Rice Leaf Blast Detected in a Blast-Susceptible Variety in Bolivar and Sunflower Counties

Rice leaf blast. Note the diamond shaped lesion that can oftentimes be confused with other rice diseases.

Leaf blast was observed and confirmed in three rice fields on Tuesday (8/6/13) afternoon.  The three fields were at different growth stages.  Two of the fields had not reached head emergence while the third field the rice had reached ripening stages.  In that particular situation blast was only observed on the field edge in a location where the flood was extremely low.  The other two fields were between booting and flowering stages.

Properly diagnosing blast is an important first step.  Other foliar diseases (e.g., brown spot, stackburn) can produce lesions that appear similar to blast.  Chemical damage can also be misdiagnosed as leaf blast so consider the products that have been applied to a field prior to observing for blast symptoms.  Generally speaking, blast lesions initially appear as small white, gray, or blue-tinged spots on leaves.  Under conducive environmental conditions, that typically include high levels of relative humidity, the spots can enlarge quickly to either oval or diamond-shaped spots.  On susceptible varieties the leaf spots can become linear with pointed ends and white centers with brown/maroon borders.  Keep in mind that blast lesions can have tremendous diversity so observing several leaves for the presence of lesions and taking into account the difference in size and shape will aid in the proper diagnosis of the disease in the field.  In the laboratory, lesions can be observed for the characteristic lemon-shaped spores when lesions cooperate.

Fungicides are not generally applied to manage leaf blast unless the disease is killing the stand.  The best management alternative is to restore or deepen the flood to make sure that all of the soil is covered.  However, it should be noted, that the sporulation from leaf blast lesions can also lead to infection of the panicle and result in neck blast.  Numerous fungicides are labeled for blast management.  However, in fields planted to susceptible varieties where leaf blast has been detected a fungicide product that contains a preventive (strobilurin) and a curative (triazole) fungicide are likely the best management option.  Based on information obtained from researchers in Arkansas and Louisiana fungicides that contain trifloxystrobin (Gem and Stratego) have superior activity on blast compared to fungicides that contain azoxystrobin (Quadris, Quilt, Quilt Xcel).  But, fungicides that contain azoxystrobin do have good activity against blast.

Application rate as well as coverage will be important to control the disease if it has already started to sporulate in the field.  In the three field situations outlined above leaf blast lesions were all actively sporulating.  Fungicide rates for blast management are as follows:

-Gem = 3.1 – 4.7 fl oz/a (pre-harvest interval = 35 days)

-Quadris = 12 – 15 fl oz/a (pre-harvest interval = 28 days)

-Quilt = 28 – 34 fl oz/a (pre-harvest interval = 35 days)

-Quilt Xcel = 21 – 27 fl oz/a (pre-harvest interval = 35 days)

-Stratego = 16 – 19 fl oz/a (pre-harvest interval = 35 days)

Please always read and follow label directions especially when it comes to pre-harvest interval (PHI).

To increase the effectiveness of the fungicide aerial applications should be made in a minimum of five gallons of water.  Follow label instructions regarding the use of an adjuvant (either COC or NIS) that will aid in coverage as well as canopy penetration.

Cultural practices can also contribute to the production of leaf blast in rice fields.  Leaf blast can be more of a problem in situations where:

(a) the field has a history of disease,

(b) a susceptible variety was planted,

(c) high nitrogen rates are used,

(d) the field in question has sandy soils,

(e) the rice was planted late (late-planted rice is more likely to encounter foliar disease problems than early-planted rice),

(f) along the edges of tree lined fields, and

(g) the rice is growing under conditions were the flood has not been maintained (upland conditions)

Last year in Louisiana, under several blast pressure, plots that contained Bengal, Caffey, CL151, CL152, CL162, CL181, CL261, Cypress, Jupiter, Rex, and Wells appeared to be more susceptible to leaf blast than other varieties.