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16
May
2013
Diseases may be a concern in late planted rice
Author: Yeshi Wamishe, Extension Rice Plant Pathologist
Four photos of rice

Stressed rice seedlings

Arkansas producers have been struggling to plant rice in the cold and wet conditions of 2013. As of the second week of May, the oldest rice I have seen was at the 3-leaf stage.  I planted my first field plots at the Rice Research and Extension Center on March 19, and the plants took a month to emerge. Since emergence, the cold weather has slowed down plant growth and the seedlings look stressed. Seedlings had several brownish spots that formed as a result of dew frozen on the leaves (photo A).  In your field, you might see similar symptoms and possibly more — including white rings that form at the soil line when seedlings encounter cold soil during emergence (photo B left).   In some cases, with cold weather, seedling leaves may turn dark brown (photo B, right). Symptoms get more complex if herbicide damage is also encountered (photo C).  Six weeks after planting, the seedlings still look unhappy and the expected solid green rice within plots is hard to see (photo D).  Instead, weeds have begun to emerge as pre-emergence herbicides lose control and seedling stands have been highly affected.

It is often said that rice is forgiving, and a significant proportion of our rice seedlings are likely to survive the harsh conditions if we finally get sunshine and warmer weather.  Modern rice varieties have the ability to tiller and fill in available space, thus compensating for early stand loss, but again only if we finally get sunshine and warmer weather. My other plots planted on April 25th emerged on the 17th day indicating soil temps are getting better.  In the third week of May several rice producers will still be planting.  With a rain shower every now and then, rice planting may continue through to the end of May or longer. Due to late planting, late emergence, or delayed growth in general, Arkansas rice in 2013 is going to be a late crop.

Some rice diseases are known to be of concern with late planted crops.  For disease to occur, it all depends on the presence of the pathogen, the susceptibility of the plant varieties, and favorable environmental conditions.  A disease occurs when the virulent pathogen propagules meet a susceptible variety in a favorable environment (see figure below).

The disease triangle: susceptible variety + virulent pathogen + favorable environment

The disease triangle

With late planting, you may be applying more nitrogen fertilizer to enhance plant growth through the short crop season. The excess nitrogen enhances development of not only the crop but also the pathogens. Excess nitrogen also makes the plants more lush and prone to pathogen attack.  As the season gets warmer and plants go into reproductive stages, water consumption by the crop increases.  When water is limited, it creates stress on the crop and the crop becomes prone to pathogen attack.  As weather is perpetually dynamic, the combination of dry- hot or wet-warm, foggy-warm, or other situations create favorable conditions for some pathogen groups to survive and reproduce unchecked.  It is important to note that reproductive stages are generally the most susceptible times for disease in crop development.

Blast (photos E & F) is one of the diseases of primary concern in late-planted rice. The pathogen attacks leaves, nodes, the collar and panicles of rice plant. If a susceptible variety such as CL 261 is planted late in the spring in a field with a history of the disease, blast is likely to prevail. These fields include those with sandier soils, river-bottom areas where dew is prominent, surrounded by trees, low in potash, and with inadequate irrigation capacity. The disease is aggravated by frequent light rains in June and July.  If leaf blast is apparent or suspected, you need to pump the water depth up to 4 inches or more. The flood needs to be held until it is time to drain the field. This will minimize the risk of neck blast.  The pathogen being seed- and air-borne, blast can affect plants early in plant growth. Therefore, you need to be prepared to spray an approved fungicide before heads have emerged. Two applications may be needed depending on the situations. Do the first application at late boot to boot-splitting and the second when heads are about 2/3 out on the main tillers. Be sure the panicle bases on the main tillers are still in the boot during the second application. The fungicides will not be as effective once the heads are fully out of the boot. Therefore, do not wait too late to apply the fungicide.

EFYeshiRice

Rice blast symptoms

Bacterial panicle blight of rice (photos G, H) is another major disease of late planted rice. In fields with a history of this disease, it should be of primary concern because there is no successful treatment for prevention or control of this disease.  In 2010 and 2011 late-planted rice fields were hit the hardest.  A planting date study in 2012 at the Rice Research and Extension Center near Stuttgart showed severe bacterial panicle disease in plots planted in the third week of May.  In this study, seeds of Bengal (a susceptible variety) were artificially inoculated with the bacteria. Rice in these test plots reached their peak reproductive stages during the hot days and nights of July.  The bacteria were present, the variety was susceptible, and the environment was favorable for the bacteria to survive and reproduce.  Later in August, the windy-rain of a Tropical Storm Isaac enhanced disease transmission between neighboring panicles.  The disease triangle was perfectly aligned and plots suffered from the disease.  We are working to understand this disease, but at this time, resistance is the best control method.  So far, the hybrids and Jupiter appear to do well in this regard.  These cultivars are not immune to the disease, but provide the best resistance of available lines.  High potash and adequate nitrogen fertilization, a “not too dense” crop canopy, and a constant deep flood throughout the season appeared to reduce the disease pressure in the past years.

GHYeshiRice

Bacterial panicle blight

Kernel smut and false smut (photos I and J) are frequently blamed for quality loss in fields with a history of the diseases. The smuts are unpredictable diseases. They may occur in hot-dry years or warm and wet years. What we do know is, they hit late-planted rice harder than early-planted rice. Under favorable conditions most of our cultivars are susceptible to smuts. False smut appears to be less sensitive to propiconazole fungicides than kernel smut.  For this reason the fungicide rate has been increased from 4 fl oz  of propiconazole to 6 fl oz rate.  Since the fungicide is for suppression of the diseases, spray earlier at mid-boot instead of waiting until boot-split.  Remember that fungicide timing and rate are important for suppression of both kernel and false smut.

IJYeshiRice

I. – Kernel smut
J. – False smut

 

 

 

 

 

 

 

 

 

Please refer to the MP154 – Arkansas Plant Disease Control Products Guide – 2013 (links below)

Rice seedling diseases

Rice fungicides

You can also contact your county agents for the rate and timing of applications.


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