Blast is one of the devastating diseases of rice. The blast pathogen is air, seed, or residue-borne. Seed and air-borne pathogens can easily move long distances. Air-borne spores usually infect young leaves. Spores from young leaves may infect collars, nodes, and panicles of rice. Sometimes when the first emerged leaves dry and die back, blast tends to “disappear” and may not be a problem later in the season. Blast has not been much of an issue in recent years in Arkansas. However, when present, it can cause dramatic yield loss on susceptible rice under favorable conditions. Although severe damage occurs when the fungus attacks the nodes, leaf blast can also do considerable damage to leaves and provide inoculum for neck blast later in the season.
This week, we got a couple of reports of leaf blast—one from Lonoke and the other from Randolph County in Arkansas. In both counties blast was detected on Jupiter, a susceptible medium grain rice variety. In the last couple of weeks, blast was reported in Louisiana. With rice blast, it is wise to be watchful to not create favorable disease conditions i.e. shallow flood or excessive use of nitrogen fertilizer. Regularly scouting blast-prone fields planted with susceptible varieties is highly recommended. For relative susceptibility of the current commercial varieties see the table below.
|Blast||S||S||No info||S||No info||R||VS|
|Variety||CL142 AR||CL181AR||CL 151||CL 152||CL 162||CL 261||COCODRIE|
|Variety||RT CL XL729||RT CL XL745||RT CL XP756||RT XL723||RT XP 753||RT XP 754||TAGGART|
|Blast||R||R||No info||R||No info||No info||MS|
|Variety||CL111||CL 131||REX||ROY J||COLORADO||DELLA-2||TEMPLETON|
Early blast disease in susceptible varieties can be well suppressed with deep flood management. Deep flood suppresses the pathogen growth and reproduction, while a drought stress or aerobic condition promotes disease development. Fungicide application may not be needed for early blast disease unless the field has a history and conditions are favorable. Once a flood is established, maintenance of a deep flood up to four inches throughout the season is highly recommended. Excess nitrogen fertilization also worsens the blast disease.
The occurrence of blast in Randolph County is in an open area at the edge of the field where the ground is high. The ground was dry to the extent of causing leaf tip necrosis (Picture 1). The field in Lonoke County is surrounded by trees. The hot spots are largely at the field edges where the dew period is long due to the shade from trees particularly on east side (Picture 2). In both fields, distinct old and young lesions have developed (Picture 3 and 4). Older lesions are lens-shaped—typical symptoms for identification. Young lesions can be confused with brown spot or chemical injury to unfamiliar eyes. As a result, laboratory diagnosis may be needed to confirm blast in the absence of older lesions.
Recent weather in Arkansas has encouraged blast disease development. Mild temperatures, extended free moisture periods on the leaves, foggy or cloudy conditions, frequent rain and slow wind promote disease development and spread.
If available, resistant varieties are better options for easy management of blast. Hybrids appear to have the best overall resistance but several Clearfield and non-Clearfield varieties also have adequate resistance under Arkansas conditions. It is important to note that resistance to blast is not permanent as the blast fungus eventually adapts to overcome resistant varieties if planted over and over under favorable blast conditions. Therefore, scouting for blast is necessary particularly in blast-prone areas. Leaf blast can start at any developmental stage of the rice crop. Scout fields for leaf blast symptoms on susceptible varieties grown in blast-prone fields and apply preventative fungicides to manage neck blast that may appear later in the season. Scout fields close to the tree lines more frequently. Fields in low lying areas, with heavy tree lines especially on the east side, that do not hold water or are difficult to water, and that are planted late can be categorized as “blast-prone fields”. Usually leaf blast prevalence in a particular field could be an early warning for neck blast later in the season. Although an exception is a possibility, you need to be prepared to apply preventative fungicides for neck blast. Fungicides applied after the head is completely out of the boot will not be effective enough to control neck blast disease. Waiting too late is a call for disaster. Susceptible varieties need to receive the fungicides at late boot stage to 10% heading followed by a second application at 50-75% heading. Remember for fungicide application during “heading” the neck should be still in the boot. That way, the effectiveness of the fungicide is maximized and the crop is rescued from disaster. Triflooxystrobin (GEM is considered slightly more effective on blast than Azoxystrobin (Quadris). To protect the crop from neck blast, the higher rates are preferred.
Below are answers by Cartwright and Wamishe to questions on blast in 2013. They are provided below in case similar situations happen.
Q. If I see leaf blast early in the season and treat field with Quadris, do I need to spray twice later in the season to control neck blast?
A. You should keep permanent flood to at least 4 inches. A deep flood is 4 inches in the shallowest part of the paddy. The effective fungicides work best if applied twice for blast. The first application should be made at late boot to beginning of panicle tip emergence and the second when panicles are 50-75% out of the boot on most of the main tillers. Higher rates are best.
Q. Do I need to worry about blast when I hear reports of its occurrence in LA, TX, or even in AR?
A. No. But scouting of fields historically prone to blast is always warranted. Leaf blast should be scouted for during June or early July. We recommend integrated disease management practices including deep flood, clean and high quality seed, resistant varieties, proper N and K fertilizer management, and disease scouting and fungicide applications when needed. Staying alert and on top of irrigation is really necessary for blast control in areas prone to the disease.
Q. If there is no blast in my field but I know my variety is susceptible to blast; do I need to spray fungicide for protection?
A. Depends. If there is truly no blast in the field, why would we spray? Accurate scouting pays off with this disease. Any blast history in the field is a good indicator of potential disease problems, so these fields should be monitored accordingly. Leaf blast and neck blast occur at different times during the season, and leaf blast tends to dry up or “disappear” after midseason in many fields. While preventative applications to all acres of susceptible varieties are simple, they are not as economical or as sustainable as IPM applications based on monitoring and field to field judgment.
Gratitude goes to Dr. Jarrod Hardke, Brent Griffin and Craig Brown for their efforts to timely report the disease situation.