Rice Disease Alert: Leaf Blast Has Started; Sheath Blight Is Bouncing

    Severe leaf blast symptoms in rice. Photo: Louisiana State University

    Blast: We received our first report of leaf blast on June 30 in 2020 from Randolph County on a medium grain rice, Titan. Although minimal, blast has been detected this season on medium grain rice—Jupiter, Titan, CLM04 in Louisiana.

    If you have not started scouting fields planted with susceptible rice for leaf blast, please do so. As observed for several years, blast in Arkansas often starts in the second or the third week of June. Right at the end of June, we received our first report.

    Cooler temperatures than required by the sheath blight fungus highly favor the rice blast fungus. However, the blast fungus is highly adaptable to survive in wide ranges of temperatures and hence, we need to be diligent and be ahead of this disease to minimize its deleterious effect.

    Blast is an unpredictable disease.  Long dew periods (leaf wetness) from 9 to 14 hours are needed for spores to germinate.  Dew, fog, shade (trees), frequent light rains, and overcast enhance prolonged leaf-wetness period. Low-lying fields also retain leaf moisture longer than higher ground.

    The blast fungus survives on rice seeds, in residues, and also on live plants. Slow wind carries pathogen spores across the field and to neighboring fields. The spores can also be carried long distances. Seedlings as early as tillering can be killed completely and heavy infections to the panicle/node can cause devastating yield loss.

    Soils that do not hold flood consistently often are prone to blast. Susceptible rice planted late and applied with excessive nitrogen fertilization are often at risk. Such fields need to be kept adequately flooded at least to 4 inches depth.

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    Field spots to scout for blast: Scouting for blast across your fields planted with susceptible or moderately susceptible rice is not realistic. Blast infection often shows up along tree lines, drier field edges, on the levees, and spots in the field where nitrogen fertilization overlaid. Early symptoms may be greyish-black spots without the typical symptoms as in Figure 1.

    You may find the typical symptoms of blast if you open the canopy and look at lower leaves as in Figure 2. If you cannot find leaves with typical leaf blast symptoms but only the dark-grey spots, you may flag it for re-visit within a week.  When more infection occurs across a field, you may see color differences as shown in Figure 3.

    If more of these are seen, chemical spraying becomes inevitable.

    Early detection helps for two reasons: 1) To raise the flood depth immediately and lower further leaf infection. As a result, you will cut cost from early fungicide application and 2) To definitively plan for protective fungicides to guard your rice from later season neck, panicle and collar blast damage.

    Fig. 1. Early symptoms of blast may look greyish-black spots and can be confusing.

    Fig. 2. Typical diamond-shaped lesions of blast are formed after the pathogen sporulated. The symptoms are more evident on the lower leaves where leaf wetness lasts longer for the spores to germinate and infection to proceed.

    Fig. 3. Color differences across the field due to new blast infections. Slow wind often carries spores across the field from focal point (areas prone for blast spores to germinate.

    Sheath blight: It is wet and warm and very favorable for the sheath blight pathogen to take off. See Figure 4 below. Our research plots were inoculated on June 24 and this picture was taken on June 29.  In 5 days, the pathogen established itself within rice tissues and lesions are seen both on the sheaths and lower leaves.

    This means the weather is just perfect for sheath blight fungus to bounce in your rice. Please continue scouting and determine the threshold for management action.

    To read on how to establish the threshold, please visit last week’s post here.

    Fig. 4. Plots were inoculated on June 24th, 2020. On June 29, the sheath blight pathogen has already established itself within rice tissues and lesions are formed both on the sheath and lower leaves.




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