UC Cooperative Extension is conducting a survey of rice diseases this fall. The objective of the survey is to document the incidence of rice diseases in Sacramento Valley rice fields so that the Air Resources Board agrees to continue to allow the rice industry to burn up to 25% of the acreage. Every five years, the Board has to re-approve the straw burning program. One of the reasons for letting growers still burn is disease control. Therefore, it is important to show that diseases are still present in rice fields.
A crew of four to five will be stopping in random fields in the major rice producing counties and inspecting for the presence of diseases in 30 sites in each field. We will look for stem rot, aggregate sheath spot, kernel smut, and blast. The precise location of each field won't be disclosed, only the general location and County. So far, we have inspected 10 fields in Butte County, and have found very low levels of blast and kernel smut. Stem rot and aggregate sheath spot are more variable; we have found fields with very little of these diseases and fields with a lot.
I got a report from a PCA early in the week of blast infections detected near Maxwell. Blast can occur at any time during the development of the crop. When it infects leaves, it is called leaf blast. If it infects the base node of the panicle, it is called neck blast, if it is on the nodes of tillers, node blast, etc. In all cases, it is the same pathogen. Usually, the first visual indication of blast infection happens in nitrogen overlap areas in the form of dead circular spots.
If you inspect these areas closely, the plants in the center are burned to the water, and the typical diamond shaped lesions can be found in the leaves of surrounding plants.
We do not recommend treating leaf blast. But presence of leaf blast is a good indication that a fungicide treatment will be needed once the crop starts heading.
Blast is a multi-cycle disease, meaning that infections produce spores that then can cause more infections later. After an infection occurs, lesions form after 14 days, and 3 days later new spores are formed. These spores can then repeat the cycle. This is why it is important to protect the panicles if leaf blast is present in a field or surrounding fields. Time your fungicide application anywhere from mid boot to panicle emergence.
All our varieties are susceptible to blast, with the exception of the new M-210. M-206 is considered less susceptible than M-104 and M-205. We have not seen the response of M-105 and M-209 in the field yet, but I suspect they are very susceptible based on their background.
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./table>
Our two year stretch of little to no blast might be over. I visited an M-209 field last week that seemed to be infected with blast. The lesions appeared to me like propanil drift burn; however, the yellow halo around the lesions made the PCA suspicious. Paul Sanchez, pathologist at the Rice Experiment Station, collected some leaf samples and was able to confirm it is blast.
According to Paul, changes in environmental conditions may be why the lesions did not look like your typical blast lesions. When conditions are favorable, the fungus' spores germinate and infect the tissue. But if conditions change and become unfavorable for the fungus growth and sporulation, the fungus dies and the tissue then has that burned look instead of the white-gray powdery look.
Some of the lesions did look more like the typical blast lesion.
M-209 is a variety closely related to M-205, a variety considered to be susceptible to blast. Since we have not seen blast in the field since M-209 was released, we do not know what level of susceptibility M-209 has. This might be the year we find out.
At this point, a fungicide treatment is not recommended. However, a treatment near heading may be appropriate considering that the field and area have a history of blast epidemics./table>
- Author: Christopher A Greer
It is still too early to know if this is going to be a year with high incidence and severity of rice blast. The most favorable conditions for sporulation, spore germination and infection ofplant tissue by the blast fungusinclude high relative humidity, free moisture on the plant tissue surface and temperatures around 82o F. Temperatures cooler or warmer than this slow down disease development but do not prohibitit. As the season progresses, watch out for mild temperatures, calm mornings and foggy orovercast skies that favor extended free moisture periods, all conditions that promote blast development.
As I have mentioned in the past, there are several factors that may predispose rice plants to infection by the rice blast fungus. First and foremost is the inherent resistance of aspecific rice variety. Our California rice varieties do differ in their tolerance to infection bythe pathogen. M-104 and M-205 appear to be the least tolerant of the most widely grown commercialvarieties while M-202 and M-206 are somewhat more tolerant. M-208 is the only commerciallyavailable rice variety in California with a specific resistance gene to race IG-1 of the blastpathogen. IG-1 was the only race of this pathogen known to exist in California until recently.Unfortunately, confirmed cases of limited leaf and neck blast inM-208 fields in recent years indicate that a new race of the pathogenhas evolved through mutation or has been introduced into California. M-208 is still resistant torace IG-1 but is not resistant to this new race.
I am more convinced than ever that water management plays a critical role in rice blast diseasemanagement. Not only does field drainage increase the risk of disease transmission fromseed to seedling but any practice which leads to aerobic conditions within the soil predisposesrice plants to rice blast disease. Drill seeding and draining for stand establishment or herbicideapplications that require a drain in water seeded systems increase the risk of infection and plant susceptibility torice blast. Additionally, rice plants grown in deeper water exhibit increased tolerance to thedisease over those grown in shallower water depths. This is apparent where we often see localizedincreased disease severities associated withhigh spots within a field or prolonged periods of field drainage. From an irrigation standpoint, maintaining a deep continuous flood isthe bestoption for minimizing the risk associated with rice blast disease.
Rice blast is a very complex disease that has the ability to increase in incidence and severityvery rapidly under favorable conditions. Growers should consult with their pest control adviser todetermine if a fungicide application(s) should be made to protect developing panicles as theyemerge from the boot.
Rice field showing severe blast