Steven Linscombe | 6/22/2012 1:01:20 AM
Rice produced in Louisiana is susceptible to a number of major and minor diseases. Sheath blight disease normally is present each year, and the severity of this disease is predicated by the particular environmental conditions of the growing season. Other diseases such as blast and bacterial panicle blight tend to be much worse in some years than others. This year appears to be a severe one for 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 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. Blast spots on leaves start as 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. This year, leaf blast began to be detected in a number of fields in early May. In several of these fields, the infection became severe enough to warrant a fungicide application. It is rare to have an infection level that severe.
Typically, the disease can be most damaging in later growth stages. On stem nodes, the host tissue turns black and becomes shriveled and gray as the plant approaches maturity. The infected area may turn dark purple to blue-gray because of the production of fungal spores. 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. Some varieties are infected where the flag leaf attaches to the sheath at the collar. The lesion turns brown or chocolate-brown to gray, and the flag leaf becomes detached from the plant as the lesion area becomes dead and dry. 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, lost or is shallow. 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.
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. Medium-grain varieties are more susceptible to blast, especially during the leaf phase, than the long-grain varieties grown in Louisiana. The medium-grain variety CL261 appears to be the most susceptible variety. The disease also has been detected in fields of Jupiter, CL151 and CL152. However, none of our currently grown varieties are immune to the diseases. 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 nitrogen levels, late planting and dry soil (loss of flood). Several physiologic races of blast disease exist, and disease development varies, depending on variety-race interactions. One of the most important cultural practices that can be undertaken to minimize the potential for blast disease is to maintain an adequate flood depth throughout the field.
Because of the prevalence of the disease this year, producers may want to consider a fungicide application to minimize the potential impact of the disease. Fungicides are available for reducing blast, but fungicide timing is critical. Leaf blast present in the field is the indicator to spray for blast. Remember, when scouting for disease also scout for rice growth stages, so fungicides can be applied in a timely manner. For a single application, apply when 50 percent to 70 percent of the heads have just begun to emerge because fungicide application before or after that growth stage will not provide good control of this disease. Under severe disease pressure, which we seem to have this year, two applications may be necessary – the first at the boot growth stage (2- to 4-inch panicle) followed by an application at heading as described above. The first application controls the leaf blast present in the field, which reduces spore production and allows the heading application a better chance of controlling the head infections.
Fungicides containing the active ingredient trifloxystrobin (Gem and Stratego) have the best blast activity, but azoxystrobin-containing fungicides (Quadris, Quilt and Quilt Excel) also have good blast activity. Gem fungicide is not readily available, but the Stratego fungicide is priced less and has propiconazole already in it for broader disease control, including Cercospora. The rate of Stratego is 16-19 oz/A, Quadris 12-15 oz/A, Quilt 28-34 oz/A and Quadris Xcel 21-28 oz/A. In the areas where the strobilurin-resistant sheath blight pathogen Rhizoctonia solani is present, the recommendation is to apply the Section 18 fungicide Sercadis, which has no blast activity, at boot and a blast-active fungicide at heading.
Permission granted June 15, 2012 by B. Leonards (LA Farm & Ranch) to republish article on www.lsuagcenter.com