Disease Management in California

Disease Management in California

There are four main diseases that affect rice in California: blast, stem rot, smut, and aggregate sheath spot. I listed them from more to less important.

While blast does not occur every year, it can be very damaging when it does. It is difficult to predict when an epidemic will happen, but I suspect that years with high relative humidity and warm nights are more favorable. Last year was one of those years. Blast was prevalent in the northwest part of the Valley, but there were reports from south Sutter County as well. Some dry-seeded fields were affected severely. The yield reduction potential of blast is high; when the disease is severe, losses can be higher than 25%.

Stem rot occurs in all regions and is present in all fields. Over the years, some fields build up inoculum and end up having more severe stem rot. Over many trials, I have found that stem rot can reduce yield up to 12%.

Like blast, smut severity fluctuates every year. In 2018, we had a severe epidemic. I think the overcast and humid conditions we had during heading because of fires on the west side produced favorable conditions for smut. Smut is probably the most difficult disease to manage because there are no signs or symptoms until grain maturity, and at that point, it is too late to do anything. When smut incidence is high, it can compromise quality and yield.

Aggregate sheath is mostly related to potassium deficiency. Over the years, I have seen some affected fields, but typically, the disease does not result in a drastic yield reduction like blast or stem rot.

Several practices can help reduce the incidence of all these diseases. Start with making sure you get a good stand, not too thin but also not too thick. Thick stands may result in more tiller diseases because of the high humidity under the canopy. Avoid excess nitrogen; only put a nitrogen topdress if needed. Excess nitrogen favors blast, stem rot, and smut. Also, check potassium levels in your fields. Potassium deficiency can increase the incidence of stem rot and aggregate sheath spot. If you have been baling, this is very important because a lot of potassium is removed with the straw.

Keep the field flooded; draining at any time increases the risk of blast. M-210 is the only variety resistant to blast. There are no varieties resistant to stem rot or smut, but varieties differ in their susceptibility. Varieties with longer developmental times like M-211 and M-209 are less susceptible to stem rot than varieties with shorter developmental times like M-105 or CM-101. Long grains are more susceptible to smut than medium or short grains; among the medium grains, M-209 is the most susceptible.

Finally, consider a fungicide application when appropriate. Azoxystrobin (the active ingredient in Quadris) can reduce the incidence of blast, stem rot, and aggregate sheath spot when applied from mid-boot to very early heading. If smut is a concern, use propiconazole (the active ingredient in Tilt) at mid-boot.

Rice Blast

Rice blast is caused by the fungus Pyricularia grisea. The disease is also called leaf blast, node blast, panicle blast, collar blast, and rotten-neck blast, depending on the portion of the plant affected. Blast has been one of the most important diseases in Louisiana, causing severe yield losses to susceptible varieties under favorable environmental conditions. Blast can be found on the rice plant from the seedling stage to maturity.

The leaf blast phase occurs between the seedling and late tillering stages. Spots on leaves start as small white, gray, or blue tinged spots that enlarge quickly under moist conditions to either oval or diamond-shaped spots or linear lesions with pointed ends with gray or white centers and narrow brown borders. Leaves and whole plants are often killed under severe conditions. Lesions on resistant plants are small brown specks that do not enlarge. On stem nodes, the host tissue turns black and becomes shriveled and gray as the plant approaches maturity.  Culms and leaves become straw-colored above the infected node. Plants lodge or break off at the infected point, or they are connected only by a few vascular strands.

Rotten-neck symptoms appear at the base of the panicle starting at the node. The tissue turns brown to chocolate brown and shrivels, causing the stem to snap and lodge. If the panicle does not fall off, it may turn white to gray, or the florets that do not fill will turn gray. Panicle branches and stems of florets also have gray-brown lesions.

Scouting a field for blast should begin early in the season during the vegetative phase and continue through the season to heading. Leaf blast will usually appear in the high areas of the field where the flood has been removed or lost. Rice is most susceptible to leaf blast at the maximum tillering stage. Areas of heavy nitrogen fertilization and edges of the fields are also potential sites of infection. If leaf blast is in the field or has been reported in the same general area and if the variety is susceptible, fungicide applications are advisable to reduce rotten-neck blast.

The pathogen overwinters as mycelium and spores on infected straw and seed. Spores are produced from specialized mycelium called conidiophores and become wind-borne at night on dew or rain. The spores are carried by air currents and land on healthy rice plants. The spores germinate under high humidity and dew conditions and infect the plant. Generally, lesions will appear four to seven days later, and additional spores are produced. Plants of all ages are susceptible.

Medium-grain varieties are more susceptible to blast, especially during the leaf phase, than the long-grain varieties grown in Louisiana. Environmental conditions that favor disease development are long dew periods, high relative humidity, and warm days with cool nights. Agronomic practices that favor disease development include excessive N levels, late planting, and dry soil (loss of flood). The disease can be reduced by planting resistant varieties, maintaining a flood, proper N fertilizer, avoiding late planting and by applying a fungicide at proper rates and timings.

Scouting for blast should begin early in the season. If leaf blast is in the field or has been reported in the same general area, and if the variety is susceptible, fungicide applications are advisable to reduce rotten-neck blast. The absence of leaf blast does not mean rotten-neck blast will not occur. Fungicide timing is critical. If a single fungicide application is used to control blast, it should be applied when 50% to 70% of the heads have begun to emerge. Application before or after this growth stage will not provide good control of this disease. This growth stage happens quickly, so it is important to scout for the crop growth stage at the same time as scouting for disease.

Allow time to obtain the fungicide and schedule the application. Under heavy blast pressure, two applications, one at boot and one at 50% to 70% heading, may be needed to effectively suppress blast.

Mind Your Disease

Most seasons, we prefer to plant more of our familiar cultivars and save limited acreage for trying out new cultivars. However, the seed situation for 2024 has left many planting different and new (or at least new to them) rice cultivars. Watching out for possible disease issues could be an important aspect of success this season. Consult the 2024 Rice Management Guide (https://www.uaex.uada.edu/farm-ranch/crops-commercial-horticulture/rice/Rice-Management-Guide.pdf) for cultivar-specific disease ratings.

Sheath blight continues to be our primary disease concern each year in rice. In general, it seems we’ve been helped in recent years by cultivar improvements toward the disease and conditions that have limited its severity.

Based on our cultivar disease ratings, sheath blight ratings are very susceptible (VS), susceptible (S), or moderately susceptible (MS). Our threshold for considering a fungicide application to manage sheath blight is based on cultivar susceptibility, disease presence (percent positive stops when scouting), and whether the disease is threatening the upper canopy.

For cultivars rated VS or S, we may consider a fungicide application if we have 35% or more positive field stops and sheath blight is threatening the upper two to three canopy leaves. For cultivars rated MS, we recommend waiting until 50% or more positive field stops and sheath blight threatening the upper canopy leaves. Even if we have a high percentage of positive stops, that alone doesn’t mean you should consider applying a fungicide.

In 2022, plenty of sheath blight could be found, but due to dry conditions, it remained low in the canopy. In contrast, the 2023 season brought extended weather at mid-season that was favorable to sheath blight, and it extended toward the upper canopy resulting in necessary fungicide applications. If we can make it to heading with the upper two to three leaves clean, then we have outrun direct yield loss.

While not a major issue in recent years, we cannot forget about the potential for issues with blast. There will likely be an increase in 2024 of acreage grown to cultivars with more blast susceptibility than any time in recent years.

Some keys to better blast prevention and management revolve around water management and timing of fungicide applications. The more susceptible a cultivar is to blast, the greater the benefit to growing it under a flooded condition and maintaining a flood depth of 4 inches or more through reproductive growth.

If conditions are favorable for blast (weather, poor flood depth, reports of blast infection in field or surrounding fields), then we can consider fungicide applications for prevention of neck and panicle blast. If attempting to manage blast with a single fungicide application, it should be made when panicles of main tillers are 30%-50% emerged but the neck is still in the boot.

Optimal prevention of neck blast through use of fungicides is achieved by making two applications: the first during late boot to 10% panicle emergence, and the second when panicles of main tillers are 50%-75% emerged from the boot.

Remember to avoid excessive nitrogen rates and apply adequate potassium fertilizer to minimize disease problems in rice. Best of luck keeping the 2024 rice crop clean, and scout before you spray!

Sheath Blight Issues in Furrow-Irrigated Rice

As I write this article April 10, Missouri is already around 30% planted as we had a big run the first week of April. Planting date trials suggest that yield potential begins to decline when we get into May, so hopefully we’ve gotten the majority of the rice crop in before you’re reading this. Projections for Missouri rice acreage are hanging around 225,000 acres, and I’m in agreeance that this will likely be what we end up with in the Bootheel.

Caused by the fungus Rhizoctonia solani, sheath blight has been an issue in furrow-irrigated rice and that issue has predominantly been at the top end of the field. The mindset of most experts earlier, including myself, was that it would be much less of a problem in furrow-irrigated rice. It’s safe to say we got that one wrong. While there is not a flood to float inoculum into contact with the sheaths, the disease inoculum is still present in the field. Counterintuitive to most earlier reasoning, reports from farmers and consultants and personal observation say that sheath blight is a much more common problem in furrow-irrigated rice than we would have thought, and in fact, the largest issue is at the top of the field.

Now let’s dive into that issue. While the sclerotia float and infect the plant at the water line in flooded rice, it can start at the soil line where a flood is not held because the sclerotia are still alive and well in that zone. Once infection occurs in furrow-irrigated rice, it may be more likely to spread due to slightly higher nitrogen rates and increased seeding rates. Also, the lack of a flood may keep the temperature higher under the canopy while the humidity remains very high due to muddy soil, creating an environment for sheath blight to thrive.

A strobilurin fungicide is still the best method of suppression for sheath blight. Avoiding a dense stand and excessive nitrogen rates are good cultural practices, but these practices may be needed to maximize furrow-irrigated rice yield potential. Growing a taller cultivar may also help, but that brings up another point: the rice is not growing as tall at the upper end of a row rice field, especially in situations with lower irrigation frequency, making it more susceptible to vertical progression of the sheath blight infection. The take-home message remains the same in furrow-irrigated rice, the upper two or three leaves must be threatened to cause significant yield reduction. Treatment threshold is 35% positive stops for a susceptible or very susceptible cultivar, and 50% positive stops for a moderately susceptible cultivar.   

Ultimately, we would never recommend an automatic fungicide application in rice. However, it is imperative to scout your furrow-irrigated rice for sheath blight infection and be ready to pull the trigger if needed. If you have any questions, please feel free to contact me and as always, God bless and eat MO rice!