessively high nitrogen rates should be avoided. The nitrogen form and timing of application can also affect disease severity. Plants utilizing ammonium nitrogen are less affected than those using nitrate. Because drainage enhances ammonium to nitrate conversion, extended drain periods should be avoided. Split applications minimize disease better than single applications. Also avoid drought stress since this is known to increase severity.

Because the disease can carry over on seed we are working on seed treatments to control primary infections from seed. We will have more to say about this later.

With regard to straw burning, diseased fields should be burned to reduce the season to season carry-over fungal population on infected crop residues. While burning will reduce the blast fungus population and help to control the disease, it will not provide complete control. This is because the fungus can carry-over on seed and other hosts, has multiple spore cycles and it's spores are dispersed by prevailing winds. We feel that burning is an important component of an overall control program that should be used in combination with all available control strategies.

We are currently trying to determined the pathogen race(s) that are present in California to determine which genes for resistance would be best to incorporate into our varieties. Resistance is probably the most effective and inexpensive way to control this disease.

Choosing The Best Wheat Variety For Your Field

Doug Munier, Jack Williams, Tom Kearney, Kent Brittan, Cass Mutters, and Steve Scardaci

Selecting the best wheat variety is one of the most important decisions made in wheat production in the Sacramento Valley. There are many varieties to choose from, but two stand out from the rest for different reasons.

Have a look at the results in Table 1. These two strip trials in Yolo and Solano counties were the only 1996 UC county trials for the Sacramento Valley. All other UC trials were destroyed because of Karnal Bunt (KB) spore contamination in the planting seed of a few varieties, although no KB disease was seen in any of the trials. Look also at the summaries in Table 2. Our UC-CE program had five Sacramento Valley wheat variety trials in 1995 and four each in 1994 and 1993. Even with the loss of most trials in 1996, the results of 15 replicated wheat variety trials for the Sacramento Valley from 1993 to 1996 can be used to make variety decisions for the 1997 crop.

After reviewing these results, you can see that there are two clear variety choices. If you're growing wheat for maximum yields and not trying to get bread quality, then RSI-5 is the best choice. As you can see in Table 2, RSI-5 was the top yielder out of a 14 to 29 different varieties, including many experimentals, in Valleywide tests. In 1995 at the Colusa Regional Trial, RSI-5 yielded 8100 lbs./acre compared to 5480 lbs./acre for Yolo. If you're after bread wheat quality, then Express is the best option for the Sacramento Valley. Express also yielded well in the Colusa trial at 6860 lbs./acre. These two varieties are relatively new, but have now been tested extensively over a wide range of conditions for several years.

Both RSI-5 and Express have excellent tolerance to the area's major diseases, including stripe rust, leaf rust and septoria tritici blotch. We have not had a stripe rust problem for many years because most varieties are resistant; however, we see the disease in our trials, so it remains important. Septoria is with us whenever we have a wet winter and can be very damaging on Klasic, Serra, Anza and Yolo. Leaf rust is becoming more prominent her, but particularly in the San Joaquin Valley, where UC-CE Specialist Lee Jackson estimated greater than 10% yield loss in 1995-96. At this level, fungicide treatments should be economic.

Some shattering occurred on RSI-5 and Express in the two Yolo/Solano County trials, but not on the order of Yecora Rojo which can shatter badly in windy areas. Express may benefit from extra nitrogen at flowering to increase protein. RSI-5 generally goes to the same markets as Anza and Yolo, and flowering application of nitrogen probably won't increase its market value. Don't use Avenge™ herbicide on RSI-5, it is sensitive.

A UC experimental variety, UC 1041, is being considered for release as a new variety by Dr. Calvin Qualset at UC Davis. This variety is particularly exciting because of its short plant height, excellent disease resistance and good potential to make bread quality. The short plant type may allow for more intensive management and even higher yields. If you are interested in this new variety, you may want to send a word of encouragement to Dr. Qualset, Genetic Resources Conservation Program, UC Davis, CA 95616-8602.

Modified: 10 Sep 1998 Comments to jayoung@ucdavis.edu